Gas phase reactions of NH 2Cl with anionic nucleophiles: nucleophilic substitution at neutral nitrogen

Abstract

Reactions of chloramine, NH 2Cl, with HO −, RO − (R = CH 3, CH 3CH 2, CH 3CH 2CH 2, C 6H 5CH 2, CF 3CH 2), F −, HS −, and Cl − have been studied in the gas phase using the selected ion flow tube technique. Nucleophilic substitution ( S N 2) at nitrogen to form Cl − has been observed for all the nucleophiles. The reactions are faster than the corresponding S N 2 reactions of methyl chloride; the chloramine reactions take place at nearly every collision when the reaction is exothermic. The thermoneutral identity S N 2 reaction of NH 2Cl with Cl −, which occurs approximately once in every 100 collisions, is more than two orders of magnitude faster than the analogous reaction of CH 3Cl. The significantly enhanced S N 2 reactivity of NH 2Cl is consistent with a previous theoretical prediction that the barrier height for the S N 2 identity reaction at nitrogen is negative relative to the energy of the reactants, whereas this barrier height for reaction at carbon is positive. Competitive proton abstraction to form NHCl − has also been observed with more highly basic anions (HO −, CH 3O −, and CH 3CH 2O −), and this is the major reaction channel for HO − and CH 3O −. Acidity bracketing determines the heat of deprotonation of NH 2Cl as 374.4 ± 3.0 kcal mol −1.