Another example is the unimolecular nucleophilic substitution (S1) reaction in organic chemistry, where it is the first, rate-determining step that is unimolecular. A specific case is the basic hydrolysis of tert-butyl bromide () by aqueous sodium hydroxide. The mechanism has two steps (where R denotes the tert-butyl radical ):
# Formation of a carbocation R−Br →  + .
# Nucleophilic attack by hydroxide ion  +  → ROH.
This reaction is found to be first-order with r = k[R−Br], which indicates that the first step is slow and determines the rate. The second step with OH is much faster, so the overall rate is independent of the concentration of OH.
In contrast, the alkaline hydrolysis of methyl bromide () is a bimolecular nucleophilic substitution (S2) reaction in a single bimolecular step. Its rate law is second-order: r = k[R−Br][].