Electrophilic aromatic substitution. Part I. The nitration of some reactive aromatic compounds in concentrated sulphuric and perchloric acids

Abstract

The nitration of benzene and some more reactive compounds in sulphuric acid (and to a smaller extent in perchloric acid) has been studied. In both media a limit is reached beyond which the introduction of further activating substituents does not increase the rate of nitration. This limit is the rate of encounter of the nitronium ions and the aromatic molecules. Up to 68% sulphuric acid the slope of the plot of the stoicheiometric second-order rate constant for the nitration of benzene against –(H_R+ log a_H2O) at 25° is unity, but above this acidity the slope increases. These facts diminish the value of a comparison of the rate of nitration of a given substrate with that of benzene as a means of studying aromatic reactivity, for the comparison would be dependent on the acidity. Further, at the higher end of the acidity range studied, the rate of nitration of benzene approaches to within a sixth of the observed encounter rate, and if the former is then to some degree encounter-controlled, the significance of comparison with other substrates is reduced.