Nucleophilic Substitution Reactions of N-Chloramines:  Evidence for a Change in Mechanism with Increasing Nucleophile Reactivity

Third-order rate constants (k Nu)H (M-2 s-1) for the hydronium ion catalyzed reactions of a range of nucleophiles with N-chlorotaurine (1) in water at 25 °C and I = 0.5 (NaClO4) are reported. The solvent deuterium isotope effects on hydronium ion catalysis of the reaction with 1 of bromide and iodide ion are (k Br)H/(k Br)D = 0.30 and (k I)H/(k I)D = 0.54, respectively. The inverse nature of these isotope effects and the absence of general acid catalysis are consistent with a stepwise mechanism involving protonation of 1 in a fast preequilibrium step. The appearance of strong catalysis by general acids for the reaction of the more nucleophilic SO3 2- and HOCH2CH2S- with the chloramine indicates a change to a concerted mechanism, with protonation of the chloramine at nitrogen and chlorine transfer to the nucleophile occurring in a single step. A rough estimate of the lifetime of the protonated chloramine in the presence of the thiolate anion suggests that the concerted mechanism is enforced by the absence of a significant lifetime of the protonated substrate in contact with the nucleophile. Theoretical calculations provide evidence against an electron-transfer mechanism for chlorination of the nucleophiles by protonated 1.