Reactive intermediate

Most chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high-energy, hence unstable, product that exists only in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods. It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it.

When a reactive intermediate is not observable, its existence must be inferred through experimentation. This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of chemical kinetics, chemical thermodynamics, or spectroscopy. Reactive intermediates based on carbon are radicals, carbenes, carbocations, carbanions, arynes, and carbynes.

Common features

Reactive intermediates have several features in common:

• low concentration with respect to reaction substrate and final reaction product
• with the exception of carbanions, these intermediates do not obey the lewis octet rule, hence the high reactivity
• often generated on chemical decomposition of a chemical compound
• it is often possible to prove the existence of this species by spectroscopic means
• cage effects have to be taken into account
• often stabilisation by conjugation or resonance
• often difficult to distinguish from a transition state
• prove existence by means of chemical trapping