Stepwise reaction
A stepwise reaction[1] is a chemical reaction with one or more reaction intermediates and involving at least two consecutive elementary reactions.
The rate law of an elementary reaction is rather simple. On the other hand, when combining multiple elementary steps, the rate law can become rather complex. Moreover, when speaking about catalytic reactions, the diffusion may also limit the reaction. In general, however, there is one very slow step, which is the rate-determining step, i.e. the reaction doesn't proceed any faster than the rate-determining step proceeds.
Organic reactions, especially when involving catalysis, are often stepwise. For example, a typical enol reaction consists of at least these elementary steps:
- Deprotonation next to (α to) the carbonyl: HC–C=O → C=C–O–
- Attack of enolate: Rδ+ + C=C–O– → R–C–C=O
Rδ+ is an electron acceptor, for example, the carbon of a carbonyl (C=O). A very strong base, usually an alkoxide, is needed for the first step.
Reaction intermediates may be trapped in a trapping reaction. This proves the stepwise nature of the reaction and the structure of the intermediate. For example, superacids were used to prove the existence of carbocations.