Method of electrochemical synthesis of ^ ⁵ -iodanes derived from iodoarenes in water-based solutions Field of Art The present invention relates to a method of electrochemical synthesis of hypervalent iodine compounds ( ^ ^x -iodanes, with “x” representing valency of the iodine atom) in solution and in particular to a method for electrochemical synthesis of oxidants possessing iodoarene (e.g. iodobenzene) or substituted iodoarene (e.g.2-iodobenzoic acid or 2-iodo-4-chlorobenzoic acid) in their structure. Background Art Hypervalent iodine oxidants have found countless applications in organic synthesis. They are considered as an interesting alternatives to heavy metal based oxidants. Their attractivity stems from environmental friendliness, low toxicity and wide range of applications. However, their synthesis can still be considered as rather problematic. Chemical methods of hypervalent iodine chemistry synthesis are based on oxidising iodoarenes by various oxidants. The most common oxidants mentioned for this purpose in patent and non- patent literature are permanganate, bromate, chlorine, persulfate or percarboxylic acids. Numerous other oxidants such as chromate, hydrogen peroxide or molecular oxygen have been used. However, the use of majority of these compounds is connected with handling risks due to their toxicity and/or instability. From this point of view an electrochemical synthesis of hypervalent iodine compounds is beneficial as it obviates the need for application of the problematic oxidation agents. On the other hand, electrosynthesis of the hypervalent iodine oxidants is much less developed. Until recently, there were practically no publications on the topic. The first and for long time the only report, where authors investigated anodic oxidation of iodobenzene and several its derivatives including iodobenzoic acids in CH3COOH/H2SO4 solutions containing up to 30% of added water using Pt working electrode was published nearly 100 years ago. It led to low yields of ^ ³ - and ^ ⁵ - iodanes accompanied always by pronounced undesired decomposition of starting materials (F. Fichter, P. Lotter, Über die elektrochemische Oxydation von Jodbenzol, o-Jodtoluol und p-Jodtoluol, Helvetica Chimica Acta, 8 (1925) 438- 442). Later, for example successful fluoridation was achieved by action of 4- (difluoroiodo)methoxybenzene prepared by electrochemical oxidation of corresponding iodide in acetonitrile solutions containing Et3N ^3HF (T. Fuchigami, T. Fujita, Electrolytic Partial Fluorination of Organic Compounds. 14. The First Electrosynthesis of Hypervalent Iodobenzene Difluoride Derivatives and Its Application to Indirect Anodic gem-Difluorination, The Journal of Organic Chemistry, 59 (1994) 7190-7192). Acetoxylation of iodobenzene in acetonitrile/CH3COOH system to iodosylbenzene diacetate ( ^ ³ -iodane) at graphite electrode was shown quite effective, however acetoxylation occurred partially on the benzene ring and isolated yield of the product was only 25% (M. Elsherbini, B. Winterson, H. Alharbi, A.A. Folgueiras-Amador, C. Génot, T. Wirth, Continuous-Flow Electrochemical Generator of Hypervalent Iodine Reagents: Synthetic Applications, Angewandte Chemie International Edition, 58 (2019) 9811-9815). Very popular for anodic oxidation of iodides to synthesis ^ ³ -iodanes are fluorinated solvents such as hexafluoroisopropanol or trifluoroethanol. In these solvents, corresponding alkoxylates are usually produced on various electrodes (glassy carbon, graphite, platinum, boron doped diamond). Some of these electrosyntheses can be operated with high selectivities and current yields. However, despite the fact, that the fluorinated solvents can be recycled, they are neither environmentally friendly nor inexpensive. More environmentally friendly environment for acetoxylation is anhydrous CH3COOH/H2SO4 solution which allows synthetizing (di)acetates of iodosylbenzene, iodosylbenzoic acids and amides of iodosylbenzoic acids (all ^ ³ -iodanes) in high yields and reasonable current efficiencies. Almost all the procedures discussed above led only to generation of ^ ³ -iodanes, which limits the potential application of these electrosynthetic methods. Oxidation of 2-iodobenzoic acid to 2-iodosylbenzoic acid and 2-iodylbenzoic acid (IBX) in aqueous electrolytes at boron doped diamond electrode do provide ^ ⁵ -iodanes. These methods are, however, also not practical, since the solubility of involved compounds limits concentration of the species in solution to a maximum of 1 mmol dm ^-3 . The most interesting method for practical application seems the very recent method based on anodic oxidation of several iodobenzene derivatives (20 mmol dm ^-3 ) in acetonitrile with 1-10% of H2O solvent with LiClO4 electrolyte at glassy carbon electrode (S.J. Folkman, R.G. Finke, J.R. Galán-Mascarós, G.M. Miyake, Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source, ACS Sustainable Chemistry & Engineering, 9 (2021) 10453-10467). Using this procedure, various ^ ³ - and ^ ⁵ -iodanes including 2-iodosylbenzoic acid, IBX, 2-iodosyl and 2-iodoxybenzenesulfonic acids, were electrochemically prepared. However, highly volatile and flammable acetonitrile based electrolyte solutions (moreover with expensive and potentially dangerous LiClO4) might still be problematic for upscale. In addition to that, for example separation of 2-iodosyl and 2- iodoxybenzenesulfonic acids was not achieved. Moreover, the type of glassy carbon used in this work was not specified, which might be problematic considering variety of commercially available glassy carbon materials. As can be seen, a need exists for easy, scalable and eco-friendly electrochemical method of hypervalent iodine compounds synthesis avoiding the use of hazardous or toxic oxidants, supporting electrolytes and solvents. Summary of Invention The present invention provides a method for production of hypervalent iodine compounds and, in particular, those containing iodobenzene moiety, without the need to add an oxidant. The method involves adding aryl iodide, solvent and potentially also electrolyte in the form of organic acid, inorganic acid, inorganic hydroxide or salt thereof into electrochemical cell possessing an anode and a cathode and, optionally, also a reference electrode. When appropriate electric field is applied by means of the anode and the cathode, the present aryl iodide is oxidized at the anode or in its vicinity yielding corresponding ^ ³ -iodanes and ^ ⁵ -iodanes. The oxidation state of iodine atom in the mixture and, particularly, the amount of product ( ^ ⁵ - iodane) may be controlled by conditions of the electrochemical reaction, such as by reaction mixture composition, electrode potential, current, electrolysis duration, applied voltage, and overall charge transported between the anode and the cathode through the reaction mixture solution. These parameters are closely interconnected and influence each other. Reaction mixture composition affects the general direction of the reaction, i.e. character of the oxidation products, solubility of the reactants and products, ionic conductivity of the electrolyte phase. In particular, concentration of the reactant in the reaction mixture affects currents (at given electrode potential) which are connected with desired oxidation process. Electrode potential or voltage affect current and character of the oxidation products. Electrolysis duration and current affects the overall transported charge which influences conversion of the reactant to the products and amount and character of the products. One or more solvent molecules may serve as reactant(s) in the anode reaction in which case the fragments of one or more solvent molecules are acting as a ligand(s) of the iodine atom in the product molecule. Definitions: alkyl = a saturated hydrocarbon chain, which may be straight, branched or cyclic or cycle- containing, and which is derived from an alkane by removal of one hydrogen atom. aryl = a functional group derived from an aromatic hydrocarbon by removal of one hydrogen atom. alkoxy = a hydrocarbon chain, which may be straight, branched or cyclic or cycle-containing, derived from an alkane by removal of one hydrogen atom, and singularly bonded to oxygen (R- O-). halogen = F, Cl, Br, I acyl = a moiety derived by the removal of one or more hydroxyl groups from an oxoacid (C2 to C6)alkylacyl = a hydrocarbon chain containing from 2 to 6 carbon atoms, which may be straight, branched or cyclic or cycle-containing, derived from an alkane by removal of one hydrogen atom, and singularly bonded to a carbonyl group, thereby forming R-C(=O)-. sulfonyl = a moiety derived from a sulfonic acid by removal of one hydroxyl group (C2 to C6)alkylsulfonyl = a hydrocarbon chain containing from 2 to 6 carbon atoms, which may be straight, branched or cyclic or cycle-containing, derived from an alkane by removal of one hydrogen atom, and singularly bonded to SO ₂ ^2- group, thereby forming R-S(=O) ₂ -. linear methoxy-(C2 to C5)alkyl = a linear hydrocarbon chain containing from 2 to 5 carbon atoms, derived from an alkane by removal of two hydrogen atoms from opposite carbon atoms, and singularly bonded to oxygen of a methoxy group (CH ₃ -O-(CH ₂ ) _n -). (C1 to C6)trialkylammonium group = a positively charged quaternary ammonium cation group, containing three alkyl groups containing from 1 to 6 carbon atoms, attached to a central nitrogen atom (N ⁺ (R3)-). (C1 to C6)alkyl-(C6 to C10)arylsulfonic acid = a sulfonic acid R-SO3H, in which substituent R is represented by aromatic hydrocarbon containing 6 to 10 carbon atoms, where one of the hydrogen atoms is removed and replaced with alkyl containing 1 to 6 carbons. semiconducting conductivity = electronic conductivity appearing in solid materials possessing band gap energy Egap > 0 (energy difference between highest energy level of valence band and lowest energy level of the conduction band), when electrons are excited from valence to conduction band; electronic conductivity of semiconductor increases with increasing temperature. metallic conductivity = metallic conductivity appears in condensed phase when the band gap energy is zero, in some