Kinetic and thermodynamic barriers to chlorine transfer between amines in aqueous solution

Third-order rate constants for the acid-catalyzed reversible reaction of N-chlorotaurine with benzylamine and dimethylamine were determined in water at 25 degrees C and I = 0.5 (NaClO4). The reaction with benzylamine shows inverse solvent deuterium isotope effects of kH/kD=0.57 and 0.47 in the forward and reverse directions, respectively. These isotope effects, together with the absence of detectable general acid catalysis for this reaction, provide evidence for a stepwise mechanism involving fast equilibrium protonation of N-chlorotaurine followed by rate-determining chlorine transfer from the protonated chloramine to benzylamine. The observation of strong catalysis by general acids of the reaction of dimethylamine with N-chlorotaurine suggests a change to a concerted mechanism with proton and chlorine transfer occurring in a single step. This change in mechanism is enforced by the absence of a significant lifetime for protonated chlorotaurine in contact with this strongly nucleophilic amine. The kinetic and thermodynamic parameters for the reaction between protonated chlorotaurine and benzylamine are used to estimate a Marcus intrinsic reaction barrier of deltaG(0)+/+ =4.1 kcal/mol for chlorine transfer between amines. Comparison of this intrinsic barrier with those reported previously for bromine transfer between carbanions points to the existence of certain similarities between halogen and proton transfer reactions.