Rate-determining Step - Nucleophilic Substitution

Nucleophilic Substitution

Another example is the unimolecular nucleophilic substitution (SN1) 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 (t-C4H9Br) by aqueous sodium hydroxide. The mechanism has two steps (where R denotes the tert-butyl radical t-C4H9)

  1. Formation of a carbocation R-Br → R+ + Br-
  2. Nucleophilic attack by one water molecule R+ + OH- → ROH.

This reaction is found to be first-order with rate = k, 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 (CH3Br) is a bimolecular nucleophilic substitution (SN2) reaction in a single bimolecular step. Its rate law is second-order, rate = k.

Read more about this topic:  Rate-determining Step

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