cases valence band and conduction band even overlap; metallic conductivity decreases with increasing temperature. anolyte = an electrolyte solution present in (or flowing through) the anode compartment. catholyte = an electrolyte solution present in (or flowing through) the cathode compartment. MSE - mercury sulfate electrode = an electrode of the second kind (a reference electrode), it possesses three phases (Hg(l), Hg2SO4(s), SO4 ^2- (aq)) in equilibrium; the corresponding redox process taking place at the Hg surface can be written as Hg2SO4(s) + 2e- 2 Hg(l) + SO4 ^2- (aq) and since the solubility equilibrium between sulfate in solution and in precipitate (Hg2SO4(s) ⇆ 2 Hg ⁺ (aq) + SO4 ^2- (aq)) is fast, the electrode potential of the MSE, EHg2SO4|Hg, is given by the activity of the sulfate in the solution phase, a _SO42- (dimensionless), i.e. EHg2SO4|Hg = E ^Hg2SO4|Hg – RT/2F ^ln(aSO42-), where R represents universal gas constant (unit Joule per Kelvin per mole, J K ^-1 mol ^-1 ), T temperature (unit Kelvin, K) and F is Faraday’s constant (unit Coulomb per mole, C mol ^-1 ) and E ^Hg2SO4|Hg = 0.640 V (at 298 K). current density, j (unit ampere per meter square, A m ^-2 ) = current I per geometric area of the working electrode AG, i.e. j = I/AG ^ ³ -iodane = a compound possessing in its structure trivalent atom of iodine. ^ ⁵ -iodane = a compound possessing in its structure pentavalent atom of iodine. electrode potential, E (unit volt, V) = a voltage of the electrochemical cell U consisting of the appropriate reference electrode (with potential at or close to equilibrium state) and investigated working electrode, lowered by the ohmic losses caused by the current I (unit ampere, A) flowing in the system with internal electric resistance R (unit Ohm, Ω), and by the liquid junction potential ΔELJ, i.e. E = U- I ^R - ΔELJ supporting electrolyte = an electrolyte electrochemically inactive within the used potential range, it is added to the solution in order to ensure or increase an ionic conductivity of the solution. electrolyte volumetric flow rate, Vflowrate (unit m ³ s ^-1 ) = an electrolyte volume flowing through the system per unit of time. The object of the present invention is thus a method for electrochemical synthesis of ^ ⁵ -iodanes, which comprises the following steps: i) providing an aqueous solution of iodoarene of general formula Ar-I, wherein Ar is (C6 to C10)aryl, which may optionally be substituted with one or more substituents selected from the group comprising (C1 to C6)alkyl, (C6 to C10)aryl, (C1 to C6)alkoxy group, -OH, -COOH, -SO3H, halogen, -NO2, -C(=O)-NHR, -C(=O)-NR2, wherein R is independently selected from the group comprising (C1 to C10)alkyl, linear methoxy-(C2 to C5)alkyl, (C1 to C6)trialkylammonium group; The electrochemical cell, suitable for the claimed method, comprises a batch or flow cell body, equipped with at least one anode (a working electrode) and at least one cathode (a counter electrode). The cell may further possess a separator of the anode and cathode compartments preventing free solution convection between anode and cathode compartment while at the same time ensuring ionic conductivity in the volume between the electrodes. The cell may be equipped with device allowing stirring of the solution, such as magnetic stirrer. The cell may be connected to a hydraulic circuit allowing electrolyte flow through the cell, or to two hydraulic circuits allowing anolyte and catholyte flow though the anode and cathode compartment, respectively. The cell may further possess the reference electrode, positioned preferably on the same side of the hydraulic circuit as the anode. Electrolysis may be operated in batch reactor, flow reactor with recirculation of electrolyte solution(s) or in a single-pass continuous reactor; ii) placing the aqueous solution of iodoarene from step i) into an electrochemical cell by filling the electrochemical cell with the iodoarene solution or flowing it through the electrochemical cell such that the iodoarene solution is in direct contact with the anode; wherein the electrochemical cell contains a cathode and an anode, wherein the cathode is the counter electrode and the anode is the working electrode and comprises a surface containing a boron doped diamond active layer possessing semiconducting or metallic conductivity, which is in direct contact with the aqueous solution of iodoarene; iii) determining the electrode potential of the anode for the desired oxidation reaction of the iodoarene to ^ ⁵ -iodane; iv) applying the electrode potential determined in step iii) to the aqueous solution of iodoarene of general formula (I) to oxidise the iodoarene of general formula Ar-I into ^ ⁵ -iodane of general formula (I), (II) and/or (III), ArI(OR ¹ )(OR ² )(OR ³ )(OR ⁴ ) ............... (I) ArI(OR ¹ )(OR ² )(O) ..............................(II) ArIO2 ................................................(III) wherein Ar is defined in step i), R ¹ , R ² , R ³ , R ⁴ are independently selected from the group comprising hydrogen; acyl, which may be attached to the same or different aromatic ring as the iodine atom; (C2 to C6)alkylacyl; sulfonyl group that may be attached to the same or different aromatic ring as iodine atom; (C2 to C6)alkylsulfonyl;. The ^ ⁵ -iodane is obtained in the form of a homogenous solution or a suspension. It is preferred to ensure convection or movement of the aqueous solution of iodoarene with respect to anode surface in step iv) by means of stirring, mixing or flowing of the electrolyte solution or by any other means. The substituents R ¹ , R ² , R ³ , and R ⁴ may be parts of solvent molecules or may represent further substituents present on the aryl of the iodoarene Ar-I. As non-limiting examples of the structures of ^ ⁵ -iodanes of general formulae (I), (II) or (III) are the following structures: wherein R ¹ is OH from water molecule and R ² is sulfonyl present as a substituent on the iodoarene; as ArI(OR ¹ )(OR ² )(O), as ArI(OR ¹ )2(OR ² )(OH) wherein R ¹ substituent come from acetic acid present in the solution and R ² from the carboxyl group attached on the aromatic ring ^; wherein R ¹ substituent comes from acetic acid present in the solution and R ² from the carboxyl group attached on the aromatic ring, R ³ is sulfonyl present as a substituent on the iodoarene and R ⁴ is OH from water molecule. Step iii) of determining the electrode potential of the anode suitable for the desired oxidation reaction can preferably be done by first measuring of linear voltammogram, (dependence of I on electrode potential E obtained by measuring I while linearly changing E in time with defined potential scan rate dE dt ^-1 ) in the electrochemical cell and with the working electrode of step ii), possessing the reference electrode (preferably selected from the group comprising Hg(l)|Hg2SO4(s)|SO4 ^2- (aq) electrode, Ag(s)|AgCl(s)|Cl-(aq) electrode, Hg(l)|Hg2Cl2(s)|Cl-(aq) electrode, Pt(l)|H2(g)|H ⁺ (aq) electrode) and containing the iodoarene solution of step i), at potential scan rate preferably in the range of from 0.05 to 0.2 V s ^-1 , preferably with active uncompensated resistance compensation. Second, the measured linear voltammogram is analyzed to determine the suitable electrode potential: The linear voltammogram contains two concave segments corresponding to oxidation of iodoarene to ^ ³ -iodane and further oxidation of the ^ ³ -iodane into ^ ⁵ -iodane. The latter concave segment always appears at higher E _p than the concave segment corresponding to ^ ³ -iodane formation. If the result of the measurement is unsatisfactory and oxidation peaks are merged and/or badly defined, it is possible to repeat the measurement of the linear voltammogram with the iodoarene concentration decreased to 50% of the original value. This can be repeated until a satisfactory result is obtained. The electrode potential for the oxidation reaction is selected in the range of from Ep-0.23 V to Ep+0.6 V of oxidation peak corresponding to λ ⁵ -iodane generation at the electrode, wherein the corresponding current at E _p as measured from the voltammogram is called the absolute peak current (Ip ^abs ). Ep can be determined as a potential at the local maximum on the concave part of the voltammogram. If the peak representing the concave part of the voltammogram is not well defined, i.e. if there is no local maximum on the concave part of voltammogram corresponding to λ ⁵ -iodane generation and Ep cannot be unambiguosly determined, the Ep can be approximated as an arithmetic average of the potentials of the nearest inflection points on the voltammogram (E _p = 0.5 ^E _inf,1 + 0.5 ^E _inf,2 ), see Figure 2; the first inflection point potential (E _inf,1 ) is defined as a potential at which the first derivative of the current in voltammogram, dI dE ^-1 , attains local maximum and, at the same time, second derivative of the current in voltammogram, d ² I dE ^-2 , attains zero value; the second inflection point (Einf,2) is defined as a potential at which the first derivative of the current in voltammogram attains local minimum and at the same time second derivative of the current in voltammogram attains zero value; it applies that E _inf,1 < E _inf,2 ; for practical determination of the Einf,1 and Einf,2, analysing only first derivative, i.e. dI dE ^-1 in the potential range of badly defined oxidation peak is necessary (Figure 2). The working electrode (anode) contains a boron doped diamond active surface, and is commercially available. The working electrode can also be prepared by chemical vapour deposition assisted by hot filament or microwaves (from gaseous mixture containing hydrocarbons, hydrogen and source of B doping atoms such as (di)borane) onto a suitable substrate such as Ti, Ta, Mo, W or Si. Free-standing boron doped diamond layer are also commercially available. Vast majority (>99 mol.