Studies in aromatic nitration : addition-elimination mechanisms in electrophilic aromatic substitution

It has been found that the nitration of suitably substituted aromatic compounds in nitric acid-acetic anhydride at 0° gives a mixture of products, some of which result from attack by the nitrating species at substituted ring positions (ipso-nitration). From this and earlier work, it is now known that ipsonitration may occur at ring positions substituted by either chloro, bromo, methyl, ethyl or methoxy groups. The product distributions suggest the factors determining the ratio of products arising from ipso-nitration and normal nitration are very sensitive to substituent changes. In general, however, provided the substituted ring position is of comparable reactivity to the available unsubstituted reaction sites, ipso-nitration is expected to be observed. Among the products that may arise from ipsonitration are nitrocyclohexadienones which rearrange to o,-nitrophenols. The rearrangement mechanism of two of these-dienones has been studied and found to be intermolecular, involving cleavage of the dienone into phenoxide and nitronium ions