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

被引:28
作者
BALIGA, BT
WHALLEY, E
机构
[1] Division of Applied Chemistry, National Research Council, Ottawa, Ont.
关键词
D O I
10.1021/j100723a029
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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.
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页码:654 / &
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