Electrophilic aromatic substitution. Part VIII. Isomer ratios and assignment of mechanism in aromatic nitration. o-xylene and biphenyl

The nitrations of o-xylene and biphenyl by nitric acid in aqueous sulphuric acid have been studied. The variation of the rates of nitration with acidity indicates that the nitronium ion is the electrophile in both cases. The ratio of 1,2-dimethyl-3-nitrobenzene to 1,2-dimethyl-4-nitrobenzene formed from o-xylene varies from 0·45 to 1·5 over the range 50–70% sulphuric acid at 25° and becomes constant above 70% sulphuric acid. Possible explanations of these data are proposed and their implications examined. The ratio of 2-nitrobiphenyl to 4-nitrobiphenyl formed from biphenyl is ca. 3·5. This suggests that previous deductions about the nitration of this substrate should be reconsidered.