General acid-base catalysis in the reversible disproportionation reaction of N-chlorotaurine

Third-order rate constants for the general base-catalysed reaction between N-chlorotaurine and its protonated form, k(T)(B) (M-2 s(-1)), and for general acid catalysis of the reverse process, k(T)(BH+) (M-2 s(-1)), have been determined in aqueous solution at 25 degrees C and I = 0.5 M (NaClO4). The slopes for the Bronsted correlations for the forward and the reverse reactions give beta = 0.55 and alpha = 0.48, respectively. These results suggest that a proton transfer is involved in the rate determining step of the reaction. The high acidity of protonated N,N-dichlorotaurine in aqueous solution rules out the possibility of a stepwise mechanism involving this species as an intermediate. We propose that disproportionation of N-chlorotaurine takes place by a concerted process, where proton and chlorine transfer occur simultaneously in the transition state. This mechanism differs from that found for the transfer of chlorine from a chloramine to an amine.