Equilibria and nitration of sulphonic acids in concentrated sulphuric acid

Abstract

Protonation equilibria of alkane- and arene-sulphonic acids in concentrated aqueous solutions of sulphuric acid have been investigated, and the pK_a values estimated using the M_c activity coefficient function. The reliability of the new values is discussed, since the results are found to disagree with the previous figures obtained by an acidity function procedure.

The equilibrium data have been applied to nitration studies of arenesulphonic acids reacting in an analogous medium. The k_2obs and k₂° rate constants were estimated in connection, respectively, with the stoichiometric (rate =k_2obs[Aromatic]_st[HNO₃]_st) and with the effective concentration of solutes (rate =k₂°[ Aromatic][NO₂⁺]).

In the range 40–90 wt% H₂SO₄, the k₂° rates are found to be independent of solvent effects. Above 90 wt% H₂SO₄, a large decrease is observed in the rates of nitration of benzenesulphonic acid compared with that of other substrates, so that the involvement of PhSO₃^–·NO₂⁺ ion-pair at high acidity is suggested.

Equilibria and nitration of phenyl sulphoxides have also been discussed, and it is suggested that it is the unprotonated species (i.e. PhSO₃^–, PhSOMe) that undergoes nitration.