Limitations of the transition state variation model. Part 8. Dual reaction channels for solvolyses of 3,4-dimethoxybenzenesulfonyl chloride

Abstract

Solvolyses of 3,4-dimethoxybenzenesulfonyl chloride (DSC) in water, D2O, CH3OD, and in aqueous binary mixtures of acetone, acetonitrile, 1,4-dioxane, ethanol, methanol, and 2,2,2-trifluoroethanol (TFE) have been investigated at 25.0 degrees C. Kinetic solvent isotope effects (KSIE) in water and in methanol and product selectivities in alcohol-water mixtures are also reported. The Grunwald-Winstein plot of first-order rate constants for the solvolyic reaction of DSC with Y-Cl shows marked dispersions into separated lines for various aqueous mixtures. With use of the extended Grunwald-Winstein equation, the I and m values obtained are 1.12 and 0.58 respectively for the solvolyses of DSC. The relatively large magnitude of I is consistent with substantial nucleophilic solvent assistance. From Grunwald-Winstein plots the rate data are dissected approximately into contributions from two competing reaction channels. This interpretation is supported for alcohol-water mixtures by the trends of product selectivities, which show a maximum for ethanol-water mixtures. From the KSIE of 1.45 in methanol, it is proposed that the reaction channel favored in methanol-water mixtures and in all less polar media is general-base catalysed and/or is possibly (but less likely) an addition-elimination pathway. Also, the KISE value of 1.35 for DSC in water is expected for S(N)2-S(N)1 processes, with minimal general base catalysis, and this mechanism is proposed for solvolyses in the most polar media.