Kinetics of the reactions of nitrous acid with olefins. Kinetic identification of probable nitrosating agents

Abstract

The overall reactivity of the following olefins towards nitrous acid in water has been found to decrease along the series 2,3-dimethylbut-2-ene (I) > 2-methylbut-2-ene (II) 2-methylprop-2-enol (III) > 2-methylprop-2-enyl chloride (IV), indicating the electrophilic nature of the reagent. At low acidity for (I) and (II) the reaction was of the second order in nitrous acid suggesting reaction via N₂O₃, whereas at higher acidities for all four olefins the reaction was of the first order in nitrous acid and also of first order in mineral acid, which is interpreted in terms of reaction via H₂N⁺O₂. Both chloride- and bromide-ion catalysis was observed, the kinetics of these catalysed reactions being consistent with the formation of and reaction by the nitrosyl halides, with nitrosyl chloride being ca. 6 times more reactive than nitrosyl bromide towards (III). The nitrosyl halide reactions showed only a small dependence upon the substituents in the olefin whereas the nitrous acidium ion (H₂N⁺O₂) had selectivity comparable with that shown in bromination. The results are compared with those obtained for the nitrosation of aromatic amines.