Electrophilic aromatic substitution. Part 28. The mechanism of nitration of some 4-substituted anisoles and phenols, and of rearrangement of the intermediate 4-nitro-4-substituted-cyclohexa-2,5-dienones

Abstract

The kinetics of nitration in sulphuric acid of 2-chloro-4-methyl-, 4-chloro-, 2,4-dichloro-, and 4-fluoroanisole and of the corresponding phenols have been determined. The reaction products from the anisoles and from 2-chloro-4-methyl- and 4-fluoro-phenol have been determined. Results for 4-methylanisole supplementary to earlier ones are also reported. Generally the anisoles give the 2-nitro-derivatives and the 2-nitrophenols, and from 2-chloro-4-methylanisole, 2-chloro-4-methyl-4-nitrocyclohexa-2,5-dienone was formed as an intermediate. The decomposition of this dienone in sulphuric acid, like those of others, changes from a non-acid-catalysed to an acid-catalysed form with increasing acidity. The first form is regarded as a decomposition into an aryloxyl radical and nitrogen dioxide which can recombine to give the 2-nitrophenol. The formation of a small amount of 2-(4-fluorophenoxy)-4-fluorophenol in the nitration of 4-fluorophenol is seen as support for this view. The acid-catalysed form is regarded as the decomposition of the protonated dienone into a phenol–nitronium ion encounter-pair which can give the nitrophenol. A consequence of the mechanism is that if the phenol were nitrated at less than the encounter rate, the phenol itself would in appropriate conditions be a product of the ipso-nitration of the original anisole. 4-Methyl-, 2-chloro-4-methyl-, and 4-chloro-phenol have been so identified. Quantitative analysis of the results allows evaluation of the partitioning of dienone decomposition between the two modes. The mechanism accounts for the formation from 2,4-dichloro-anisole of both 2,4-dichloro-6- and 2,4-dichloro-5-nitroanisole, but only 2,4-dichloro-6-nitrophenol.