Solvolyses of para-substituted benzoyl chlorides in trifluoroethanol and in highly aqueous media

Abstract

Rates of solvolyses of para-Z-substituted benzoyl chlorides (Z = OMe, Me, Cl, or NO₂) are reported for aqueous trifluoroethanol mixtures. Additional kinetic data for highly aqueous binary mixtures (with acetone and methanol cosolvents) were obtained from an improved rapid-mixing (0.4 s) conductimetric technique. These data and additional literature data (including those for carboxylic acids) are interpreted using the equation log(k/k₀)_RCI=mY_CI, where m is the sensitivity of the substrates (RCI) to solvent ionizing power (Y_CI). A satisfactory Hammett–Brown correlation with σ⁺ for solvolyses in trifluoroethanol–water shows the rate-enhancing effects of electron donation. These results support previous evidence that solvolyses of acid chlorides (possibily including p-nitrobenzoyl chloride) can proceed by an S_N2 process in which positive charge develops on the carbonyl group. An independent nucleophilically assisted process is carbonyl addition, in which negative charge develops on the carbonyl group; this process appears to be particularly favourable in methanol and in high percentage methanol–water mixtures, and has a lower sensitivity to solvent ionizing power (m) than the S_N2 process. Competition between the two processes leads to curved mY_CI plots for a wide range of alcohol–water mixtures, the extent and position (solvent composition) of curvature changing in the expected order: OMe, Me, H, CI, NO₂. The results are not consistent with a solvation effect as the cause of the curved mY_CI plots.