Aromatic sulphonation. Part XLIX. Sulphonation of anthracene and some meso-substituted hydrocarbon derivatives: mechanism of methyl side-chain sulphonation

Abstract

The sulphonation of anthracene and 9-phenyl-, 9-methyl-, and 9,10-dimethyl-anthracene with dioxan–SO₃ in dioxan has been studied at 40°. With all substrates substitution products and no addition products are formed. With the first two substrates sulphonation occurs predominantly at the meso-position, the substitution distribution being 66 ± 89-, 26 ± 31-, and 8 ± 2% 2-substitution with anthracene, and 67 ± 410- and 33 ± 4% 4-substitution with 9-phenylanthracene. With the meso-methylated anthracenes not ring but methyl substitution takes place, the only products being 9-anthrylmethanesulphonic acid with 9-methylanthracene, and 10-methyl-9-anthrylmethanesulphonic acid and 9,10-bis-sulphomethylanthracene with 9,10-dimethylanthracene. It is proposed that the methyl sulphonation occurs by initial addition of SO₃ to the opposite meso-position and subsequent loss of SO₃ from the methyl sulphonated σ-complex. A similar but more complex mechanism is proposed to explain the methyl sulphonation of 7-isopropyl-1,4-dimethylazulene.