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 > –CH2But; for the secondary alkylbenzenes the order is -cyclobutyl > -cyclopentyl > -cyclohexyl =–CHMe2–CHMePh > –CHPh2. The variations in fo/fp are ascribed to variations in fo 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%.