What Is a Reaction Intermediate?

An intermediate or reaction intermediate is a substance formed during a middle step of a chemical reaction between reactants and the desired product. Intermediates tend to be extremely reactive and short-lived, so they represent a low concentration in a chemical reaction compared with the amount of reactants or products. Many intermediates are unstable ions or free radicals.

Example in a chemical equation:

A + 2B → C + E

The steps could be

A + B → C + D
B + D → E

The D chemical would be an intermediate chemical.

A real-world example of chemical intermediates are oxidizing radicals OOH and OH found in combustion reactions.

Chemical Processing Definition

The term "intermediate" means something different in the chemical industry, referring to a stable product of a chemical reaction that is then used as a starting material for another reaction. For example, benzene and propylene may be used to make the intermediate cumene. Cumene is then used to make phenol and acetone.

Intermediate vs Transition State

An intermediate is different from a transition state in part because an intermediate has a longer lifetime than a vibrational or transition state.