coreactants

Coreactants are reagents that participate directly in a chemical reaction alongside the primary substrates and are consumed during the process. They typically provide or accept electrons, donate or accept protons, or help stabilize reactive intermediates, enabling transformations that would not proceed with the main reactants alone. Coreactants are distinct from catalysts, which cycle through a state without being consumed; however, in some mechanistic descriptions the same species can play multiple roles depending on context.

In redox- and photoredox-related chemistry, coreactants often act as sacrificial electron donors or acceptors. For example, in photoredox catalysis, an excited photocatalyst can oxidize a substrate, and a sacrificial coreactant such as a tertiary amine or a Hantzsch ester reduces the oxidized catalyst back to its ground state, thus closing the catalytic cycle. In other schemes, oxidants like persulfate or molecular oxygen function as coreactants that accept electrons and enable oxidative transformations. Proton donors or acceptors can also serve as coreactants in proton-coupled electron transfer steps, facilitating bond-making events.

Coreactant choice influences reaction efficiency, selectivity, and scope; their redox potentials, stability, and compatibility with other reagents determine whether a given transformation proceeds smoothly. They are typically consumed in stoichiometric amounts, with by-products identified from the reaction mechanism. The term appears across disciplines, including organic synthesis, electrochemistry, and materials science, where coreactants enable radical formation, electron transfer steps, and other essential steps that enable product formation or catalyst turnover.