%) of carbon contained in the resulting doped diamond layer possesses sp ³ hybridisation. The doping element is introduced in order to ensure electronic conductivity of the material. The amount of boron in diamond significantly affects the material electronic conductivity and conductivity type. For example, in the case of boron doped diamond electrode, valence band conductivity is present at boron doping levels below roughly 10 ¹⁹ B atoms cm ^-3 . At doping levels above 3 ^10 ²⁰ B atoms cm ^-3 , a metallic conductivity is present. Between these two boron concentrations, a hopping conduction is present, i.e. material behaves like a p-type semiconductor. The boron doped diamond electrodes possessing the hopping conduction and preferably metallic conductivity are most suitable for the electrochemical applications of the present invention. Such electrodes are commercially available under various commercial names, for example DIACHEM ^® electrodes can be purchased from CONDIAS GmbH, NeoCoat ^® -Electrodes from NeoCoat SA, Diafilm™ EP from Element Six. Boron doped diamond offers several advantages compared to glassy carbon: 1. it is more chemically stable, meaning that it can operate in more harsh environment where glassy carbon surface would be significantly damaged and would lost its function; 2. it is more electrochemically stable allowing to be operated at more extreme electrode potentials than glassy carbon; 3. it possess significantly lower activity towards oxygen evolution reaction and hydrogen evolution reaction in aqueous environment resulting in wider potential window in aqueous solutions allowing to perform useful electrochemical reactions in wider potential range, i.e. at potentials, where water would be oxidized or reduced when using glassy carbon or where glassy carbon surface would be damaged after few minutes/hours of operation. This makes it possible to significantly increase faradaic efficiency of the desired electrode processes and sometimes even obtain products that could not be obtained using different electrodes including glassy carbon (J.V. Macpherson, A practical guide to using boron doped diamond in electrochemical research, Phys.Chem.Chem.Phys.17 (2015) 2935-2949). Faradaic efficiency (dimensionless) for the desired product (DP) at given time (t) αDP ^t is defined by the equation α _DP ^t = Q _DP ^t /Q ^t , where Q _DP, ^t is the electric charge that was consumed for the desired electrode process during electrolysis within time and Q ^t (unit Coulomb, C) is the overall charge consumed during electrolysis within t (unit second, s); Q ^t can be calculated by integrating current, I (unit Ampere, A) in time, ^^ ^t = ∫ _^ ^௧ ^^ ^^ ^^ , and the actual value it is often available in standard electrochemical software controlling electrochemical experiments. Q ^t = QDP, ^t can be calculated using equation Q _DP ^t = z _DP ^n _DP, ^t ^F, where z _DP represents number of electrons required to oxidize one reactant species (molecule, ion, radical) to the desired product, nDP,t is number of mols of desired product produced during electrolysis within given t, F is Faraday’s constant (F value is approximately 96485 C mol ^-1 ). Moreover, the present method allows to prepare ^ ⁵ -iodanes in significantly higher concentrations compared to previously described electrochemical synthesis method allowing to produce cca 20 mM ^ ⁵ -iodanes, whereas the method according to the present invention is capable of producing over 50 mM ^ ⁵ -iodanes. The electrochemical cell in step ii) can be selected from a batch electrochemical cell, a flow electrochemical cell with recirculation of the electrolyte solution and a single pass flow electrochemical cell. In one embodiment, the concentration of iodoarene in the solution in step i) is at least 1 µM, preferably at least 1 mM, more preferably at least 25 mM, even more preferably in the range of from 30 mM to 1 M, most preferably in the range of from 40 mM to 200 mM. In one embodiment, the aqueous solution of iodoarene in step i) may further contain at least one solvent selected from the group comprising (C2 to C6)carboxylic acid, (C1 to C6)alkyl- (C6 to C10)arylsulfonic acid, (C2 to C6)alkylsulfonic acid, acetonitrile. Preferably, the concentration of the further solvent is at most 85 vol.% Preferably, the aqueous solution of iodoarene in step i) further contains acetic acid. In one embodiment, the aqueous solution of iodoarene in step i) further contains acetic acid at the concentration in the range of from 50 vol.% to 75 vol.% In one embodiment, the aqueous solution of iodoarene in step i) may further contain a supporting electrolyte selected from the group comprising (C2 to C6)carboxylic acid, (C1 to C10) alkylsulfonic acid, (C1 to C10)alkylphosphonic acid, wherein the (C2 to C6)carboxylic acid, (C1 to C10) alkylsulfonic acid and (C1 to C10)alkylphosphonic acid may further be independently substituted with one or more halogen atoms; inorganic oxoacid of boron, sulfur, nitrogen, phosphorus or halogen, hydrogen halide, inorganic hydroxide of alkali metal, alkali earth metals, ammonium hydroxide, wherein one or more hydrogen atoms may be substituted with (C1 to C6)alkyl, and salts of any of the above mentioned acids with any of the above mentioned hydroxides. Preferably, the electrolyte is selected from the group comprising H2SO4, H3PO4, HNO3, HCl, arylsulfonic acid, arylphosphonic acid, 4-toluenesulfonic acid, halogenoacetic acid, trihalogenoacetic acid; LiOH, NaOH, KOH, tetraalkylammonium hydroxide; and salts of any of the above mentioned acid with Li ⁺ , Na ⁺ , K ⁺ , NH4 ⁺ , tetraalkylammonium cation with (C1 to C6)alkyls. More preferably, the concentration of the electrolyte in the aqueous solution of iodoarene is in the range of from 0.03 to 5 M, preferably in the range of from 0.1 to 2 M. The presence of supporting electrolyte enhances ionic conductivity in the solution. In one embodiment, the aqueous solution of iodoarene in step i) further contains sulfuric acid as the supporting electrolyte, more preferably at the concentration in the range of from 0.01 M to 2 M. In one preferred embodiment, the aqueous solution of iodoarene in step i) further contains sulfuric acid as the supporting electrolyte and acetic acid as further solvent. More preferably, the aqueous solution of iodoarene in step i) contains 0.2 to 2 M sulfuric acid and 60 to 70 vol.% of acetic acid. In one preferred embodiment, the iodoarene is iodobenzene, which may optionally be substituted with one or more substituents selected from the group comprising (C1 to C6)alkyl, (C1 to C10)aryl, (C1 to C6)alkoxy group, -OH, -COOH, -SO3H, halogen, -NO2, -C(=O)-NHR, -C(=O)-NR2, wherein R is independently selected from the group comprising (C1 to C10)alkyl, linear methoxy-(C2 to C5)alkyl, (C1 to C6)trialkylammonium group. In one embodiment, the iodoarene is iodonaphtalene, substituted with one or more substituents selected from the group comprising (C1 to C6)alkyl, (C1 to C10)aryl, (C1 to C6)alkoxy group, -OH, -COOH, -SO3H, halogen, -NO2, -C(=O)-NHR, -C(=O)-NR2, wherein R is independently selected from the group comprising (C1 to C10)alkyl, linear methoxy-(C2 to C5)alkyl, (C1 to C6)trialkylammonium group. More preferably, the iodoarene is selected from the group comprising 2-iodobenzesulfonic acid, 2-iodobenzoic acid and iodobenzene. In one embodiment, the potential during electrolysis is in the range of from 0.5 V to 3 V vs. MSE, preferably in the range of from 1.0 V to 2.5 V vs. MSE, more preferably in the range of from 1.5 V to 2.05 V vs. MSE, more preferably in the range of from 1.6 V to 1.95 V vs. MSE. In one embodiment, the electrical current density during electrolysis is in the range of from 2 mA cm ^-2 to 50 mA cm ^-2 , preferably in the range of from 3 mA cm ^-2 to 15 mA cm ^-2 . In one embodiment, the electrolysis is performed at room temperature. 5 In one embodiment, the duration of the electrolysis is at least 30 minutes, preferably in the range of from 1 hour to 10 hours, more preferably in the range of from 2 hours to 4 hours. The electrochemical cell, suitable for the claimed method, comprises a batch or flow cell body, 10 equipped with at least one anode as the working electrode and at least one cathode as the counter electrode. The electrochemical cell may further possess a separator of the anode and cathode compartments preventing free solution convection between anode and cathode compartment and ensuring ionic conductivity in the volume between the electrodes. The cell may further contain a reference electrode, positioned preferably on the same side of the hydraulic circuit as 15 the anode. The cell may be connected to a hydraulic circuit allowing electrolyte flow through the cell. The cell may be connected to two hydraulic circuits allowing anolyte and catholyte flow though the anode and cathode compartment, respectively. Electrolysis may be operated in batch reactor, flow reactor, flow reactor with recirculation of reactant mixture or single pass flow reactor. It is also preferable to measure the current and voltage difference between anode 20 and cathode during the electrolysis. The knowledge of thus determined voltage and current ranges may be effectively utilized when selecting current and voltage to be applied when electrolysis is controlled by current or voltage instead of electrode potential. Under these conditions, the presence of reference electrode in the cell is not required. 