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). Fromt his and earlier work ,it is now known that ipso-nitration 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 an 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 ipso-nitration are nitrocyclohexadienones which rearrange to o-nitrophenols. The rearrangements mechanism of two of these dienones has been studied and found to be intermolecular, involving cleavage of the dienone into phenoxide and nitronium ions.