reductant

A reductant, or reducing agent, is a substance that donates electrons to another species in a redox reaction, thereby reducing that species and itself being oxidized. The species that accepts electrons is the oxidant or oxidizing agent. In redox notation, the reductant is the one whose oxidation state increases as it loses electrons.

Strength and selectivity of a reductant are described by standard redox potentials. A stronger reductant tends to have a more negative standard oxidation potential (or a more positive standard reduction potential for its conjugate oxidation couple), meaning it is more readily oxidized and more capable of reducing other species. The choice of reductant depends on the target redox chemistry, solvent, and reaction conditions.

Common examples and contexts:

- Elemental metals such as zinc, iron, and aluminum can act as reductants, especially in metal extraction and cementation processes.

- Molecular hydrogen (H2) and carbon monoxide (CO) are reductants in various metal-oxide reductions and industrial smelting.

- Hydride donors such as sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) are widely used reductants in organic synthesis.

- Organic and biological reductants include ascorbic acid (vitamin C) and biological carriers such as NADH and FADH2, which donate electrons in metabolic pathways.

Applications span metallurgy, chemical synthesis, environmental chemistry, and biochemistry. Reductants are chosen to balance reactivity, selectivity, and compatibility with the reaction medium.