EFFECT OF PRESSURE ON RATE OF HYDROLYSIS OF METHYL AND ISOPROPYL BROMIDES

The rate constants for the spontaneous hydrolysis of methyl and isopropyl bromides in water have been measured to an accuracy of about 0.2% up to 3 kbars at three temperatures. The shape of the rate-constant-pressure curve for methyl bromide shows clearly that a second mechanism occurs at high pressures. The high-pressure mechanism has perhaps more covalent attachment of water in the transition state, i.e., is more SN2-like, than the low-pressure mechanism. The results for isopropyl bromide are consistent with, but do not prove, a similar but more reluctant change of mechanism with pressure. The activation volume for both reactions becomes less negative with increasing temperature at 1 bar, but this trend reverses at about 500 bars. The activation enthalpy and entropy vary with pressure in a complicated fashion, partly although not entirely connected with the change of mechanism. The constant-volume heat capacity of activation is about -93 cal deg-1 mol-1 for both reactions, which is about twice the constant-pressure heat capacity. It varies much less with pressure but much more with temperature than the constant-pressure heat capacity. A detailed interpretation of this unexpected behavior is premature at present.