Aromatic sulphonation. Part 65. On the sulphonation of 1-methylnaphthalene and polymethylbenzenes

The isomer distributions for the mono-, di-, and tri-sulphonation of 1-methylnaphthalene in respectively 90.1, 98.4, and 110% sulphuric acid at 25°C have been determined. The degree of sulphonation at the 2-position is smaller for sulphonation in 98.4 than 90.1% H₂SO₄. The results are discussed in terms of sulphonation by H₃SO ₄⁺, H₂S₂O₇, and H ₂S₄O₁₃ as sulphonating entities and compared with those of the polymethylbenzenes. The mechanistic conclusion that the sulphonation with H₃SO₄⁺ has a 'later' transition state than that with H₂S₂O₇ is in agreement with the results of simple Hückel MO calculations. The deviating behaviour for the 2-substitution of 1-methylnaphthalene is ascribed to enhanced steric control, because of the shorter C(1)-C(2) bond distance in 1-methylnaphthalene as compared with toluene. The isomer distribution for the mono-, and disulphonates resulting from 1-methylnaphthalene with SO₃ in nitromethane is also reported.