Acylarylnitrosamines. Part 12. Decomposition of N-nitrosoacetanilide and p-nitro-N-nitrosoacetanilide in bromotrichloromethane. An answer to the long-standing problem of the mechanism of ‘anomalous’ dual halogen abstractions

Abstract

Whereas phenyl radicals from dibenzoyl peroxide or phenylazotriphenylmethane abstract bromine almost exclusively, rather than chlorine, from bromotrichloromethane, decomposition at 50 °C of N-nitrosoacetanilide (NNA), also an established source of phenyl radicals, leads to both bromo- and chloro-benzene thus providing a further example of ‘anomalous’ halogen abstraction involving NNA and polyhalogenomethanes. Reaction at 20 °C leads to the precipitation of a mixture of benzenediazonium chloride and acetate, the bromide being absent, and to the formation of bromobenzene and little chlorobenzene. Heating this mixture of diazonium salts at 50 °C in bromotrichloromethane leads to mainly chlorobenzene and some phenyl acetate. Also formed in the reaction of NNA with bromotrichloromethane are significant amounts of carbon dioxide (23 mol per 100 mol NNA), acetic anhydride (14.5), and acetic acid (62). Reaction in the presence of the radical trap, 1,1-diphenylethene (1 mol. equiv.), leads to suppression of chlorobenzene, i.e. the reaction is restored to normality. Isolation of 9-phenylphenanthrene is evidence of competing formation of benzyne in this reaction. These results point to the occurrence of abstraction of bromine by phenyl radicals from bromotrichloromethane to give trichloromethyl, which is scavenged by phenyldiazo-oxyl (1) to give trichloromethylbenzenediazoate (2); this in turn gives the observed benzenediazonium chloride, and hence chlorobenzene, and phosgene. Reaction of phosgene with NNA leads to more diazonium chloride, carbon dioxide, and acetic anhydride. In support of this, silver benzenediazoate reacts with bromotrichloromethane to give benzenediazonium chloride. The reaction in the presence of 1,1-diphenylethene leads to an increase in yield of bromobenzene thus pointing to decomposition of phenyldiazo-oxyl, in this case, via electron transfer with the radical Ph₂Ċ·CH₂·CCl₃, to give a phenyl radical.

p-Nitro-N-nitrosoacetanilide also reacts with bromotrichloromethane via dual or ‘anomalous’ abstraction of halogen, superimposed on scrambling of aromatic substituents via nucleophilic displacement by chloride and acetate of the activated p-nitro-group in the p-nitrobenzenediazonium ion.

Attention is drawn to the relevance of these results to other anomalous halogen abstraction reactions of NNA in chloroform and in carbon tetrachloride.