Equilibrium and Rate Data on Some Arsenite Transesterification Reactions

Abstract

The arsenite transesterification reaction As(OCH3)3+ HOR\ightleftharpoonsAs(OCH3)2OR+HOCH3(with R equaling benzyl and t-butyl) has been studied in acetonitrile solvent. With benzyl alcohol, K, ΔH°, and ΔS° are 3.6 (25°C), 5.0 kJ mol−1 and 27 J mol−1 K−1 respectively; with t-butyl alcohol, the values are 0.36 (25°C), 4.0 kJ mol−1, and 5 J mol−1 K−1 respectively. The rate law for the approach to equilibrium in the case of benzyl alcohol is: \frac−d[As(OCH3)3]dt=kf′[As(OCH3)3][HOCH2Ph]2−kr′[As(OCH3)2(OCH2Ph)][HOCH3][HOCH2Ph]. A mechanism which explains the unusual law has been postulated. The slower t-butyl alcohol reaction proceeds by a different mechanism which apparently involves catalysis by small amounts of water in the solvent. The steric effects are very significant.

Reaction kinetics of several alkyl arsenite hydrolyses

“…Deviations from linearity were small in all cases but qualitatively similar in that the initial rates were somewhat more rapid and the final rates somewhat slower than expected for a first-order process. The most reasonable explanation would be that case a almost applies but with one or both of the final steps of the hydrolyses being approximately [5][6][7][8][9][10] times as rapid as the first step. Thus the observed deviations can be attributed to the presence of significant concentrations of the acidic intermediates.…”