Coreactant

A coreactant is a chemical species that participates in a chemical reaction together with the principal reactant, providing a necessary counterpart to enable the transformation. Coreactants are typically consumed during the reaction and are not regenerated, unlike catalysts. They often participate in electron, proton, or radical transfer, or take part in bond-forming steps that would not proceed without their involvement.

In photochemical and electrochemical reactions, coreactants commonly act as electron donors or acceptors, or as sacrificial agents that drive redox cycles of catalysts or electrodes. For example, in visible-light photocatalysis, a tertiary amine may serve as a coreactant by donating an electron to an excited catalyst, with the resulting species subsequently processed. In other systems, a net oxidant such as a quinone can accept an electron from the substrate, functioning as a coreactant. Coreactants can also serve as co-substrates that participate directly in the catalytic cycle to enable bond formation through radical or ionic pathways.

Coreactants are distinct from catalysts, which are not consumed in the transformation and are regenerated, and from the primary substrates targeted by the reaction. The term appears in discussions of reaction mechanisms across fields such as photoredox chemistry, electrochemistry, and radical reactions, as well as in some contexts of polymerizations and biochemical processes. Synonyms include co-reactant or co-substrate, though usage may vary by discipline.