25 In one preferred embodiment, the electrochemical cell is a divided batch electrochemical cell containing a cylindrical body, preferably made from polytetrafluoroethylene, and a lid containing a hole for the reference electrode and a hole for the counter electrode, preferably placed inside a glass tube containing a porous frit. The cylindrical cell body is pressed against the working electrode, representing bottom of the electrochemical cell. The working electrode 30 can be attached to the cylindrical body by a plurality of screws. A sealing is arranged between the cylindrical body and the working electrode, the sealing is preferably made of expanded polytetrafluoroethylene ring. The electrochemical cell is preferably equipped with a stirring means, such as a magnetic stirrer. In one preferred embodiment, the electrochemical cell is a divided flow cell with reservoirs of the electrolyte solutions and the electrolyte solutions are recirculated through the cell. The electrolyte solutions are circulated between the cell and reservoirs using preferably a peristaltic pump. The electrochemical cell is a filter-press type cell. Parts of the cell body in contact with 5 electrolyte solutions, except for a sealing, are preferably made of polyvinylchloride. Electrode compartments are preferably separated by cation-exchange membrane made preferably of perfluorinated sulfonated polymer. The active parts of the electrodes are defined by an opening in frames which are present on both sides of the separator of electrode compartments and which possess internal flow distribution channels allowing electrolyte inflow and outflow. These 10 frames are from the backside connected to the external hydraulic circuit (tubing) allowing anolyte and catholyte flow through the respective compartments while one side of each compartment is represented by the separator and the other one by the electrode. The electrode compartments are preferably equipped by turbulizers enhancing mass transport towards/from electrode surfaces. The sealing present between electrodes and adjacent frame and between the 15 frames and the membrane is preferably made of expanded polytetrafluoroethylene. The anode and cathode are contacted from the backside by the current collector made preferably of copper. Current collectors are isolated from aluminium endplates by the polyvinylchloride blocks. The whole cell is screwed together by a plurality of screws. If present, a reference electrode is preferably part of the anolyte hydraulic circuit. 20 In a preferred embodiment, the aqueous solution of iodoarene comprises at least 15 vol. % of water, preferably from 20 to 100 vol. % of water, more preferably from 25 to 90 vol. % of water, more preferably from 30 to 80 vol. % of water. 25 In a preferred embodiment, the aqueous solution of iodoarene further comprises acetic acid and optionally acetonitrile, preferably the aqueous solution contains at least 15 vol.% of water, up to 85 vol. % of acetic acid and optionally up to 10 vol.% of acetonitrile. More preferably, the aqueous solution of iodoarene further comprises from 15 to 50 vol. % of water, from 50 to 85 vol. % of acetic acid and from 0 to 10 vol. % of acetonitrile. 30 In a most preferred embodiment, the aqueous solution of iodoarene comprises water or a mixture of water with acetic acid (at least 20 vol.% of H2O) or a mixture of water with sulfuric acid (from 0.2 to 2M H ₂ SO ₄ ). As mentioned above, the working electrode is a boron doped diamond electrode, preferably selected from the group comprising free-standing boron doped diamond and boron doped diamond on Ti, Ta, Nb, Mo or Si substrate. Preferably, the cathode active surface (active surface of the counter electrode) is selected from the group comprising Pt, Au, Ir, Ru, Ni, Al, Ti, Ta, Nb, Mo, Fe, Cu, Zn; eventually, Ti, Ta, Nb, and/or Mo may be coated with Pt, Au, Ir, Ru or mixture of thereof; boron doped diamond, graphite, glassy carbon, graphitic felt material, alloy containing Fe (> 80 wt.%), C (< 2.1 wt.%), Ni (<1 wt.%), V(<2 wt.%), Cr (<wt.15%), Mn (<wt.1 %), or any combination of these. In one embodiment, the electrochemical cell further comprises a reference electrode, preferably selected from the group comprising Hg(l)|Hg2SO4(s)|SO4 ^2- (aq) electrode, Ag(s)|AgCl(s)|Cl-(aq) electrode, Hg _(l) |Hg ₂ Cl _2(s) |Cl- _(aq) electrode, Pt _(l) |H _2(g) |H ⁺ _(aq) electrode. Preferably, the concentration of the SO4 ^2- and Cl- in the homogeneous solution phase corresponds to a maximum solubility of the given salt. It may also be lower but it should be at least of 0.01 M. Activity of H ⁺ (aq) (aH+) is preferably in the range of from 10 to 10 ^-14 (pH is defined as pH = -log aH+, and pH values can vary in the range between -1 to 14). The present method may be used to synthetize various ^ ⁵ -iodanes preferably derived from iodobenzene or iodobenzene substituted on the benzene ring, as defined above. In particular, one or more hydrogen atoms on benzene ring may be replaced preferably by alkyl, aryl, alkoxy group, carboxylic group, sulfo group, halogen atom, -OH, nitro group, amido group, amide, N- amide, trialkylammonium group, or any combination of these. The present invention is based on the following subsequent reactions at the anode: a) formation of ^ ³ -iodane ArI + R ¹ -OH + R ² -OH ^ ArI(OR ¹ )(OR ² ) + 2 e- + 2 H ⁺ b) formation of ^ ⁵ -iodane ArI + R ¹ -OH + R ² -OH + R ³ -OH + R ⁴ -OH ^ ArI(OR ¹ )(OR ² )(OR ³ )(OR ⁴ ) + 4 e- + 4 H ⁺ or ArI + R ¹ -OH + R ² -OH + H2O ^ ArI(OR ¹ )(OR ² )(O) + 4 e- + 4 H ⁺ or ArI + 2 H2O ^ ArIO2 + 4 e- + 4 H ⁺ wherein reactants, intermediate products and products can undergo acid-base reactions (protonation and deprotonation) in the solution. Typically, the following reaction occurs at the cathode: 2 H ⁺ + 2e- ^ H2 The overall reaction is: ArI + R ¹ -OH + R ² -OH ^ ArI(OR ¹ )(OR ² ) + H2 (during synthesis of ^ ³ -iodane) ArI + R ¹ -OH + R ² -OH + R ³ -OH + R ⁴ -OH ^ ArI(OR ¹ )(OR ² )(OR ³ )(OR ⁴ ) + H2 or ArI + R ¹ -OH + R ² -OH + H2O ^ ArI(OR ¹ )(OR ² )(O) + 2 H2 or ArI (during synthesis of ^ ⁵ -iodane); wherein Ar, R ¹ , R ² , R ³ , R ⁴ are as defined above. In one embodiment, no solvent other than water nor supporting electrolyte is present in the aqueous solution of iodoarene. In this embodiment, the iodoarene must contain at least one substituent as defined above, capable of dissociation, which thus serves as an electrolyte. The most suitable substituents are selected from the group comprising -OH, -COOH, -SO3H, - C(=O)-NHR, -C(=O)-NR2, wherein R is independently selected from the group comprising (C1 to C10)alkyl, linear methoxy-(C2 to C5)alkyl, (C1 to C6)trialkylammonium group. Such embodiment is advantageous because it contains no remaining solvent nor supporting electrolytes other than water, which can be easily evaporated, and no further purification of the product is necessary. In one embodiment, no solvent other than water is present in the aqueous solution of iodoarene, however the solution further contains a supporting electrolyte as defined above. In such embodiment, the iodoarene needs not to contain further substituents, and the electrochemical synthesis can be performed even with non-substituted iodoarenes, such as iodobenzene. If no substituents other than iodine are present on the aryl of iodoarene, the ^ ⁵ -iodane formed by the above described electrochemical method is ArIO ₂ and/or ArI(OH) ₂ (O) and/or ArI(OH) ₄ . In the embodiment wherein the iodoarene contains a substituent on the aryl of iodoarene, e.g. – SO3H substituent, then the ^ ⁵ -iodane formed by the above described electrochemical method may be ArIO ₂ and/or ArI(OH)(OR ¹ )(O) and/or ArI(OH) ₃ (OR ¹ ), wherein R ¹ may represent e.g. sulfonyl attached to the same aromatic ring as iodine. Optimization procedure of determination of the electrolysis conditions: The conditions suitable for the electrolysis (choice of the electrode potential, voltage, current, duration of the electrolysis, amount of electrolyte) depend on the type of the reactor (electrolytical cell): a) In one embodiment, the electrolytical cell is a batch reactor or a flow reactor with recirculation of the electrolyte solution. The electrolysis in the batch reactor or in the flow reactor with recirculation of the electrolyte solution can be operated in potentiostatic mode, potentiodynamic mode or combination of these (with the anode potential controlled with respect to reference electrode). The desired anode reaction requires appropriate electrode potential. The approximate estimation of operating potential is preferably determined on the basis of linear voltammogram measured in the electrolyser immediately prior to electrolysis with the stationary electrolyte solution of the same composition as will be used for electrolysis while taking into account electrochemical reactions corresponding to the individual current peaks. The linear voltammogram is preferably recorded with uncompensated resistance compensation and at potential scan rate in the range of from 0.05 to 0.2 V·s ^-1 . The details of such linear voltammogram measurement are known to those skilled in the art. The electrode potential value or range of electrode potential values for electrolysis is preferably selected in such a way so that it is in the potential range of the peak corresponding to the λ ⁵ -iodane generation or at higher potentials, preferably in the range of from Ep - 0.23 V to Ep + 0.6 V, and it is even more preferably selected in such a way so that the desired electrode reaction is limited by the mass transport of the iodoarene molecule towards the electrode surface while, at the same time, minimizing extent of other undesired processes such as λ ⁵ -iodane overoxidation, supporting electrolyte or solvent oxidation. The maximum selected potential for electrolysis should not exceed potential at which current in linear voltammogram reaches value 3 ^Ip ^abs . Ip ^abs stands for absolute peak current and corresponds to the current at Ep. If the applied electrode potential is significantly below Ep, the rate of the reactant oxidation is low and substantial amount of λ ³ -iodanes instead of λ ⁵ -iodanes is produced. The potential and duration of electrolysis can be determined experimentally as follows: Electrolysis is operated in the potential range determined above and samples of the electrolyte are taken for analysis. The first electrolyte (anolyte in the case of divided electrolyser) sample is preferably taken when the charge (Q ^t ) consumed in the electrolysis reaches value corresponding to 4 electrons per each molecule of iodoarene present at the beginning in the reaction mixture, Q ^t,1 is calculated as Q ^t,1 = 4 ^F ^niodoarene ⁰ , where, niodoarene ⁰ is number of mols of iodoarene present in the electrolyte at the beginning of the electrolysis, and F is Faraday’s constant (unit Coulomb per mole). After this point, samples of electrolyte (anolyte), should be taken, while it is preferable that Q ^t,s+1 at each following sample is not higher than 1.2 ^Q ^t,s at the previous sample, wherein upper index “s” represents sample number, i.e. s ^ 1. Electrolyte sample analysis should provide information about the λ ³ -iodane and λ ⁵ -iodane content and preferably also about the iodoarene content. Sample analysis is preferably performed by means of iodometry titration, ¹ H NMR, HPLC with UV-vis detection or their combination. The electrolysis should preferably be terminated when at least one the following conditions is fulfilled: 1. ciodoanere ^t + cλ3 ^t < 0.1 ^ciodoarene ⁰ , where ciodoanere ^t and ciodoarene ⁰ represent the concentrations of iodoarene at the given t and the beginning of electrolysis in the electrolyte, respectively and cλ3 ^t represents concentration of λ ³ -iodane at the given t; 2. αDP ^t (Faradaic efficiency of the desired product - λ ⁵ -iodane - as defined above) at the given t drops below 20% of the value determined at time when the charge consumed during the electrolysis reached value Q ^t,1 = 4 ^F ^niodoarene ⁰ 3. concentration of the desired product (λ ⁵ -iodane) at given t in the electrolyte reaches 0.9 ^ciodoarene ⁰ . 