Electrophilic aromatic substitution. Part 22. The nitration of some reactive aromatics in methanesulphonic acid, and the question of positional selectivity in encounter rate nitrations of substituted naphthalenes and 1,2-diphenylethanes

Abstract

Nitrations in aqueous methanesulphonic acid are shown by the steep acidity dependence of the rate constants, the identification of a kinetic form zeroth-order in the concentration of the aromatic, and the existence of a limiting rate constant identified as the encounter rate constant, most probably to involve the nitronium ion.

2-Methyl-, 2-methoxy-, and 2-methoxy-6-methyl-naphthalene react at the encounter rate. In the last compound the ratio of reactivities of C-1 and C-5 is 6.5 : 1 (smaller than that reported by other workers because of the suppression of nitrosation). Comparison amongst the three compounds suggests that in the nitration of 2-methoxy-6-methylnaphthalene the rate-controlling step is the irreversible formation of an encounter pair which is sufficiently long-lived and mobile to allow selection between positions of differing reactivities in the following product-controlling step of σ-complex formation. The case resembles that of the nitration of 1,2,4-trimethylbenzene reported earlier.

Mesitylene, 3,5-dimethoxytoluene, and 1-(3,5-dimethoxyphenyl)-2-(3,5-dimethylphenyl)ethane react at the encounter rate (with the two methoxylated compounds account has to be taken of the effect upon their performances of ring protonation). In the unprotonated diarylethane the ratio of reactivities (1.8) of the methoxylated and and the methylated rings is very close to the ratio of reactivities (1.4–1.5) of the mononuclear compounds. The small selectivity between the two rings in the unprotonated diarylethane shows that the compound is nitrated by the formation of two non-interconverting encounter pairs, formation of which is both rate- and (so far as intranuclear behaviour is concerned) product-determining.

Although 1-(3,5-dichlorophenyl)-2-(3,5-dimethylphenyl)ethane also reacts at the encounter rate (being nitrated only in the methylated ring), comparison with the previously mentioned diarylethane shows that a statistical factor (not exactly 2 because of the slightly differing efficiencies of the two rings in the dimethoxylated compound in forming productive ion pairs) has to be taken into account in comparing the two compounds.

The behaviour of these various compounds in no way requires the assumption of bonding forces between electrophile and aromatic in the encounter pair, but the performance of 1-(3,5-dimethoxyphenyl)-2-(3,5-dimethylphenyl)-ethane could be very easily understood if the two encounter pairs were π-complexes, or involved some other kind of stabilisation.