Aromatic sulphonation. Part XXXVI. The sulphuric acid sulphonation of some o-dialkylbenzenes, benzocycloalkenes, and their 4-sulphonic acids. Formation of polyxylylene from benzocyclobutene

Abstract

Isomer distributions for the sulphonation of two o-dialkylbenzenes, indane, tetralin, and 6,7,8,9-tetrahydro-5H-benzocycloheptene with concentrated aqueous sulphuric acid have been determined. The 3-:4-sulphonic acid ratio decreases in the order tetralin > indane > o-xylene > 6,7,8,9-tetrahydro-5H-benzocycloheptene > o-diethylbenzene. The partial rate factors for 3-substitution by the H₂S₂O₇ sulphonation mechanism exhibit the order tetralin ≃ indane > o-xylene > o-diethylbenzene ≃ 6,7,8,9-tetrahydro-5H-benzocycloheptene. The results of tetralin and indane are accounted for by means of a theory recently advanced to explain the Mills–Nixon effect for, e.g., protiodetritiation. The reversed order of indane and o-xylene for sulphonation as compared with protiodetritiation is ascribed to hyperconjugative stabilization of the transition state for sulphonation of indane at the 3-position. Both the lower 3 : 4 ratio and the lower f₃ of o-diethylbenzene than of o-xylene and of the benzocycloheptene than of tetralin are ascribed to steric hindrance in the case of the former substrates. Sulphonation of the corresponding arene-4-sulphonic acids leads to the sole formation of the arene-3,5-disulphonic acids. In ca. 100% H₂SO₄ the rate order is rather similar to that of the parent hydrocarbons. The reaction of benzocyclobutene with 81–98% H₂SO₄ yields polyxylylene.