4. cλ3 ^t > cDP ^t (cλ3 ^t is the concentration of λ ³ -iodane at the given t, and cDP ^t is the concentration of λ ⁵ -iodane at the given t) and at the same time αDP ^t < 0.05 If electrolysis was terminated due to condition number 1 (ciodoanere ^t + cλ3 ^t < 0.1 ^ciodoarene ⁰ ) and yield of the desired product is at least 81 mol. %, then electrolysis is optimized. If electrolysis was terminated due to condition number 1 (ciodoanere ^t + cλ3 ^t < 0.1 ^ciodoarene ⁰ ) and yield of the desired product was below 81 mol. %, the electrosynthesis should be repeated with electrode potential lowered by 0.02 V. If electrolysis was terminated due to condition number 2, and at the same time, yield of the desired product is at least 81 mol. % then electrolysis is optimized. If electrolysis was terminated due to condition number 2, and at the same time, yield of the desired product is below 81 mol. % and at the same time c _iodoanere ^t + c _λ3 ^t > 0.12 ^c _iodoarene ⁰ , then electrolysis should be repeated with an increased electrode potential (the electrode potential should be increased by about 0.01 to 0.05 V). Alternatively, the electrolysis can be continued with electrode potential increased by 0.01 to 0.05 V. If electrolysis was terminated due to condition number 2, and at the same time, yield of the desired product is below 81 mol. % and at the same time c _iodoanere ^t + c _λ3 ^t < 0.12 ^c _iodoarene ⁰ , then electrolysis should be repeated with electrode potential decreased by about 0.01 to 0.05 V. If electrolysis was terminated due to condition number 3, then electrolysis is optimized. If electrolysis was terminated due to condition number 4, and at the same time ciodoanere ^t + cλ3 ^t + cDP ^t > 0.8 ^ciodoarene ⁰ , then the electrolysis should be repeated with electrode potential increased by about 0.01 to 0.05 V. If electrolysis was terminated due to condition number 4, and at the same time ciodoanere ^t + cλ3 ^t + cDP ^t < 0.8 ^ciodoarene ⁰ , then electrolysis is optimized. Thus optimized electrolysis can be used for λ ⁵ -iodane electrosynthesis. Preferably, the demands on analytical instrumentation during optimization can be significantly reduced if it is confirmed (or at least assumed) that below certain electrode potential and under the conditions used, neither iodoarene, nor λ ³ - and λ ⁵ -iodane undergo any undesired processes at the anode or in solution leading to formation of iodine containing compounds other than starting iodoarene, corresponding λ ³ -iodane and corresponding λ ⁵ -iodane. If no such undesired processes occurs, analysis of the electrolyte (anolyte) composition can be performed by only the iodometry titration. Iodometry titration is based on oxidation of iodide anion by either λ ³ - iodane or λ ⁵ -iodane to I2 according to the following reactions while both oxidants are reduced to starting iodoarene: ^ λ ⁵ -iodane + 4 I- ^ 2 I ₂ + iodoarene ^ λ ³ -iodane + 2 I- ^ I ₂ + iodoarene Using this titration procedure does not allow exact distinguishing between λ ³ -iodane and λ ⁵ - iodane. Instead, it allows calculating the overall titrated yield of iodane calculated using formula: Xiodane ^t,s = VKI ^t,s ^cKI/(4 ^ ciodoarene ⁰ ^Vsample), where VKI ^t represents volume of KI consumed during titration of sample assigned by number “s”, cKI stands for the molar concentration of KI used for the titration, ciodoarene ⁰ is the initial molar concentration of iodoarene prior to electrolysis, and V _sample is the volume of the sample taken for iodometric titration. For example, in the case of complete conversion of iodoarene to λ ⁵ -iodane, X _iodane ^t = 1. Then the rules for the electrolysis termination should be as follows: The electrolysis at the given conditions should preferably be terminated when at least one the following conditions is fulfilled: 5. Xiodane ^t ^ 0.9 6. Xiodane ^t,s+1 = Xiodane ^t,s 7. I ^t < I ^t=0 /2 and Xiodane ^t < 0.3 (I ^t and I ^t=0 represent current I at time t and at the beginning of the electrolysis, respectively) 8. I ^t < I ^t=0 /4 and Xiodane ^t < 0.5 If electrolysis was terminated due to condition number 5, then electrolysis is optimized. If electrolysis was terminated due to condition number 6 and at the same time X _iodane < 0.9, the electrolysis should be repeated with potential increased by about 0.01 to 0.1 V. Or alternatively, electrolysis can be continued with potential increased by about 0.01 to 0.1 V. If electrolysis was terminated due to conditions number 7 and 8, the electrolysis should be repeated with potential increased by about 0.01 to 0.15 V. Alternatively, electrolysis can be continued with potential increased by about 0.01 to 0.15 V. The current during the optimized electrolysis (I = f(t)) and cell voltage during the optimized electrolysis ((U = f(t))) should be recorded. Then, the electrolysis can be performed by controlling cell voltage or current, while the electrolysis is performed under the same conditions as before, except that the reference electrode is removed from the system, or it is not part of electric circuit. It is preferable that the profiles of the current or cell voltage used for controlling the electrolysis are close to those obtained during optimized electrolysis with electrode potential control. It is also possible to simplify the current or cell voltage profile, and apply for example constant current or cell voltage while the values of current and cell voltage should be within the limits observed during electrolysis with electrode potential control. However, in this case, the optimization is required. The principle of electrolysis optimization is analogous to that described above. It involves application of constant current density or constant voltage and following the electrolyte (anolyte) composition by analysing electrolyte samples as described above. In one embodiment, the electrolysis can be controlled by voltage. In this embodiment, the first optimization experiment should be started with voltage measured at the end of optimized electrolysis with controlled electrode potential. Electrolyte composition is followed to optimize the electrolysis time. This should also provide result comparable to that obtained for optimized electrolysis with electrode potential control, though the time of the electrolysis is usually prolonged. If the electrolysis time is too long, optimization is repeated at cell voltage increased by 0.1-15 % as long as reasonable time and λ ⁵ -iodane yield are achieved. In one embodiment, the electrolysis can be controlled by current. In this embodiment, the first optimization experiment should be started with current measured at the end of optimized electrolysis with controlled electrode potential. Electrolyte composition is followed to optimize the electrolysis time. In general, application of this current should provide result comparable to that obtained for optimized electrolysis with electrode potential control, though the time of the electrolysis is prolonged. If the electrolysis time is too long, optimization is repeated with current density increased by 10-25 %. This is repeated for as long as acceptable λ ⁵ -iodane yield is achieved and electrolysis time is acceptable. Alternative method of current optimization is based on an application of current equal to 50 % of that measured at the beginning of the electrolysis with controlled electrode potential. This current is retained until the cell voltage reaches value observed at the end of electrolysis with controlled electrode potential. When this cell voltage is reached, the current is decreased by 10-25 % of the original value. Again, this current is retained until the cell voltage reaches value observed at the end of electrolysis with controlled electrode potential. This procedure is repeated until the desired λ ⁵ - iodane yield is achieved, while the lowest applied current should be equal to that measured at the end of the electrolysis with controlled electrode potential. This current is applied until the desired λ ⁵ -iodane yield is achieved. b) In one embodiment, the electrolytical cell is a single pass flow reactor. In this arrangement, the solution containing iodoarenes passes through the electrolysis cell only once, i.e. it is not recirculated. Therefore, high conversion of iodoarene to λ ⁵ -iodane compound should be achieved in one pass. The most important parameters in this case are appropriate electrode potential of the anode and electrolyte volumetric flow rate (dm ³ s ^-1 ), Vflowrate. One necessary conditions required for achieving the desired product yield at given current is that maximum volumetric flow rate is, Vflowrate, max = I/(4 ^F ^ciodoarene), wherein I is electrical current, F is Faraday’s constant, and c _iodoarene is concentration of iodoarene in the solution. If gas is the main product of the cathode reaction, and electrolyser does not possess separator of the anolyte and catholyte, it is preferable to operate electrolysis at elevated pressure in the range of from 2 to 20 bar, which will reduce volume of the gaseous product and improve faradaic efficiency of electrolysis. If gas is the main product of the cathode reaction, and electrolyser possesses separator of the anolyte and catholyte, it is preferable to increase catholyte volumetric flow rate by the