Electrosynthesis

Electrosynthesis is a method used in electrochemistry to create chemical compounds within an electrochemical cell.

Instead of relying solely on traditional chemical reagents, this process uses an applied electric field to activate reactants in situ. This means the reaction happens right where the materials are located. This technique is highly valued because it can offer improved selectivity and higher yields compared to ordinary redox reactions. It is also closely aligned with the principles of Green Chemistry. This is because electrosynthesis can improve energy efficiency, safety, and atom economy while reducing waste.

The mechanism of electrosynthesis relies on a specific experimental setup. A basic cell requires a potentiostat, which controls the electricity, and two electrodes. These electrodes act as the sites where chemical changes occur. One electrode is the anode, where organic oxidations take place. The other is the cathode, where compounds undergo reduction.

The reactants are typically dissolved in a solvent. To ensure electricity flows well, an electrolyte is added to minimize electrical resistance. In protic conditions, scientists might use alcohol-water mixtures with soluble salts or acids. In aprotic conditions, they might use organic solvents like acetonitrile with electrolytes such as lithium perchlorate.

Cell design is another critical part of the process. An undivided cell allows everything to mix in one space. However, a divided cell uses a semiporous membrane to separate the anode and cathode chambers.

Common membrane materials include sintered glass, porous porcelain, or polypropylene. This separation allows ions to diffuse through while restricting the flow of reactants and products. This design simplifies the "workup," or the process of collecting the final product. For example, reducing nitrobenzene to phenylhydroxylamine requires a divided cell. This is because phenylhydroxylamine is susceptible to oxidation at the anode if it is not kept separate.

Choosing the right electrode materials is vital for a successful reaction. The composition and surface area of the electrodes can be decisive. In aqueous environments, competing reactions can occur, such as forming oxygen at the anode or hydrogen at the cathode. To prevent this, a graphite anode and a lead cathode might be used because they have high overpotentials for those specific gases. Other materials used include platinum, magnesium, mercury, or stainless steel. Some processes even use a sacrificial electrode, such as zinc or lead, which is actually consumed during the reaction.

Scientists manage the reaction by choosing between constant potential or constant current. This choice involves a trade-off between experimental ease and current efficiency. Current efficiency is the ratio of Coulombs consumed to form products versus the total Coulombs passed through the cell. Constant potential is often more efficient. In this mode, the current decreases over time as the substrate is depleted near the electrode. Conversely, under constant current, the potential across the cell increases as the substrate concentration drops. This increase in potential can drive unwanted side reactions, which lowers the overall efficiency.

There are many specific types of reactions used in electrosynthesis. Anodic oxidations include the Kolbe electrolysis, where two carboxylic acids decarboxylate and their remaining radicals couple.

Another method is the Shono oxidation, which oxidizes amides into N-acyliminium ions. In industrial settings, the cathodic hydrodimerization of activated olefins is used to produce adiponitrile from acrylonitrile.

Other notable processes include the Markó–Lam deoxygenation of alcohols and the electrofluorination of hydrocarbons. The latter method was invented in the 1930s and uses liquid HF at voltages near 5–6 V with nickel anodes to create perfluorinated derivatives.

Electrosynthesis has significant connections to both industry and environmental science. It is used in the commercial production of propiolic acid and acetylenedicarboxylic acid. It also plays a role in pharmaceutical synthesis, such as the electrochemical carboxylation used to promote the production of Ibuprofen. Beyond making products, the process has potential for wastewater treatment through electrooxidation.

Even carbon dioxide can be used as a feedstock. For instance, a copper complex can help reduce carbon dioxide to oxalic acid. These diverse applications show how electricity can transform simple molecules into complex, useful substances.