Acyl–oxygen versus aryl–oxygen bond scission in reactions of benzenethiolate with nitrophenyl esters of carboxylic acids

Abstract

A number of nitrophenyl esters of acetic and benzoic acid have been found to undergo both carbonyl carbonoxygen and aryl carbon–oxygen scission in reaction with sodium benzenethiolate in ethanol. The ratio of C_co–O to C_Ar–O scission has been determined for the reactions of eight ester substrates: o-nitrophenyl acetate (ONPA), o-nitrophenyl benzoate (ONPB), p-nitrophenyl acetate (PNPA), p-nitrophenyl benzoate (PNPB), 2,4-dinitrophenyl acetate (2,4-DNPA), 2,4-dinitrophenyl benzoate (2,4-DNPB), 2,6-dinitrophenyl benzoate (2,6-DNPB), and 2,4,6-trinitrophenyl benzoate (2,4,6-TNPB). The fraction of C_co–O scission varies according to the substrate from 100% for the mononitrophenyl esters to 0% for 2,4,6-TNPB. The overall rate constants k_totfor the reactions of these esters with sodium benzenethiolate have also been measured spectrophotometrically (u.v.) in ethanol at 22°k_tot was separated into two composite rate constants k_co and k_Ar corresponding to the cleavage of C_co–O and C_Ar–O bonds respectively. k_co increases in the series ONPB < PNPB < ONPA < 2,6-DNPB ⩽ PNPA < 2,4-DNPB < 2,4-DNPA and k_Ar in the series 2,6-DNPB < 2,4-DNPA < 2,4-DNPB < 2.4,6-TNPB. Both k_co and k_Ar decrease with increasing ionic strength. The reactions of 2,4-DNPA and of 2,4-DNPB with sodium benzenethiolate show a decrease of both k_co and k_Ar on changing the solvent from ethanol to methanol, the solvent effect being slightly more pronounced for Ar than for CO attack.