Transition-State Variation in the Solvolyses of para-Substiuted Phenyl Chloroformates in Alcohol-Water Mixtures

Solvolyses of para-substituted phenyl chloroformates in water, <TEX>$D_2O,;CH_3OD$</TEX>, 50% <TEX>$D_2O-CH_3OD$</TEX>, and in aqueous binary mixtures of acetone, ethanol, methanol are investigated at 25.0 ℃. Product selectivities are reported for a wide range of ethanol-water and methanol-water solvent compositions. These data are interpreted using the Grunwald-Winstein relationship, Hammett equation, and quantum mechanical model. Grunwald-Winstein plots of the first-order rate constants for phenyl chloroformates with <TEX>$Y_{Cl}$</TEX> (based on 1-adamantyl chloride) show marked dispersions into three separate curves for the three aqueous mixtures with a small m value and a rate maximum for aqueous alcohol solvents. To account for these results, third-order rate constants, <TEX>$k_{ww},;k_{aw},;k_{wa}$</TEX>, and <TEX>$k_{aa}$</TEX> were calculated from the observed <TEX>$k_{ww};and;k_{aa}$</TEX> values together with <TEX>$k_{aw};and;k_{wa}$</TEX> calculated from the computer fit. The kinetic solvent isotope effects determined in water and methanol are consistent with the proposed mechanism of the general base catalyzed carbonyl addition-elimination for para-substituted phenyl chloroformates solvolyses based on mass law and stoichiometric salvation effect studies. This study has shown that the quantum mechanical model predicts transition state variation correctly for <TEX>$S_N2;like;S_AN$</TEX> reaction mechanism of para-substituted phenyl chloroformates.