Limitations of the transition state variation model. Part 2. Dual reaction channels for solvolyses of 2,4,6-trimethylbenzenesulphonyl chloride

Abstract

Rate constants for solvolyses of 2,4,6-trimethylbenzenesulphonyl chloride (1) are reported for aqueous binary mixtures with acetone, acetonitrile, dioxane, ethanol and methanol. Kinetic solvent isotope effects in water and in methanol and product selectivities in alcohol–water mixtures are also reported. Additional Y_Cl values have been determined for aqueous acetonitrile and dioxane from rate constants for solvolyses of 1-adamantyl chloride. Contrary to earlier reports, correlations between logarithms of rate constants for solvolyses of 1vs. Y or Y_Cl are approximately bilinear. From these plots the rate data are dissected into contributions from two competing reaction channels, and this interpretation is supported for alcohol–water mixtures by the trends of product selectivities, which show maxima close to the solvent compositions where there are breaks in the rate–rate profiles. Greater rate constants for 40% v/v ethanol–water than for 97% w/w trifluoroethanol–water (solvents of approximately equal ionizing power) show the importance of nucleophilic solvent assistance (S_N2 character) even for the reaction channel favoured in more polar media. Hence, in agreement with an earlier consensus, neither of the reaction channels corresponds to an S_N1 mechanism. From the kinetic solvent isotope effect of 1.68 in methanol, it is proposed that the reaction channel favoured in less polar media is general-base catalysed and/or is possibly an addition–elimination pathway.