Electrophilic aromatic substitution. Part II. The nitration of some reactive aromatic compounds by nitric acid in sulpholan and nitromethane

Abstract

The nitration in sulpholan, and to a lesser extent in nitromethane, of a number of aromatic compounds has been studied. In sulpholan zeroth-order, mixed-order, and first-order kinetics were observed with mesitylene, toluene, and benzene, respectively. A comparison is made of sulpholan with nitromethane and acetic acid, other solvents in which zeroth-order nitration has been observed.

By adding water to sulpholan and to nitromethane it was possible to produce media in which all the compounds examined were nitrated according to the first-order law; rates relative to that of benzene were established. The relative rate of nitration of mesitylene in sulpholan, established by the competitive method, was the same as that in 7·5% aqueous sulpholan, established by the kinetic method.

In both 7·5% aqueous sulpholan and 15% aqueous nitromethane a limit to the rate of nitration is reached which is about 300 times the rate for benzene under the same conditions. Thus, m-xylene and more reactive compounds are all nitrated at about the same rate, which is the rate of encounter of the nitronium ions and the aromatic molecules. The factors which determine the point in a series of aromatic compounds at which control of rate by encounter sets in are discussed.

It is shown that a very reactive compound such as anthanthrene is extremely susceptible to nitration through nitrosation, though under appropriate conditions nitration according to the zeroth-order law could with difficulty be observed. Thus, nitration by nitronium ion and through nitrosation have both been observed. These results, combined with the establishment for organic media of a limiting rate of nitration set by encounter control, make it probable that the very high substrate selectivities reported by other authors for nitration of very reactive compounds in acetic anhydride refer, in fact, to nitration through nitrosation.