Aromatic sulphonation. Part 79. Sulphonation of some (poly)methylanthracenes with dioxan–sulphur trioxide complex in dioxan

Abstract

The reactions of 2-MeA (2-methylanthracene), 2,3-Me₂A, 2,3,6,7-Me₄A, 1-MeA, 1,2-Me₂A, 1,3-Me₂A, and 1,4-Me₂A with 1.5 equiv. dioxan–SO₃ complex in dioxan at 40 °C lead in general to the formation of monosulphonic acids. The first three substrates yield the meso-sulphonic acids and 1-MeA and 1,2-Me₂A products with the sulphonic acid group at the 4- and 10-position. The reaction of 1,3-Me₂A at 17 °C leads to an equilibrium mixture of unconverted substrate and the 1,3-dimethylanthracenium-4-, -9-, and -10-sulphonate σ-complexes. On warming this reaction mixture to 40 °C, 1,3-dimethylanthracene-4- and -10-sulphonic acid and the sultone (1) are formed. 1,4-Me₂A at 17 °C yields a mixture of unconverted substrate, and 1,4-dimethylanthracenium-9- and possibly -1-sulphonate σ-complexes. Warming this mixture to 40 °C leads to the formation of 1,4-dimethylanthracene-2-, -5-, and -6-sulphonic acid. The positional order for sulphonation is in agreement with that predicted by the localization energies, provided that allowance is made for steric factors.