Why do hydrolysis reaction rates typically increase at higher pH?

bentleyt@expert.cc.purdue.edu

Hydrolysis reactions are a large and diverse set. Here is a very simple example. Methyl halides undergo hydrolysis according to

CH₃X + H₂O CH₃OH + H⁺ + X^- (rate constant k₁)

but they also react with hydroxide in a reaction very familiar to first semester organic students:

CH₃X + OH^- CH₃OH + X^- (rate constant k₂)

The first reaction is known to be first order, and the second is second order, so the rate law is

dx/dt = k₁(CH₃X) + k₂(CH₃X)(OH^-)

where x is the concentration of methyl halide reacted at time t. You can use a similar explanation for many other substances that react with both water and hydroxide.

For a review of the kinetics of typical hydrolysis reactions, see H. Wendt, Chimia, 27, 575 (1973).

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why do hydrolysis reaction rates typically increase at higher pH?