We report on a nucleophilic study of esters RC (= X)-Y-Ar in which the electrophilic center has been modified by replacing O by S in the leaving group or carbonyl center: 4-nitrophenyl acetate (1), S-4-nitrophenyl thioacetate (2), 4-nitrophenyl benzoate (3), and O-4-nitrophenyl thionobenzoate (4). The studies include O and S nucleophiles as well as α nucleophiles in H2O at 25.0±0.1° C. The sulfur nucleophile (4-chlorothiophenoxide, 4-ClPhS) exhibits ...