chemicalreduction

Chemical reduction is a type of redox reaction in which a species gains electrons or undergoes a decrease in oxidation state. In practice, reductions are described relative to oxidation; the substance that donates electrons is the reducing agent, and the substance that accepts electrons is the oxidant. Reductions occur in redox couples with simultaneous oxidation of another species.

Common chemical reducing agents include metal hydrides such as sodium borohydride and lithium aluminium hydride, which transfer hydride to carbonyls and other substrates. Molecular hydrogen (H2) acts as a reducing agent in catalytic hydrogenation. Metals such as zinc, iron, tin, and aluminum can reduce metal ions in acidic or complex media. Organic reductions include hydrogenation of unsaturated bonds, the Birch reduction of arenes using alkali metals in liquid ammonia, and carbonyl-to-alkane transformations by Wolff-Kishner or Clemmensen methods.

Reductions can be carried out electrochemically, where electrons are supplied at the cathode, or chemically, using reagents that transfer electrons without an external power source. The choice of reducing agent and conditions depends on the substrate, the desired level of chemoselectivity, and the tolerance of other functional groups.

Applications of reduction processes span metal refining and deposition, organic synthesis, materials processing, and environmental chemistry, where reductants transform pollutants or convert metal ions to less soluble forms. In biology, many reductions are coupled to oxidations in metabolic pathways via cofactors such as NADH.

Related topics include oxidation, redox reactions, reducing agents, and oxidation state.