factor in the range of from 5 to 30 compared to anolyte volumetric flow rate. This will increase the gas removal rate from the cathode compartment and improve faradaic efficiency of electrolysis. In one embodiment, the electrolysis is controlled by constant electrode potential. The optimization experiment should be started in potential range identified by means of linear voltammetry as described above. Preferably, at first, the electrode potential corresponding to Ep is applied and held constant. The Vflowrate is varied between Vflowrate, max and Vflowrate, max /10. Each Vflowrate value is held constant for time corresponding to 3 ^Vinternal/Vflowrate, where Vinternal represents internal volume of the electrolyser (or internal volume of anode compartment in the case of divided electrolyser). After that the sample of the electrolyte is collected for analysis, and the V _flowrate is changed. The sample analysis is performed as described above. When all selected Vflowrate values are tested, electrode potential value is increased by about from 0.02 to 0.1 V and the whole procedure is repeated. The optimized conditions for electrolysis are selected based on the desired product yield, faradic efficiency and production rate. The current and cell voltage should be recorded during optimization. Since the single pass flow cell in operated in steady state mode, each electrode potential value at given Vflowrate corresponds to unique current and cell voltage. Consequently, the optimization procedure can be used for selecting appropriate current and cell voltage, if the electrolysis is to be controlled by constant current or constant cell voltage. Brief description of Figures Figure 1: Electrochemical cell scheme Figure 2: Example of peak potential E _p value selection for electrolysis in the case of not well defined current peak on the linear voltammogram. The lower plot shows dependence of I on E, the upper plot shows dependence of the first potential derivative of the current dI dE ^-1 on E. The badly defined current peak of interest (peak corresponding to λ ⁵ -iodane) is marked by the rectangle. The two arrows pointing downwards show section of the inflection points. The arrow pointing upwards show section of Ep value, the corresponding current is assigned as Ip ^abs Figure 3: Determination of peak potential E _p from linear voltammogram using first current derivative dI dE ^-1 , Example 1. Voltammogram was measured with potential scan rate of 0.1 V s ^-1 in stationary electrolyte in batch divided electrolyser. Figure 4: Determination of peak potential Ep from linear voltammogram, Example 2 Voltammogram was measured with potential scan rate of 0.1 V s ^-1 in stationary electrolyte. Figure 5: Scheme of divided electrochemical flow cell Figure 6: Scheme of setup of divided electrochemical flow cell with reservoirs of the electrolytes Figure 7: Determination of peak potential E _p from linear voltammogram, Example 3 Figure 8: Current I and cell voltage U during electrolysis, Example 3 Examples Measurements and methods ¹ H NMR spectra were recorded with an Agilent 400 MR DDR2 spectrometer at 399.94 MHz using DMSO-d ₆ as an external standard using the PRESAT pulse sequence minimising the solvent signals. Chemical shifts are given in parts per million, coupling constants in Hz. Example 1 Electrochemical synthesis of 2-iodylbenzensulfonic acid in batch electrolyser The electrochemical cell 1 used in this experiment is depicted in Figure 1 and comprised polytetrafluoroethylene cylindrical body 2 with height of 10 cm with internal diameter of 4 cm and a lid 3 possessing a hole for the reference electrode 4 and a hole for a glass tube 5. The glass tube 5 was equipped with a porous frit 6 and contains a counter electrode 7. The cell body 2 was pressed against the working electrode 8, representing bottom of the electrochemical cell. The working electrode 8 was attached to the cylindrical body 2 by four screws (not shown). A sealing 9 was arranged between the cylindrical body 2 and the working electrode 8. The sealing 9 was made of expanded polytetrafluoroethylene ring of 1 mm thickness. Bottom part of the porous frit 6 representing bottom end of the glass tube 5 with the counter electrode 7 was arranged 1.5 cm above the surface of the working electrode 8. The electrochemical cell was equipped with a magnetic stirring bar 10 (24 mm ^ 5 mm). Electrolysis was performed in a three-electrode arrangement using an electrochemical workstation (Zennium Pro, ZAHNER, Germany) controlled by Thales software. Boron-doped diamond electrode Diachem® BDD (Condias, Germany) was used as the working electrode 8, and Pt flag placed in glass tube with porous frit was used as the counter electrode 7. A Hg(l)|Hg2SO4(s)|K2SO4(sat) (mercury sulfate electrode, MSE) single junction electrode was used as the reference electrode 4. The used boron-doped diamond electrode possesses a boron concentration in the range of from 3.3 ^10 ²⁰ to 9 ^10 ²⁰ B atoms cm ^-3 leading to material with metallic conductivity and conductance of about 10 ² -10 ³ S·cm ^-1 . In order to determine the oxidation state of iodine in the resulting mixture (after electrolysis), potentiometric iodometry titration was performed using EasyPlus titrator (Mettler Toledo) equipped with potentiometric electrode (EM40-BNC, Mettler Toledo). The electrolyte sample (Vsample = 0.5 ml) was dissolved in 25 ml of 5 M H2SO4 and titrated by KI solution (cKI = 20 mM). 30 ml of reactant solution (0.030 M 2-iodobenzensulfonic acid in 0.2 M H ₂ SO ₄ in distilled water) was placed into the batch electrolyser depicted in Figure 1. The electrolyte (reactant solution) was agitated by a magnetic stirring bar 10 revolving at speed of 250 revolutions per minute. Electrolysis was performed at room temperature. Suitable electrode potentials and times were determined, and the determined electrode potentials were applied to the reactant solution of 2-iodobenzensulfonic acid to oxidise it to 2-iodylbenzensulfonic acid. The electrode potential during electrolysis was held at 1.6 V for 30 minutes, followed by potential hold at 1.7 V for 30 minutes, followed by potential hold at 1.85 V for 70 minutes. These potentials and times were determined using the optimization procedure described below: First, the linear voltammogram was measured (0.1 V s ^-1 , stationary electrolyte) before the electrolysis in 30 mM 2-iodobenzenesulfonic acid in 0.2M H2SO4. It was not possible to distinguish the individual peaks in voltammogram. Therefore, the linear voltammogram was re-measured in 15 mM 2-iodobenzenesulfonic acid in 0.2M H ₂ SO ₄ solution. The result is shown in Figure 3. There are two peaks visible on the voltammogram. The first dominant peak with Ep ≈ 1.5 V vs. MSE corresponds to formation of λ ³ -iodane formation. Ep of the second, badly defined peak, corresponding to λ ⁵ -iodane formation, was identified to be E _p ≈ 1.71 V vs MSE. This determination was performed by calculating an arithmetic average of the potentials of the nearest inflection points on the voltammogram (Ep = 0.5 ^Einf,1 + 0.5 ^Einf,2), see Figure 3; the first inflection point potential (Einf,1) corresponds to the potential at which the first derivative of the current in voltammogram, dI dE ^-1 , attains local maximum and, at the same time, second derivative of the current in voltammogram, d ² I dE ^-2 , attains zero value, thus 1.66 V; the second inflection point (Einf,2) corresponds to the potential at which the first derivative of the current in voltammogram attains local minimum and at the same time second derivative of the current in voltammogram attains zero value, thus 1.76 V. For the electrolysis, the potential of 1.6 V vs MSE was selected, falling within the range of from Ep-0.23 V to Ep+0.6 V. After 30 minutes of electrolysis at 1.6 V vs. MSE, the I ^t < I ^t=0 /2 value and Xiodane ^2,s = 0.25. The potential was increased to 1.7 V, after 30 minutes the Xiodane ^t,4 = 0.45 was achieved while I ^t < I ^t=0 /4. Finally, the potential was increased to 1.85 V, after 70 minutes leading to X _iodane ^t,6 = 0.96 was achieved. A sample of the resulting solution was taken for ¹ H NMR analysis. ¹ H NMR spectrum of the solution after electrolysis showed practically pure 2-iodylbenzensulfonic acid (98-99 mol.%) with only minor traces of 2-iodobenzenesulfonic acid (2-1 mol.%), suggesting that anolyte solution after electrolysis contained 2-iodylbenzensulfonic acid at concentration of about 29 mM. This corresponds to Xiodane ^t,NMR ^ 0.98 (Xiodane ^t,NMR represents yield of iodane at time t determined by NMR). Results of ¹ H NMR analysis and titration were in very good agreement. The resulting mixture is ready to be used as a highly efficient oxidating agent. The small content of iodoarene needs not be separated from the resulting mixture as it is the product of reduction of λ ⁵ -iodane during oxidation reactions where λ ⁵ -iodane is used as the oxidating agent. ¹ H NMR (H ₂ O, DMSO-d ₆ as the external standard): δ = 7.93 (ddd, ³ J _H-H = 7.9 Hz, ³ J _H-H = 7.1 Hz, ⁴ JH-H = 1.1 Hz, 1 H), 8.04 (m, 2 H), 8.28 (dd, ³ JH-H = 8.2 Hz, ⁴ JH-H = 0.9 Hz, 1 H) ppm. Example 2 Electrochemical synthesis of 2-iodylbenzoic acid in divided batch electrolyser The procedure was the same as in Example 1 except that 50 mM 2-iodobenzoic acid solution in 2 M H2SO4 in mixture of 60 vol.% CH3COOH with 40 vol.% H2O was used and the electrochemical cell was a divided batch electrochemical cell 1 containing a cylindrical body 2 made from polytetrafluoroethylene, and a lid 3 containing a hole for the reference electrode 4 and a hole for the counter electrode 7 placed inside a glass tube 5 containing a porous frit 6. The cylindrical cell body 2 is pressed against the working electrode 8, representing bottom of the electrochemical cell 1. The working electrode 8 was attached to the cylindrical body 2 by a plurality of screws, and a sealing 9 of expanded polytetrafluoroethylene ring was arranged between the cylindrical body 2 and the working electrode 8. The electrochemical cell was equipped with a magnetic stirring bar 10. The electrode compartments were separated by ceramic porous frit 6. The reactant solution was electrolyzed at constant potential of 1.95 V vs. MSE for 3 hours followed by constant potential of 2.0 V vs. MSE for 75 minutes. Xiodane ^t = 0.905 was achieved. This procedure for electrolysis was determined by the following procedure: First, 50 mM 2-iodobenzoic acid solution in 2 M H2SO4 in mixture of 60 vol.