Reaction intermediate

This article is about short-lived chemical species. For industrial chemicals primarily used to produce other chemicals, see chemical intermediate.

In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall reaction. For example, consider this hypothetical reaction:

A + B → C + D

If this overall reaction comprises two elementary steps thus:

A + B → X
X → C + D

then X is a reaction intermediate.

The phrase reaction intermediate is often abbreviated to the single word intermediate, and this is IUPAC's preferred form of the term.

In most non-biological cases, a reaction intermediate is also a reactive intermediate: a short-lived, high-energy species too reactive for isolation. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation. Instead, reactive intermediates are only observable through fast spectroscopic methods. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.

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