Polar effects in nucleophilic substitutions at aromatic and carbonyl carbon atoms. Kinetics of the reactions of substituted arenethiolates with 2,4-dinitrophenyl acetate and benzoate

Abstract

2,4-Dinitrophenyl acetate (DNPA) and benzoate (DNPB) are cleaved by sodium benzenethiolate in 95% aqueous ethanol with scission of both the C_CO–O and the C_Ar–O bond. Substituent effects in the nucleophile have been studied by measurement of the nucleophilic reactivity of a series of substituted arenethiolates toward these two substrates. For both esters the percentages of C_CO–O bond scission decrease with increasing nucleophilicity of the arenethiolate. The overall rate constants, k_tot, were dissected into k_CO and k_Ar corresponding to the cleavage of C_CO–O and C_Ar–O bonds respectively. Both k_CO and k_Ar increase with increasing nucleophilicity of the arenethiolate. Logarithmic plots of k_CO and k_Ar against the acidity constants K_a of the corresponding meta-substituted arenethiols are rectilinear for both esters and give β_CO 0.44 and β_Ar 0.69 for DNPA, β_CO 0.53 and β_Ar 0.65 for DNPB. Proton basicity, carbon basicity, and nucleophilicity of the arenethiolate anions have been compared.