Pseudo-first order reactions

Certain reactions which are expected to be of higher order follow first order kinetics. Consider the hydrolysis of methyl acetate:

The rate law is

The reaction is expected to follow second order kinetics; however, it obeys first order.

The reason is that the solvent water is present in such large excess that the change in its concentration is negligible compared to the initial one — its concentration remains constant. Thus $\mathrm{[H_2O]} = \text{constant} = k''$. The rate law becomes

$\text{rate} = k'\,\mathrm{[CH_3COOCH_3]}\,k'' = k\,\mathrm{[CH_3COOCH_3]}$ …