% CH3COOH with 40 vol.% H2O was placed in the electrochemical cell described above and a linear voltammogram was measured (0.1 V s ^-1 , stationary electrolyte). The result is shown in Figure 4. There are two peaks visible on the voltammogram. The first peak with Ep ≈ 1.27 V vs. MSE corresponds to formation of λ ³ -iodane formation. Ep of the second, badly defined peak, corresponding to λ ⁵ -iodane formation, was identified to be E _p ≈ 1.77 V vs MSE. This determination was performed by calculating an arithmetic average of the potentials of the nearest inflection points on the voltammogram (Ep = 0.5 ^Einf,1 + 0.5 ^Einf,2), see Figure 4; the first inflection point potential (Einf,1) corresponds to the potential at which the first derivative of the current in voltammogram, dI dE ^-1 , attains local maximum and, at the same time, second derivative of the current in voltammogram, d ² I dE ^-2 , attains zero value, thus 1.69 V; the second inflection point (Einf,2) corresponds to the potential at which the first derivative of the current in voltammogram attains local minimum and at the same time second derivative of the current in voltammogram attains zero value, thus 1.84 V. For the electrolysis, the potential of 1.95 V vs MSE was selected, which corresponds to Ep+0.18 V. The overall titrated iodane yield in the 4 ^th sample (taken after 3 h of electrolysis) was equal to that in 3 ^rd sample, X _iodane ^t,4 ^ X _iodane ^t,4 = 0.615. Electrolysis was terminated. Sample of anolyte was taken for ¹ H NMR analysis and ¹ H NMR spectrum was immediately recorded. It showed that approx.75 mol.% of reactant was converted to 2-iodosylbenzoic acid and 24 mol.% to 2-iodylbenzoic acid. This corresponds to Xiodane ^t,NMR = 0.24+0.5 ^0.75 = 0.615. Results of ¹ H NMR analysis and titration were in very good agreement. The electrolysis was continued with the potential increased to E = 2.00 V vs. MSE, which corresponds to Ep+0.23 V, and after another 75 minutes of electrolysis, X _iodane ^t = 0.905 was achieved. This corresponds to 81 mol.% yield of 2-iodylbenzoic acid and 19 mol. % yield of iodosylbenzoic acid. The resulting mixture is ready to be used as a highly efficient oxidating agent. The small content of λ ³ -iodane needs not be separated from the resulting mixture as it is the product of reduction of λ ⁵ -iodane during oxidation reactions where λ ⁵ -iodane is used as the oxidating agent. 2-Iodylbenzoic acid (1-Hydroxy-1-oxo-1λ ⁵ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H ₂ O, DMSO-d6 as the external standard): δ = 7.90 (t, ³ JH-H = 7.4 Hz, 1 H), 8,08 (t, ³ JH-H = 7.5 Hz, 1 H), 8.19 (d, ³ JH-H = 8.2 Hz, 1 H), 8.30 (d, ³ JH-H = 8.1 Hz, 1 H) ppm.2-Iodosylbenzoic acid ((1- Hydroxy-1λ ³ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.70 (td, ³ JH-H = 7.4 Hz, 1 H), 7.89 (dd, ³ JH-H = 8.2 Hz, ⁴ JH-H = 0.6 Hz, 1 H), 7.96 (ddd, ³ JH-H = 8.5 Hz, ³ JH-H = 7.2 Hz, ⁴ JH-H = 1.4 Hz, 1 H), 8.10 (dd, ³ JH-H = 7.6 Hz, ⁴ JH-H = 1.4 Hz, 1 H) ppm.2-Iodobenzoic acid ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.17 (td, ³ J _H-H = 7.8 Hz, ⁴ J _H-H = 1.7 Hz, 1 H), 7.42 (ttd, ³ J _H-H = 7.7 Hz, ⁴ J _H-H = 1.1 Hz, 1 H), 7.76 (dd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.95 (dd, ³ JH-H = 7.9 Hz, ⁴ JH-H = 1.0 Hz, 1 H) ppm. Example 3 Electrochemical synthesis of 2-iodylbenzoic acid in divided flow electrolyser with reservoirs of electrolytes The electrochemical cell 22 used in this experiment is depicted in Figure 5. The electrochemical cell 22 was a divided flow cell with reservoirs of the electrolyte solutions 23 and 24 and electrolyte solutions were recirculated through the cell 22. The electrochemical cell 22 was a filter-press type cell of rectangular shape with outer construction dimensions of 14 cm ^ 13 cm ^ 6.5 cm (height ^ width ^ thickness). The parts of the cell body in contact with electrolyte solutions, except for sealing 9, were made of polyvinylchloride. The reaction region was a rectangle of 5 cm height, 4 cm width (defining the working electrode 8 geometric area of 20 cm ² ) and thickness of 0.75 cm, corresponding to the distance between the working 8 and counter 7 electrodes. The two electrode compartments were separated by a cation-exchange membrane 11 based on perfluorinated sulfonated polymer (Nafion® 117) with dimensions 7 cm ^ 6 cm (height ^ width) and thickness of about 0.015 cm (in dry state). The distance between membrane 11 and electrodes (7 and 8) was 3.75 mm. The volumes between the individual electrodes (7 and 8) and membrane 11 were equipped with four turbulizer meshes 12 made of polypropylene. The turbulizer meshes 12 were 0.09 cm thick and possessed 0.1 cm ^ 0.1 cm openings. The membrane 11 was clamped between two polyvinylchloride frames 13 of 2.75 mm thickness with internal flow distribution channels allowing electrolyte inflow and outflow. Both frames 13 were from the backside connected to two polyvinylchloride tubes (14, 15, 16, 17) of 5 cm length and internal diameter of 0.6 cm which could be connected to an external tubing 25 (not shown in Figure 5) connecting the cell 22 with anolyte reservoir 23 and catholyte reservoir 24. The external tubing 25 (internal diameter 3.175 mm, outer diameter 6.35 mm) is preferably made of polypropylene-based thermoplastic elastomer. The connection between polyvinylchloride tubes (14, 15, 16, 17) and external tubing 25 is ensured by polypropylene fitting. The sealing 9, present between electrodes (7, 8) and adjacent frames 13 and between the frames 13 and the membrane 11 was made of expanded polytetrafluoroethylene. The anode 8 and cathode 7 were contacted from the backside by the current collector 18 made of copper. In order to ensure better electronic contact between the current collector 18 and the anode 8, the graphitic felt 19 of 2 mm thickness was placed between these two components. The current collectors 18 were isolated from the aluminum endplates 20 by polyvinylchloride blocks 21. The whole electrochemical cell 22 was screwed together by eight screws (not shown) located at the sides of the aluminium endplates 20 outside of the active zone. Anolyte was entering the electrochemical cell 22 via the tube 15 and leaving via the tube 14. Catholyte was entering the electrochemical cell via the tube 17 and leaving via the tube 16. Reference electrode 4 was connected to the external tubing 25 in the vicinity of the anolyte output tube 14. The whole experimental setup is depicted in Figure 6. It consists of peristaltic pump 27 (323, Watson Marlow), electrochemical cell 22, anolyte reservoir 23 and catholyte reservoir 24, both possessing lids 26 with holes for tubing 25. Electrolysis was performed in a three-electrode arrangement using an electrochemical workstation (Zennium Pro, ZAHNER, Germany) controlled by Thales software. Boron-doped diamond electrode Diachem® BDD (Condias, Germany) was used as the working electrode 8, and platinized Ti plate as and the counter electrode 7. A Hg(l)|Hg2SO4(s)|K2SO4(sat) (mercury sulfate electrode, MSE) single junction electrode was used as the reference electrode 4. The boron-doped diamond electrode Diachem® BDD (Condias, Germany) possesses a boron concentration in the range of from 3.3 ^10 ²⁰ to 9 ^10 ²⁰ B atoms cm ^-3 leading to material with metallic conductivity and conductance of about 10 ² -10 ³ S·cm ^-1 . Procedure for potentiometric iodometry titration of iodane was the same as in Example 1. 27 ml of 50 mM 2-iodobenzoic acid solution in 1 M H2SO4 in the mixture of 70 vol.% CH3COOH with 30 vol.% H2O was used as anolyte.23 ml of 1 M H2SO4 in mixture of 70 vol.% CH ₃ COOH with 30 vol.% H ₂ O was used as catholyte. Both electrolytes were circulated with volumetric flowrate of 150 ml min ^-1 . The electrolysis was operated at constant potential of 2.05 V, after for 4 h 50 min, Xiodane ^t,4 = 0.90 was achieved. This procedure for electrolysis was determined by the following procedure: First, the linear voltammogram was measured (0.1 V s ^-1 , stationary electrolytes) before the electrolysis in 50 mM 2-iodobenzoic acid in 1 M H2SO4 in mixture of 70 vol.% CH3COOH with 30 vol.% H2O. The result is shown in Figure 7. There are two peaks visible on the voltammogram. The first peak with Ep ≈ 1.45 V vs. MSE corresponds to formation of λ ³ -iodane formation. Ep of the second, badly defined peak, corresponding to λ ⁵ -iodane formation, was identified to be Ep ≈ 2.03 V vs MSE (the determination of Ep was performed analogously to the previous examples). For the electrolysis, the potential of 2.05 V vs MSE was selected, corresponding to E _p +0.02V. The overall titrated iodane yield in the 4 ^th sample was X _iodane ^t,4 = 0.89 and in 5 ^th sample Xiodane ^t,4 = 0.904 (taken after 4 h 50 min of electrolysis). This corresponds to 81 mol.% yield of 2-iodylbenzoic acid and 19 mol. % yield of iodosylbenzoic acid. Electrolysis was terminated. Anolyte sample was taken for ¹ H NMR analysis.200 μl of H ₂ O was added to 400 μl of the sample to move the signal of acidic -OH signals out of aromatic protons to enable PRESAT analysis. ¹ H NMR spectrum was measured approx.3 hours after it was collected and it revealed that the reaction mixture contained approx.4 mol % of iodine compound in the form of 2-iodobenzoic acid, 63 mol.% in the form of 2-iodosylbenzoic acid, 33 mol.% in the form of 2-iodylbenzoic acid. This corresponds to Xiodane ^t,NMR = 0.33+0.5 ^0.63 = 0.65. This is value lower than Xiodane ^t,4 determined by titration immediately after sample collection. This can be attributed to decomposition of the inherently instable 2-iodylbenzoic acid in the solution causing its decomposition to 2-iodosylbenzoic acid and partly also to 2- iodobenzoic acid. According to ¹ H NMR spectrum, no other iodine compounds except for 2- iodylbenzoic acid, 2-iodosylbenzoic acid and 2-iodobenzoic acid were present in the sample. The current and voltage during electrolysis are shown in Figure 8. The resulting mixture used preferably immediately after electrolysis is ready to be used as a highly efficient oxidating agent. The small content of λ ³ -iodane needs not be separated from the resulting mixture as it is the product of reduction of λ ⁵ -iodane during oxidation reactions where λ ⁵ -iodane is used as the oxidating agent. 