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Concept
Reaction intermediate
Summary
In chemistry, a reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) but is consumed in further reactions in stepwise chemical reactions that contain multiple elementary steps. Intermediates are the reaction product of one elementary step, but do not appear in the chemical equation for an overall chemical equation.
For example, consider this hypothetical stepwise reaction:
A + B -> C + D
The reaction includes two elementary steps:
A + B -> X
X -> C + D
In this example, X is a reaction intermediate.
The IUPAC Gold Book defines an intermediate as a compound that has a lifetime greater 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 condition distinguishes true, chemically distinct intermediates from vibrational states or such transition states which, by definition have lifetimes close to that of molecular vibration.
Kinetically, intermediates are often consumed quickly in a step-wise mechanism. The designation of "fast" or "slow" consumption speed is relative, and a relative intermediate are sometimes separated from a reaction intermediate based on being relatively short-lived. Reactive intermediates are an unstable type of reaction intermediate, and are usually short-lived, high-energy, and seldom isolated. They do not remain in the product mixture due to their short lifetime, in contrast to other reaction intermediates.
Cations, often carbocations, serve as intermediates in various types of reactions to synthesize new compounds.
Carbocations are formed in two major alkene addition reactions. In a HX addition reaction, the pi bond of an alkene acts as a nucleophile and bonds with the proton of an HX molecule, where the X is a halogen atom. This forms a carbocation intermediate, and the X bonds to the positive carbon that is available, as in the example reaction shown below.
Official source
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