Illustrated Glossary of Organic Chemistry

Reactive intermediate: A reaction intermediate that has a short lifetime because it is unstable and reacts quickly. A reactive intermediate should not be confused with a transition state; a reactive intermediate is a saddle point on an energy profile whereas a transition state is an energy maximum.

The tert-butyl carbocation is a reactive intermediate. In this SN1 reaction mechanism the tert-butyl carbocation is highly unstable. Once formed, it reacts quickly and cannot be isolated.

An energy profile showing the relationship between reactants, products, transition state, and reactive intermediate for the SN1 reaction shown above.