Aromatic sulphonation. Part XLIV. Sulphonation of some meta-substituted benzenesulphonic acids with fuming sulphuric acid. Preparation of arene-1,2-disulphonic anhydrides

Abstract

Isomer distributions for the sulphonation of various 3-substituted benzenesulphonic acids in fuming sulphuric acid at 25° have been determined. The isomer distributions are dependent on the sulphuric acid concentration and approach a constant ratio below 104 and above 115% H₂SO₄. The degree of substitution at the 6-position and the f₆:f₅ partial rate factor ratio run parallel with the ability of conjugative electron release of the substituent at the 3-position. The size of this substituent further determines the f₄:f₆ ratio. The preparation of some 4-substituted benzene-1,2-disulphonic anhydrides is described. They hydrolyse rapidly with the exception of 3,4,5,6-tetramethylbenzene-1,2-disulphonic anhydride. This compound is also formed more rapidly from the corresponding disulphonic acid than the other anhydrides, probably as a result of a decrease in overcrowding. The ¹H n.m.r. and i.r. spectral data of the intramolecular anhydrides are compared with those of some intermolecular arenesulphonic anhydrides.