Trends in selectivity. Evidence from rates and products for simultaneous reaction channels in solvolyses of benzoyl chloride and substituted derivatives

Abstract

Rates of solvolyses of para-Z-substituted benzoyl chlorides (Z = OMe, Me, or Cl) are reported for highly aqueous binary mixtures (with acetone, ethanol, and methanol cosolvents). Product data (ester and acid) are reported for a wide range of compositions of the aqueous binary alcohol mixtures (for Z = Me, H, or Cl). As water is added to alcohol, the selectivity S([ester][water]/[acid][alcohol] initially increases but then reaches a plateau and/or decreases. This reversal of the trend in S occurs at different solvent compositions for each substrate (Z = Me, H, or Cl) and is very close to the region of mechanistic change predicted previously from rate–rate profiles; these predictions have now been refined by replacing solvolyses of 1-adamantyl chloride by p-methoxybenzoyl chloride as the model S_N1 process. The results provide new and sharply defined evidence for mechanistic change in nucleophilic substitutions at acyl carbon, supporting the co-existence of two different mechanisms or reaction channels, each of which could show variable structures of transition states. A quantitative dissection of observed rates into contributions from two reaction channels is described, and an attempt is made—with partial success—to calculate variations in S values occurring as the mechanism changes. Trends in S values are calculated successfully.