Editing Rate-determining step (section)

==Nucleophilic substitution==
Another example is the [[unimolecular nucleophilic substitution]] (S<sub>N</sub>1) reaction in organic chemistry, where it is the first, rate-determining step that is [[molecularity|unimolecular]]. A specific case is the [[base (chemistry)|basic]] [[hydrolysis]] of [[tert-butyl bromide]] ({{chem|t-C|4|H|9|Br}}) by aqueous [[sodium hydroxide]]. The mechanism has two steps (where R denotes the tert-butyl radical {{chem|t-C|4|H|9}}):
# Formation of a [[carbocation]] R−Br → {{chem|R|+}} + {{chem|Br|-}}.
# [[Nucleophile|Nucleophilic]] attack by hydroxide ion {{chem|R|+}} + {{chem|OH|-}} → ROH.

This reaction is found to be [[Rate equation#First-order reactions|first-order]] with ''r'' = ''k''[R−Br], which indicates that the first step is slow and determines the rate. The second step with OH<sup>−</sup> is much faster, so the overall rate is independent of the concentration of OH<sup>−</sup>.

In contrast, the alkaline hydrolysis of [[methyl bromide]] ({{chem|CH|3|Br}}) is a [[bimolecular nucleophilic substitution]] (S<sub>N</sub>2) reaction in a single [[molecularity|bimolecular]] step. Its rate law is [[Rate equation#Second-order reactions|second-order]]: ''r'' = ''k''[R−Br][{{chem|OH|-}}].