 Reaction intermediate

A reaction intermediate is formed during a reaction mechanism and consumed in a subsequent step, cancelling from the net chemical equation. It can accumulate, as I does in the consecutive first-order reaction mechanism. A reaction intermediate can also connect transformations in energy coupling. Unlike a catalyst, it need not be present before conversion begins.^[ref]

A reaction intermediate is distinct from a transition state: the former can persist between steps, whereas the latter describes passage through a barrier. The existing potential energy surface and energy profile provide the configuration-space picture; the time-dependent concentration of an intermediate instead measures an ensemble of reacting entities. Low concentration or short lifetime can make detection difficult, so failure to detect a proposed reaction intermediate is not by itself proof of absence. Conversely, detecting a chemical species does not establish that it carries the proposed route to the final reaction products; it could belong to a competing route.

A reaction intermediate is a species that is formed during the course of a chemical reaction but is not present in the final products. It exists transiently and is usually unstable, often having a shorter lifespan than the reactants and products. Intermediates play a crucial role in the mechanism of a reaction, as they can provide insight into how reactants transform into products through various steps. In a multi-step reaction, intermediates are typically produced in one step and consumed in subsequent steps.