Aromatic sulphonation. Part 62. Sulphonation of biphenyl in concentrated aqueous sulphuric acid

The homogeneous (mono)sulphonation of biphenyl in a large excess of aqueous sulphuric acid (81.5–96.8 wt. %) at 25 °C has been studied, and rates and isomer distributions have been determined. The sulphonation is first-order with respect to the substrate, and takes place predominantly (>90%) at the para-position. Up to an acid concentration of 86 wt. % H2SO4 the active sulphonating entity is H3SO4+. For this type of sulphonation Phfp= 600 ± 300, from which it was calculated that ρ+(H3SO4+)=–10.0 ± 1.2. The high degree of para-substitution indicates substantial conjugative stabilization of the corresponding σ-complex. The relatively low degree of ortho-substitution may be explained in terms of steric hindrance by the adjacent ortho-hydrogen atom of the phenyl substituent to the formation of the σ-complex leading to the ortho-sulphonic acid. The syntheses of the three biphenylmonosulphonic acids and the six biphenyldisulphonic acids containing one sulpho-group per phenyl ring, required as reference compounds for the multicomponent u.v. analysis, are reported.