2-Iodylbenzoic acid (1-Hydroxy-1-oxo-1λ ⁵ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.90 (t, ³ JH-H = 7.4 Hz, 1 H), 8,08 (t, ³ JH-H = 7.5 Hz, 1 H), 8.19 (d, ³ J _H-H = 8.2 Hz, 1 H), 8.30 (d, ³ J _H-H = 8.1 Hz, 1 H) ppm.2-Iodosylbenzoic acid ((1- Hydroxy-1λ ³ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.70 (td, ³ JH-H = 7.4 Hz, 1 H), 7.89 (dd, ³ JH-H = 8.2 Hz, ⁴ JH-H = 0.6 Hz, 1 H), 7.96 (ddd, ³ JH-H = 8.5 Hz, ³ JH-H = 7.2 Hz, ⁴ JH-H = 1.4 Hz, 1 H), 8.10 (dd, ³ JH-H = 7.6 Hz, ⁴ JH-H = 1.4 Hz, 1 H) ppm.2-Iodobenzoic acid ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.17 (td, ³ J _H-H = 7.8 Hz, ⁴ J _H-H = 1.7 Hz, 1 H), 7.42 (ttd, ³ J _H-H = 7.7 Hz, ⁴ J _H-H = 1.1 Hz, 1 H), 7.76 (dd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.95 (dd, ³ JH-H = 7.9 Hz, ⁴ JH-H = 1.0 Hz, 1 H) ppm. Example 4 Electrochemical synthesis of 2-iodylbenzoic acid in divided flow electrolyser with reservoirs of electrolytes The procedure was the same as in Example 3 except that electrolysis was performed at constant voltage and reference electrode was absent. The electrolysis was operated at constant voltage of U = 3.2 V for 2 hours, Xiodane ^t = 0.91 was achieved. This procedure for electrolysis was determined by the following procedure: The electrolyte solutions in this Example are of the same composition as in Example 3, where U = 3.2 V (cell voltage) was determined at the end of the electrolysis (Figure 8). Therefore, U = 3.2 V was used for electrolysis in Example 4. The overall titrated iodane yield in the 3 ^th sample was Xiodane ^t,3 = 0.81 and in 4 ^th sample Xiodane ^t,4 = 0.91 (taken after 2 h of electrolysis). This corresponds to 82 mol.% yield of 2-iodylbenzoic acid and 18 mol. % yield of iodosylbenzoic acid. Electrolysis was terminated.200 μl of H2O was added to 400 μl of the sample to move the signal of acidic -OH signals out of aromatic protons to enable PRESAT analysis. Anolyte sample was taken for ¹ H NMR analysis. ¹ H NMR spectrum was measured approx.2 hours after it was collected and it revealed that the reaction mixture contained approx. 2 mol.% of iodine compound in the form of 2-iodobenzoic acid, 48 mol.% of iodine compound in the form of 2-iodosylbenzoic acid, 50 mol.% in the form of 2-iodylbenzoic acid. This corresponds to Xiodane ^t,NMR = 0.5+0.5 ^0.48 = 0.74. This is value lower than Xiodane ^t,4 determined by titration immediately after sample collection. This can be attributed to decomposition of inherent instability of the 2-iodylbenzoic acid in the solution causing its decomposition to 2- iodosylbenzoic acid. According to ¹ H NMR spectrum, no other iodine compounds except for 2-iodylbenzoic acid, 2-iodosylbenzoic acid and 2-iodobenzoic acid were present in the system. The resulting mixture used preferably immediately after electrolysis is ready to be used as a highly efficient oxidating agent. The small content of λ ³ -iodane needs not be separated from the resulting mixture as it is the product of reduction of λ ⁵ -iodane during oxidation reactions where λ ⁵ -iodane is used as the oxidating agent. 2-Iodylbenzoic acid (1-Hydroxy-1-oxo-1λ ⁵ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.90 (t, ³ JH-H = 7.4 Hz, 1 H), 8,08 (t, ³ JH-H = 7.5 Hz, 1 H), 8.19 (d, ³ J _H-H = 8.2 Hz, 1 H), 8.30 (d, ³ J _H-H = 8.1 Hz, 1 H) ppm.2-Iodosylbenzoic acid ((1- Hydroxy-1λ ³ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.70 (td, ³ JH-H = 7.4 Hz, 1 H), 7.89 (dd, ³ JH-H = 8.2 Hz, ⁴ JH-H = 0.6 Hz, 1 H), 7.96 (ddd, ³ JH-H = 8.5 Hz, ³ JH-H = 7.2 Hz, ⁴ JH-H = 1.4 Hz, 1 H), 8.10 (dd, ³ JH-H = 7.6 Hz, ⁴ JH-H = 1.4 Hz, 1 H) ppm.2-Iodobenzoic acid ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.17 (td, ³ JH-H = 7.8 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.42 (ttd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.1 Hz, 1 H), 7.76 (dd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.95 (dd, ³ JH-H = 7.9 Hz, ⁴ JH-H = 1.0 Hz, 1 H) ppm. Example 5 Electrochemical synthesis of 2-iodylbenzoic acid in divided flow electrolyser with reservoirs of electrolytes The procedure was the same as in Example 3 except that electrolysis was performed at controlled current and reference electrode was absent. The electrolysis was operated at controlled current I = 250 mA for 32 min, followed by I = 200 mA for 15 min, followed by I = 150 mA for 37 min, followed by I = 100 mA for 29 min followed by I = 60 mA for 46 min, after which Xiodane ^t = 0.9 was achieved. This procedure for electrolysis was determined by the following procedure: The electrolyte solutions in this Example are of the same composition as in Example 3, where current at the beginning of the electrolysis was approximately 500 mA. Therefore, I = 500/2 mA = 250 mA was applied at first. After 32 min of electrolysis, cell voltage of U = 3.2 V was reached. The current was decreased to I = 200 mA (i.e. by 20%). After 15 min of electrolysis, cell voltage of U = 3.2 V was reached. The current was decreased to I = 150 mA (i.e. by 20% of the original value). After 37 min of electrolysis, cell voltage of U = 3.2 V was reached. The current was decreased to I = 100 mA (i.e. by 20% of the original value) yielding X _iodane ^t,5 = 0.83. The anolyte sample was taken for ¹ H NMR analysis.200 μl of H2O was added to 400 μl of the sample to move the signal of acidic -OH signals out of aromatic protons to enable PRESAT analysis. ¹ H NMR spectrum was measured immediately after it was collected and it revealed that the reaction mixture contained approx.1 mol.% of iodine compound in the form of 2- iodobenzoic acid, 29 mol.% of iodine compound in the form of 2-iodosylbenzoic acid, 69 mol.% in the form of 2-iodylbenzoic acid. This corresponds to X _iodane ^t,NMR = 0.69+0.5 ^0.29 = 0.84. This is result is in very good agreement with than Xiodane ^t determined by titration immediately after sample collection. According to ¹ H NMR spectrum, no other iodine compounds except for 2-iodylbenzoic acid, 2-iodosylbenzoic acid and 2-iodobenzoic acid were present in the system. Finally, the last applied current was 60 mA which was applied for 45 min, after which Xiodane ^t,6 = 0.92 was achieved. This corresponds to 84 mol.% yield of 2-iodylbenzoic acid and 16 mol.% yield of iodosylbenzoic acid. The resulting mixture used preferably immediately after electrolysis is ready to be used as a highly efficient oxidating agent. The small content of λ ³ -iodane needs not be separated from the resulting mixture as it is the product of reduction of λ ⁵ -iodane during oxidation reactions where λ ⁵ -iodane is used as the oxidating agent. 2-Iodylbenzoic acid (1-Hydroxy-1-oxo-1λ ⁵ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H ₂ O, DMSO-d6 as the external standard): δ = 7.90 (t, ³ JH-H = 7.4 Hz, 1 H), 8,08 (t, ³ JH-H = 7.5 Hz, 1 H), 8.19 (d, ³ JH-H = 8.2 Hz, 1 H), 8.30 (d, ³ JH-H = 8.1 Hz, 1 H) ppm.2-Iodosylbenzoic acid ((1- Hydroxy-1λ ³ -benzo[d][1,2]iodaoxol-3(1H)-one, ¹ H NMR (H ₂ O, DMSO-d ₆ as the external standard): δ = 7.70 (td, ³ JH-H = 7.4 Hz, 1 H), 7.89 (dd, ³ JH-H = 8.2 Hz, ⁴ JH-H = 0.6 Hz, 1 H), 7.96 (ddd, ³ JH-H = 8.5 Hz, ³ JH-H = 7.2 Hz, ⁴ JH-H = 1.4 Hz, 1 H), 8.10 (dd, ³ JH-H = 7.6 Hz, ⁴ JH-H = 1.4 Hz, 1 H) ppm.2-Iodobenzoic acid ¹ H NMR (H2O, DMSO-d6 as the external standard): δ = 7.17 (td, ³ JH-H = 7.8 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.42 (ttd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.1 Hz, 1 H), 7.76 (dd, ³ JH-H = 7.7 Hz, ⁴ JH-H = 1.7 Hz, 1 H), 7.95 (dd, ³ JH-H = 7.9 Hz, ⁴ JH-H = 1.0 Hz, 1 H) ppm. Comparative Example 6 Electrochemical synthesis of 2-iodylbenzoic acid in batch electrolyser The electrosynthesis of 2-iodylbenzoic acid was performed according to procedure described in (S.J. Folkman, R.G. Finke, J.R. Galán-Mascarós, G.M. Miyake, Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source, ACS Sustainable Chemistry & Engineering, 9 (2021) 10453-10467). The electrosythesis was performed in batch electrolyser described in Example 1, except that Ag|AgNO3(0.01 M), LiClO4 (0.1 M) (Ag|Ag ⁺ ) in acetonitrile was used as a reference electrode and GC (Sigradur G) was used as the working electrode. Electrolyte consisted of 20 mM 2-iodobenzoic acid in 0.1M LiClO4 in mixture of 99 vol.% acetonitrile with 1 vol.% of H2O. Electrolysis was performed at 1.8 V vs. (Ag|Ag ⁺ ) for 2 hours. Two electrolyte samples were taken for analysis during electrolysis (after 3600 s and after 6000 s) and one sample was collected after the electrolysis. In all three cases, iodometric titration showed that no hypervalent iodine compounds were formed (the average oxidation state of iodine in the mixture was 1). The discrepancy between this result and results published in S.J. Folkman, R.G. Finke, J.R. Galán-Mascarós, G.M. Miyake, Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source, ACS Sustainable Chemistry & Engineering, 9 5 (2021) 10453-10467) was surprising, and can likely be attributed to the fact that authors in the mentioned work used different type of glassy carbon electrode. However, as they did not specify the exact type of the working electrode, it is not possible to repeat the results published therein without further investigation and experiments requiring inventive skills. 10 List of reference signs: 1, 22 - electrochemical cell 2 - cylindrical body 3 - lid of the electrochemical cell 4 - reference electrode 15 5 - glass tube 6 - frit 7 - counter electrode (cathode) 8 - working electrode (anode) 9 - sealing 20 10 - magnetic stirring bar 11 - cation-exchange membrane 12 - turbulizer meshes 13 - polyvinylchloride frames 14, 15, 16, 17 - polyvinylchloride tubes 25 18 - current collector 19 - graphitic felt 20 - aluminum endplates 21 - polyvinylchloride blocks 23, 24 - reservoirs of the electrolyte solutions 30 25 - external tubing 26 - lid of electrolyte reservoirs 27 - peristaltic pump