Electrosynthesis

Electrosynthesis uses an electrochemical cell to create chemical compounds. By activating reactants in place with an applied electric field, it can sometimes achieve better selectivity and higher yields than standard redox reactions. This approach may align more closely with green chemistry principles—improving energy efficiency, waste reduction, safety, or atom economy—though it can still rely on hazardous solvents, electrolytes, conductivity aids, or sacrificial reagents, which must be weighed. The field is both an active area of scientific research and has industrial applications; electrooxidation, for instance, is explored for wastewater treatment.

The typical experimental setup includes a galvanic cell, a potentiostat, and two electrodes. Solvent and electrolyte combinations are chosen to minimize electrical resistance. Under protic conditions, mixtures like alcohol-water or dioxane-water are common, with a soluble salt, acid, or base as electrolyte. Aprotic conditions often use organic solvents such as acetonitrile or dichloromethane, with electrolytes like lithium perchlorate or tetrabutylammonium salts. Electrode composition and surface area can be critical. In aqueous systems, competing reactions—oxygen formation at the anode and hydrogen at the cathode—can be managed by using a graphite anode and lead cathode, due to their high overpotentials for those gases. Other electrode materials include platinum, magnesium, mercury (as a liquid pool), stainless steel, or reticulated vitreous carbon. Some reactions employ a sacrificial electrode, such as zinc or lead, which is consumed during the process. Cell designs may be undivided or divided. In divided cells, a semiporous membrane (e.g., sintered glass, porous porcelain, polytetrafluoroethene, or polypropylene) separates the cathode and anode chambers, allowing ion diffusion while restricting product and reactant flow, which simplifies workup. An example requiring a divided cell is the reduction of nitrobenzene to phenylhydroxylamine, as the product is prone to oxidation at the anode.

Reactions occur at the electrode surfaces: organic oxidations at the anode, reductions at the cathode. Radical intermediates are often involved. The initial reaction happens at the electrode surface, after which intermediates diffuse into the solution for secondary reactions. Yield is measured both as chemical yield and current efficiency—the ratio of coulombs used to form products versus total coulombs passed. Side reactions lower current efficiency. The potential drop between electrodes determines the reaction rate constant. Electrosynthesis can be run at constant potential or constant current. Constant potential uses current more efficiently because the current decreases over time as the substrate near the working electrode depletes (stirring helps reduce the diffusion layer). Under constant current, the potential rises as substrate concentration falls to maintain the fixed rate, which can drive side reactions outside the target voltage.

Anodic oxidations include carbon-carbon coupling, such as the oxidation of a carbanion leading to coupling, as in the synthesis of the tetramethyl ester of ethanetetracarboxylic acid from a malonate ester. A well-known example is the Kolbe electrolysis, where two carboxylic acids decarboxylate and the remaining radicals couple. A variation, the non-Kolbe reaction, occurs when a heteroatom (nitrogen or oxygen) is at the α-position; the intermediate oxonium ion is trapped by a nucleophile, often the solvent. Primary aliphatic amines can be anodically oxidized to nitriles. α-Amino acids form nitriles and carbon dioxide via oxidative decarboxylation at silver oxide anodes (the AgO forms in situ from Ag₂O oxidation). Cyanoacetic acid is produced from cathodic reduction of carbon dioxide and anodic oxidation of acetonitrile. Amides can be oxidized to N-acyliminium ions, which are captured by nucleophiles—this is the Shono oxidation, as in the α-methoxylation of N-carbomethoxypyrrolidine. Alcohols can be oxidized to carboxylic acids; for example, propiolic acid is made commercially by oxidizing propargyl alcohol at a lead dioxide electrode, and 1,4-butynediol is oxidized to acetylenedicarboxylic acid.

Cathodic reductions include the Markó–Lam deoxygenation, where an alcohol is almost instantaneously deoxygenated by electroreducing its toluate ester. Adiponitrile is prepared by dimerizing acrylonitrile: two molecules of CH₂=CHCN, two electrons, and two protons yield NC(CH₂)₄CN. This cathodic hydrodimerization of activated olefins is used industrially. The cathodic reduction of arene compounds to 1,4-dihydro derivatives resembles a Birch reduction; industrial examples include the reduction of phthalic acid and of 2-methoxynaphthalene. The Tafel rearrangement, named for Julius Tafel, was once an important method.