THE EFFECT OF POLARIZABILITY ON REACTIVITY OF 4-NITROPHENYL BENZOATE AND ITS SULFUR-CONTAINING ANALOGS WITH ANIONIC NUCLEOPHILES IN ETHANOL

2nd-order rate constants have been measured spectrophotometrically for the reactions of 4-nitrophenyl benzoate (1), S-4-nitrophenyl thiobenz oate (2) and 1-nitrophenyl thionbenzoate (3) with alkoxides, aryloxide s and thioaryloxides in absolute ethanol at 25.0 +/- 0.1 degrees C. Th e substitution of O by polarizable S in the leaving group has little a ffected the reactivity of 2 toward the charge localized species (e.g. EtO(-) and CF3CH2O-), while the effect of the similar replacement in t he carbonyl group has led to a decrease in reactivity by a factor of 1 0. However, the reactivity of these esters toward charge delocalized a ryloxides has been found to be in the order 1 < 3 less than or equal t o 2. The effect of replaced sulfur atom on reactivity becomes more sig nificant for the reaction with polarizable thioaryloxides, i.e. the re activity increases in the order 1 < 2 much less than 3. The difference in reactivity for the present system is attributed to a polarizabilit y effect.