Electrophilic aromatic substitution. Part 18. Nitration of acetanilide and some analogues: a reconsideration

Abstract

The kinetics of nitration and the proportions of products formed, in sulphuric acid of a range of concentrations, have been measured for 3,4-dihydroquinolin-2(1H)-one, indolin-2-one N-o-tolylacetamide, methyl N-phenylcarbamate, methyl N-phenylacetimidate, and p-methoxyacetanilide. Comparison with the behaviour of acetanilide and anisole leads to the conclusion that acetanilide is nitrated as the free base and not as the cation, in contradiction of earlier arguments. The marked increase in o : p ratio observed in the nitration of acetanilide and its homologues as the acidity of the reaction medium is lowered is attributed to decreasing hydrogen bonding. The behaviour of 3,4-dihydroquinolin-2 (1H)-one and methyl N-phenylcarbamate removes the possibilityof an earlier favoured special mechanism for the ortho-substitution. involving initial attack by the electrophile at carbonyl oxygen.

The utility of rate vs. acidity plots as criteria in deciding whether a base reacts as such or as the corresponding cation is discussed. For weak bases, especially those containing oxygen functions, the occurrence of hydrogen bonding can render the criterion ambiguous or anomalous, and the dichotomy of free base and cation is too simple. In such cases, structure permitting, the kinetic ambiguity or anomaly can be accompanied by marked acidity dependence of product ratios.