Kinetic and Thermodynamic Barriers to Chlorine Transfer betweenAmines in Aqueous Solution

摘要

Third-order rate constants for the acid-catalyzed reversible reaction of N-chlorotaurine withbenzylamine and dimethylamine were determined in water at 25 °C and I = 0.5 (NaClO_4). Thereaction with benzylamine shows inverse solvent deuterium isotope effects of k_H/k_D= 0.57 and 0.47in the forward and reverse directions, respectively. These isotope effects, together with the absence ofdetectable general acid catalysis for this reaction, provide evidence for a stepwise mechanisminvolving fast equilibrium protonation of N-chlorotaurine followed by rate-determining chlorinetransfer from the protonated chloramine to benzylamine. The observation of strong catalysis bygeneral acids of the reaction of dimethylamine with N-chlorotaurine suggests a change to a concertedmechanism with proton and chlorine transfer occurring in a single step. This change in mechanism isenforced by the absence of a significant lifetime for protonated chlorotaurine in contact with thisstrongly nucleophilic amine. The kinetic and thermodynamic parameters for the reaction betweenprotonated chlorotaurine and benzylamine are used to estimate a Marcus intrinsic reaction barrier ofΔG_0 = 4.1 kcal/mol for chlorine transfer between amines. Comparison of this intrinsic barrier withthose reported previously for bromine transfer between carbanions points to the existence of certainsimilarities between halogen and proton transfer reactions.