Electrosynthesis

electrosynthesis [i¦lek·trō′sin·thə·səs]

(chemistry)

A reaction in which synthesis occurs as the result of an electric current.

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Electrosynthesis 

a method of producing complex organic and inorganic compounds through the use of electrolysis. In electrosynthesis, electrons are gained or lost in a multistep (rather than single-step) process as stable or unstable intermediate products are formed. A specific electrode potential corresponds to each step in the process.

The steps involved in electrosynthesis are represented by the equations

(1) R + nH⁺ + ne^– → RH_k + (n – k)H⁺ + (n – k)e^– → RH_k+r + (n – k – r)H⁺ + (n – k – r)e^– → RH_n

(2) R′ + nOH^– – ne^– → R′O_k + (n – k)OH^– + kH⁺ + (n – k)e^– → R′O_k+r + (n – k – r)OH^– + (k + r)H⁺ – (n – k – r)e^– → R′O_n + nH⁺

where R and R’ are starting materials, RH_n and R’O_n are final products, and n, k, and r are the number of electrons (e^–) that participate in the electrochemical reactions.

The reactions in equation (1) occur at the cathode and are called electroreduction (electrochemical reduction) reactions. The reactions in equation (2) occur at the anode and are called electrooxidation (electrochemical oxidation) reactions. The intermediate and final products may participate in various electrochemical reactions on the electrode surface.

If the desired product of the electrosynthesis is formed in an intermediate step, the electrolysis must be performed at the electrode potential that corresponds to that step. The product may be rapidly removed from the sphere of the reaction by distillation, by extraction, or by addition to a compound that does not enter into electrochemical conversions.

The yield of electrosynthesis may vary as a result of various chemical reactions with the starting, intermediate, and final compounds. For example, certain oxidizing agents obtained on the anode may decompose in the solution, thus losing the active oxygen, or they may undergo another process, such as hydrolysis or disproportionation. The role of chemical reactions in the solution is determined on the basis of the current concentration, which is expressed in amperes per liter (A/l) and is the electric current passing through a unit volume of the electrolyte. When performing an electrosynthesis in which side reactions cause a decrease in the yield of the desired product, a high current concentration (up to several hundred A/l) must be used.

Starting materials that dissociate ionically in solution and organic compounds that possess polar functional groups undergo electroreduction or electrooxidation with greatest ease. Many neutral organic compounds do not have sufficient reactivity and do not enter into reactions on the electrode surface. In such cases, indirect electroreduction or electrooxidation is performed in the solution by means of ions of metals or nonmetals with variable valence that serve as catalyst carriers. The general process may be described by the equations

where R is the starting material, K is the catalyst carrier, C is the final product, z is the oxidation state, and n is the number of electrons participating in the reaction. In such a process, electrolysis leads to the regeneration of the chemical reducing agent or oxidizing agent on the electrodes. These agents react with the starting material either chemically or electrolytically and thus convert it into the desired product.

Electrosynthesis has been applied in the production of a number of valuable organic and inorganic compounds. For example, the oxygen-containing compounds of chlorine in different oxidation states can be produced by electrooxidation.

The electrosynthesis of sulfuric acid and sulfates serves as the basis for an industrial method of obtaining persulfuric acid and persulfates, which are the salts of the acid (seePEROXYSULFATE); from persulfuric acid and some of its salts hydrogen peroxide can be produced. Potassium permanganate is obtained by the electrooxidation of manganate or by the anodic dissolution of ferromanganese alloys. Manganese dioxide is produced on a large scale by the electrolysis of solutions of manganese sulfate in sulfuric acid.

Electrosynthesis is also used in the production of various organic compounds (seeKOLBE REACTION). Electrochemical fluorination is used for the industrial production of a number of perfluorocarbon compounds. Many other compounds, including tetraethyllead, are also produced electrochemically.

REFERENCES

Prikladnaio elektrokhimiia, 3rd ed. Edited by A. L. Rotinian. Leningrad, 1974.
Fioshin, M. Ia. Uspekhi v oblasti elektrosinteza neorganicheskikh soedinenii. Moscow, 1974.
Prikladnaia elektrokhimiia, 2nd ed. Edited by N. T. Kudriavtsev. Moscow, 1975.
Tomilov, A. P., M. Ia. Fioshin, and V. A. Smirnov. Elektrokhimicheskii sintez organicheskhikh veshchestv. Leningrad, 1976.
Fioshin, M. Ia., and V. N. Pavlov. Elektroliz v neorganicheskoi khimii. Moscow, 1976.
Elektrokhimiia organicheskikh soedinenii. Moscow, 1976. (Translated from English.)

M. IA. FIOSHIN