Reaction intermediate
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A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) and reacts further to give the directly observed 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. Reactive intermediates are usually short lived and are very seldom isolated. Also, owing to the short lifetime, they do not remain in the product mixture.
For example, consider this hypothetical stepwise reaction:
- A + B → C + D,
The reaction includes these elementary steps:
- A + B → X*
- X* → C + D
The chemical species X* is an intermediate.
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[edit] Definition
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 that is formed (directly or indirectly) from the reactants and reacts further to give (either directly or indirectly) the products of a chemical reaction.
The lifetime distinguishes true, chemically distinct intermediates from vibrational or conformational states (conformers). Their potential energy with respect to reactants or previous intermediates is defined to have a minimum of depth greater than available thermal energy arising from temperature, with an exact value RT, where R is gas constant and T is temperature.
Many intermediates are short-lived and have a high reactivity, thus having a low concentration in the mixture. When the necessary conditions of the reaction no longer prevail, these intermediates react further and no longer remain in the reaction mixture. Common examples are oxidizing radicals (OOH and OH) found in combustion reactions, which are so reactive that a high temperature is required to constantly produce them to compensate reaction, or the combustion reaction will cease.
There are some operations where multiple reactions are run in the same batch. For example, in an esterification of a diol, a monoester product is formed first, and may be isolated, but the same reactants and conditions promote a second reaction of the monoester to diester. The lifetime of such an "intermediate" is considerably longer than the lifetime of the intermediates of the esterification reaction itself.
[edit] 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
[edit] 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
[edit] 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
- para-quinone methides, ortho-quinone-methides
[edit] 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.
