Kinetics and mechanism of the decomposition of N , N ‐dihalopeptides

The tripeptide alanylalanylalanine reacts with either HOCl or HOBr to form a N , N ‐dihalopeptide. These compounds decompose to the N ‐halo‐ N ′‐(α‐iminoketo)peptides plus halide ion. The rate constants for these reactions in 0.4‐M NaCl were determined to be\documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{l} \begin{array}{*{20}c}{{\rm Cl:}} \hfill & {k_4 = 1.52 \times 10^{15} e^{ - 19,600/RT} \,{\rm min}^{{\rm - 1}} \,({\rm pH}\,{\rm 6}{\rm .7)}} \hfill \\\end{array} \\ \begin{array}{*{20}c}{{\rm Br:}} \hfill & {k_4 = 1.06 \times 10^{15} e^{ - 18,400/RT} \,{\rm min}^{{\rm - 1}} \,({\rm pH}\,{\rm 6}{\rm .6)}} \hfill \\\end{array} \\ \end{array} $$\end{document}where the activation energies are in cal/mol. The N ‐halo‐ N ′‐(α‐iminoketo)‐peptides slowly decompose either by a two‐step hydrolysis reaction to form ammonia, halide ion, and an N ‐(α‐acylketo)peptide, or by direct decomposition to a nitrile, an isocyanopeptide, and a halide ion.