Chemical intermediate

A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) and reacts further to give the directly observed products of a chemical reaction. Most chemical reactions are stepwise, that is they take more than one elementary step to complete. An intermediate is the reaction product of each of these steps, except for the last one, which forms the final product. Reactive intermediates are usually short lived and are very seldom isolated. Also, owing to the short lifetime, they do not remain in the product mixture.

For example, consider this hypothetical stepwise reaction:

The reaction includes these elementary steps:

The chemical species X* is an intermediate.

The IUPAC Gold Book defines an intermediate as a molecular entity (atom, ion, molecule...) with a lifetime appreciably longer than a molecular vibration that is formed (directly or indirectly) from the reactants and reacts further to give (either directly or indirectly) the products of a chemical reaction. The lifetime condition distinguishes true, chemically distinct intermediates from vibrational states or such transition states which, by definition have lifetimes close to that of molecular vibration, and thus, intermediates correspond to potential energy minima of depth greater than available thermal energy arising from temperature, (RT, where R is gas constant and T is temperature).

Many intermediates are short-lived and highly reactive, thus having a low concentration in the reaction mixture. As is always the case when discussing chemical kinetics, definitions like fast/slow short/long-lived are relative, and depend on the relative rates of all the reactions involved. Species that are short-lived in one reaction mechanism, can be considered stable in others and molecular entities that are intermediates in some mechanisms can be stable enough to be detected, identified, isolated or used as reactants in (or be the products of) other reactions. Reaction intermediates are often free radicals or unstable ions. Oxidizing radicals (OOH and OH) found in combustion reactions are so reactive that a high temperature is required to constantly produce them, in order to compensate their disappearance, or the combustion reaction will cease.

...
Wikipedia