TRANSITION-STATE VARIATION IN THE SOLVOLYSES OF PARASUBSTITUTED PHENYL CHLOROFORMATES IN ALCOHOL-WATER MIXTURES

Solvolyses of pars-substituted phenyl chloroformates in water, D2O, CH 3OD, 50% D2O-CH3OD, and in aqueous binary mixtures of acetone, ethanol , methanol are investigated at 25.0 degrees C. Product selectivities a re reported for a wide range of ethanol-water and methanol-water solve nt compositions. These data are interpreted using the Grunwald-Winstei n relationship, Hammett equation, and quantum mechanical model. Grunwa ld-Winstein plots of the first-order rate constants for phenyl chlorof ormates with Y-Cl (based on l-adamantyl chloride) show marked dispersi ons into three separate curves for the three aqueous mixtures with a s mall m value and a rate maximum for aqueous alcohol solvents. To accou nt for these results, third-order rate constants, k(ww), k(aw), k(wa) and k(aa) were calculated from the observed k(ww) and k(aa) values tog ether with k(aw), and k(wa) calculated from the computer fit. The kine tic solvent isotope effects determined in water and methanol are consi stent with the proposed mechanism of the general base catalyzed carbon yl addition-elimination for para-substituted phenyl chloroformates sol volyses based on mass law and stoichiometric solvation effect studies. This study has shown that the quantum mechanical model predicts trans ition state variation correctly for S(N)2 like SAN reaction mechanism of para-substituted phenyl chloroformates.