N-nitroso compounds. Part 1. Structure and decomposition of N-nitroso-2-arylimidazolines in aqueous acidic media

Abstract

Kinetic measurements for the acid-catalysed decomposition of N-nitroso-2-arylimidazolines are reported. Reactions are first-order in both [substrate] and [H⁺]. Two products are formed; an oxazoline, which is the product of hydrolysis of the amidine moiety, and the parent imidazoline formed by denitrosation of the substrate. These products arise from two competing pathways both of which are acid catalysed. The solvent isotope effects for the denitrosation, k_NO^D+/k_NO^H+, and amidine hydrolysis, k_A^D+/k_A^H+, are 3.1 and 3.5, respectively. The denitrosation pathway, but not amidine hydrolysis, is catalysed by nucleophilic anions, and a value of 1.7 for the Swain–Scott constant, s, is obtained. In the absence of nucleophilic anions, amidine hydrolysis is preferred over denitrosation, k_A^H+ being twice as large as k_NO^H+ at 25 °C. Substitutents in the 2-aryl ring affect the rate of decomposition giving Hammett ρ values of 0.7 for denitrosation and 1.0 for amidine hydrolysis, which reflect the proximity of the reacting centres to the substituents. Values of the activation parameters are ΔH_NO^‡ 74 kJ mol^–1, ΔH_A^‡ 74 kJ mol^–1, ΔS_NO^‡–48 J K^–1 mol^–1 and ΔS_A^‡–43 J K^–1 mol^–1. The data are interpreted in terms of a fast equilibrium protonation of the substrate, followed by competitive attack at the protonated substrate, either of water or nucleophilic anions at the nitroso nitrogen atom, or of water at the amidine carbon atom. Protonation is required to activate the substrate, the substrate being recovered from neutral or alkaline solutions unchanged. The mechanism is discussed with reference to the analogous reactions of N-nitrosoamines and N-nitrosoamides.