Aromatic sulphonation. Part XLV. Degree of ortho-substitution and partial rate factors in the sulphonation of some phenyl- and diphenylalkanes with sulphuric acid; evidence for conformational control of ortho-substitution

Abstract

The degree of ortho-substitution for the sulphonation of some alkylbenzenes, diphenylalkanes, and triphenylmethane with concentrated aqueous sulphuric acid has been determined. The partial rate factor ratios for o- and p-substitution by the H₂S₂O₇ sulphonation mechanism of the primary alkylbenzenes exhibit the order –CH₃ > –CH₂Me > –CH₂CH₂R > –CH₂CHRR –CH₂Ph > –CH₂Bu^t; for the secondary alkylbenzenes the order is -cyclobutyl > -cyclopentyl > -cyclohexyl =–CHMe₂–CHMePh > –CHPh₂. The variations in f_o/f_p are ascribed to variations in f_o and are explained in terms of retardation of the formation of the 2-alkyl-1-benzenium-1-sulphonic acid σ-complex and/or retardation of the conversion of the corresponding sulphonate σ-complex into the o-alkylbenzenesulphonic acid as result of repulsive steric interaction between the adjacent alkyl and sulpho-groups. The degreee of retardation correlates qualitatively with that expected from conformational analysis of the 2-alkyl-1-benzenium-1-sulphonic acid σ-complexes and the σ-alkylbenzenesulphonic acids.

With diphenylmethane the degree of ortho-substitution for the introduction of the first and second sulpho-group is the same. With 1,1-diphenylethane two o,p′-disulphonic acid conformers are formed. The amount of o,p′,p″-trisulphonic acid formed with triphenylmethane is 3 ± 1%.