Effect of Nonleaving Group on the Reaction Rate and Mechanism: Aminolyses of 4-Nitrophenyl Acetate, Benzoate and Phenyl Carbonate

Abstract

Second-order rate constants have been determined spectrophotometrically for the reaction of phenyl 4-nitrophenyl carbonate with a series of primary amines in <TEX>$H_2O$</TEX> containing 20 mol % DMSO at 25.0 <TEX>${\circ}$</TEX>C. The Bronsted-type plot is linear with a <TEX>${\beta}_{nuc}\;0.69 {\pm} 0.04$</TEX>, which is slightly smaller than the <TEX>${\beta}_{nuc}$</TEX> values for the reactions of 4-nitrophenyl acetate ( <TEX>$\beta_{nuc}= 0.82 {\pm} 0.03$</TEX>) and benzoate ( <TEX>$\beta_{nuc} = 0.76 {\pm} 0.01$</TEX>), indicating that the reaction proceeds through a tetrahedral zwitterionic intermediate <TEX>$T^{\pm}$</TEX>. The carbonate is more reactive than the corresponding acetate and benzoate. The changing Me (or Ph) to PhO has resulted in a decrease in the <TEX>${\beta}_{nuc}$</TEX> value without changing the reaction mechanism but an increase in the reactivity. The electronic effect of the substituent in the nonleaving group appears to be responsible for the enhanced reactivity of the carbonate compared with the corresponding acetate and benzoate.