SUBSTITUENT EFFECTS IN HYDROXAMATE NUCLEOPHILE DISPLACEMENTS AT CARBONYL CENTERS.

Abstract : The 'charge effect' previously discussed for nucleophilic displacements on isopropyl methylphosphonofluoridate by non-alpha but not by alpha nucleophiles was investigated in analogous reactions occurring at carbonyl centers. The catalytic coefficients determined for a variety of neutral and cationic para substituted benzohydroxamates in reaction with both p-nitrophenylbenzoate and p-nitrophenyl-p-nitrobenzoate were plotted in both Bronsted and Hammett linear free energy relationships. These and other data appear to be consistent with the conclusion that the 'charge effect' is a reflection of the differences in the mechanism of charge dispersal in the respective transition states for alpha and non-alpha nucleophiles engaged in nucleophilic attack at unsaturated centers. (Author)