REACTION OF P-TOLYLSULFONYLMETHYL BENZENESULFONATE WITH HYDROXIDE ION IN HIGHLY AQUEOUS T-BUOH-H2O - ANALYSIS OF SOLVENT EFFECTS ON THE INITIAL STATE AND TRANSITION-STATE

Second-order rate constants and isobaric activation parameters (ΔH‡, ΔS‡) have been measured for the bimolecular attack of hydroxide ion at the sulfonate sulfur atom of p-tolylsulfonylmethyl benzenesulfonate (1) in highly aqueous t-BuOH-H2O (nH2O = 0.900-1.000; 25 °C). Whereas ΔG‡ increases smoothly with decreasing nH2O, ΔH‡ and -TΔS‡ exhibit mirror image behavior and pass through extrema located around nH2O = 0.95. Transition state free energies, enthalpies, and entropies of transfer from water to t-BuOH-H2O have been derived from the experimental thermodynamic quantities of activation and corresponding transfer functions for 1 and OH-. Transfer enthalpies for OH- have been calculated using the extrathermodynamic assumption ΔHtθ(Ph4As+) = ΔHtθ(Ph4B-). The different sign of ΔHtθ for Na+ and OH- is related to the opposite effect of these ions on the hydrogen-bond structure of water. The solvation changes involved in transferring the reactants into the transition state are discussed in terms of contributions due to cavitation, nonpolar interactions, and hydrogen-bond interactions. It is proposed that changes in hydrophobic hydration are of principal importance in determining the response of ΔH‡ and ΔS‡ to changes in solvent composition in the highly aqueous binary system. © 1979 American Chemical Society.