Reaction intermediate

The IUPAC Gold Book^[1] defines a reaction intermediate or an intermediate as a molecular entity (atom, ion, molecule...) with a lifetime appreciably longer than a molecular vibration (corresponding to a local potential energy minimum of depth greater than RT; R being the gas constant and T is temperature) that is formed (directly or indirectly) from the reactants and reacts further to give (either directly or indirectly) the 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).

For example in a hypothetical stepwise reaction:

A + 2B → C + D + E,

That includes these elementary steps:

A + B → C + X

X → D + Y

B + Y → E

The chemical species X and Y are intermediates.

Reactive intermediates are usually short lived and are very seldom isolated

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The main carbon reactive intermediates

• Carbocations and their stabilized equivalents such as oxonium ions
• Carbanions and their stabilized equivalents such as enolates
• Free radicals
• Carbenes

Common features of carbon intermediates

• 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

Other reactive intermediates

• Deprotonated or hydrated forms of the compound, such as the tetrahedral intermediate in esterification
• Arynes
• Carbenes
• Carbenoid
• Carbocations
• Carbyne
• Free radicals
• Nitrenes, nitrenium ions

References

• Carey,Francis A.; Sundberg, Richard J.; (1984). Advanced Organic ChemistryPart A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press.ISBN 0-306-41198-9.
• MarchJerry; (1885). Advanced Organic Chemistry reactions, mechanisms andstructure (3rd ed.). New York: John Wiley & Sons, inc. ISBN 0-471-85472-7
• Gilchrist T.C.;Rees C.W.; (1969) carbenes, nitrenes and arynes. Nelson. London.

Notes

1. ^ IUPAC Goldbook definition of intermediate