The Mechanism of Aromatic Sulphonation

It has recently been shown that in aromatic nitration and halogenation in strongly acid media the substituting agent is a positively charged ion. Although a similar attacking entity has been postulated for sulphonation, little experimental work has been done on that line. The present research is a study of the kinetics of aromatic sulphonation in fuming sulphuric acid coupled with a study of the nature of the medium and the effect of changing acid concentration on the rate of nitration. In Part I, the ionization constants of a series of aromatic nitro-compounds are given. These were measured by a spectropho tome trie method and used to determine the value of Hammett's acidity function, H0, for the range of acid concentrations from 99% H2SO4 to 36% SO3 (108.2% H2SO4). It is shown that the change in pKa between nitrobenzene and various meta- and para-substituted nitrobenzenes agrees very well with the theoretical values predicted from an electrostatic model. It is also demonstrated that the acidity function H+ , defined for the addition of a proton to a positively-charged base, is a few tenths of a logarithmic unit higher than the H0 scale in anhydrous sulphuric acid and that the difference decreases in media of higher ionic strength. Finally, a comparison is made of these results and available cryoscopic measurements. In Part II, measurements on the kinetics of sulphonation of nitrobenzene, p-nitrotoluene and p-halogensubstituted phenyltrimethylammonium ions are given for the concentration range 4 to 40% SO3. The reactions were followed spectrometrically and were for the most part carried out at 2