Aromatic sulphonation. Part 64. Sulphonation of biphenylmonosulphonic acids and some other biphenyls, containing a deactivating substituent, in concentrated sulphuric acid

Abstract

The (homogeneous) sulphonation of a number of biphenyl derivatives containing a deactivating substituent, viz. the three biphenylsulphonic acids, the three nitrobiphenyls, and some other mainly 4-substitutions biphenyls, in concentrated aqueous sulphuric acid at 25°C has been studied, and rates and/or isomer distributions have been determined. The sulphonation is first-order with respect to the substrate and takes place in the unsubstituted ring, predominantly (>96%), with the exception of biphenyl-2-sulphonic acid, at the 4′-position. From the rate measurement it was concluded (i) that the substrate species undergoing sulphonation are mainly the biphenylsulphonate anions, and unprotonated 4-nitro- and 4-acetyl-biphenyl and biphenyl-4-carboxylic acid, and (ii) that the sulphonating entity is H₂S₂O₇. Partial rate factors for these sulphonations and the σ⁺ value for the substituents C₆H₄X (X = 2-SO₃^–, 3-SO₃^–, 4-SO₃^–, 4-NO₂, 4-COMe, and 4-CO₂H) are reported. The very low partial rate factors for 2′-substitution are explained in terms of steric hindrance between the incoming sulpho-group and the adjacent hydrogen atom or group X at the 2-position of the C₆H₄X substituent to the formation of the σ-complex leading to the 2′-sulphonic acid. The f₄, values of biphenyl-3- and -4-sulphonate anions are substantially greater than unity, indicating enhanced inter-ring mesomeric stabilization in the σ-complexes leading to 4′-substitution as compared with the substrates. The f₄, value is appreciably smaller for 4-nitrobiphenyl than for biphenyl-4-sulphonate which is ascribed to the larger –M effect of NO₂ as compared with the SO₃^– substituent.

The very low f₃, values of the 3- and 4-substituted biphenyls are explained in terms of the absence of inter-ring mesomeric stabilization in the σ-complexes leading to 3′-substitution.