Reactive intermediate
In chemistry a reactive intermediate is a short-lived, high energy, highly reactive molecule. 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. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.[1][2][3][4]
Most chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high energy, yet stable, product that only exists in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product 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.
An example of a reaction mechanism would be as follows.
Given the overall observable chemical equation: A + 2B → C + D + E,
the proposed elementary steps might be:
- A + B → C + X
- X → D + Y
- B + Y → E.
Here X and Y are the reactive intermediates. They are formed in one elementary step and destroyed in a subsequent step. Note that the algebraic sum of the proposed elementary steps must equal the observed chemical equation.
When a reactive intermediate is not an 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.
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[edit] The main carbon reactive intermediates.
based on carbon
[edit] Common features
- 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
- nitrenes
- Phosphinidenes
- carbyne
- carbenoid
- arynes
- Keto anions
- tetrahedral intermediates in carbonyl addition reactions
[edit] References
- ^ Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. ISBN 0-306-41198-9.
- ^ March Jerry; (1885). Advanced Organic Chemistry reactions, mechanisms and structure (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.
- ^ Reactive intermediate chemistry , Robert A. Moss,Matthew Platz,Maitland Jones
