Role of Amide Nitrogen in Water Chlorination:  Proton NMR Evidence

A large fraction of the dissolved amino-N in natural waters and wastewaters is contained in amide groups, for example, in proteins. Whether this N pool reacts with and modifies the chemical behavior of chlorine during water disinfection is unclear. To investigate this issue, water-suppressed, proton NMR spectra have been obtained for aqueous, acetylated glycine, alanine, and alanylalanylalanine before and after treatment with NaOCl at near-neutral pH. N-Chlorination of N -acetylglycine (aceturic acid) induced cis−trans rotation about the amide bond at ambient temperature. N-Bound Cl + induced a measurable downfield chemical shift in the acetyl methyl resonance in N -acetylglycine, ∂δ = 0.15 ppm (cis) and ∂δ = 0.3 ppm (trans), and in N -acetylalanine methyl resonances, acetyl methyl ∂δ = 0.3 ppm (trans) and side chain methyl ∂δ = 0.2 ppm. Chlorination of N -acetylalanylalanylalanine produced ∂δ values similar to N -acetylalanine. The spectral effects were reversible, the original spectra being regenerated upon dechlorination with sulfite. Negligible substrate decomposition was observed. Rate constants for chlorination of N -acetylalanine near neutral pH are k f = 1.58 × 10 - 3 M - 1 s - 1 and k b = 7.57 × 10 - 7 s - 1 where K eq = 2.1 × 10 3 . Because of both sluggish formation kinetics and an unfavorable equilibrium constant, N -chloramides are predicted to be unimportant under typical disinfection conditions.