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=mYCI, where m is the sensitivity of the substrates (RCI) to solvent ionizing power (YCI). 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 SN2 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 SN2 process. Competition between the two processes leads to curved mYCI 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 mYCI plots.