Effect of solvent on reactivity and basicity: Aminolyses of p-nitrophenyl acetate in H2O and in DMSO

Abstract

Second-order rate constants (kN) have been measured spectrophotometrically for the reactions of p-nitrophenyl acetate with a series of alicyclic secondary amines in H2O and in DMSO at 25.0 ± 0.1 °C. The solvent change from H2O to DMSO resulted in rate enhancements. The effect of solvent on reactivity was found to be most significant for the reaction with piperazinium ion and least significant for the reaction with piperidine. The pKa values of the conjugate amines studied were also determined in DMSO. It was found that piperazinium ion is more basic in DMSO than in H2O by 1.04 pKa units while piperidine is less basic in DMSO by 0.50 pKa units. The Brønsted-type plot was linear with a large slope (βnuc = 0.76-0.82) for both reactions run in H2O and in DMSO, indicating that the aminolyses proceed through rate-determining breakdown of the addition intermediate. The difference in pKa values determined in DMSO and in H2O (ΔpKa = pKa in DMSO - pKa in H2O) showed a linear correlation with the difference in the second-order rate constants determined in DMSO and in H2O (Δlog kN = log kN in DMSO - log kN in H2O) with a slope close to unity, suggesting that ΔpKa is fully responsible for the rate enhancement in DMSO. Copyright © 2002 John Wiley & Sons, Ltd.