Aromatic sulphonation. Part 53. Kinetics and mechanism of sulphonation of ω-phenylalkane-1-sulphonic acids in sulphuric acid. Comparison of the side-chain SO3H and OSO3H substituent effects

Abstract

The sulphonation of a series of sulphonic acids of the general structure Ph[CH₂]_nSO₃H (n= 1,2,3–6) in concentrated aqueous sulphuric acid at 25 °C has been studied, and partial rate factors are reported. With benzenesulphonic acid the SO₃H substituent exhibits a –M effect in addition to the combined inductive and direct field effect. The results of the ω-phenylalkane-1-sulphonic acids are explained in terms of the competing deactivating effect of the SO₃H group (a combined relayed inductive and direct field effect) and the activating hyperconjugative electron release of the benzylic CH₂ group. The former effect decreases with increasing n, whereas the latter effect only slightly increases upon increasing n from 1 to 2.

As judged from a comparison with the f_p of the sulphates of the general structure Ph[CH₂]_nOSO₃H (n 2) the electronic effects of the [CH₂]_nOSO₃H and the [CH₂]_nSO₃H groups are about the same.