EQUILIBRIA AND NITRATION OF SULFONIC-ACIDS IN CONCENTRATED SULFURIC-ACID

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 k2obs and k2-degrees rate constants were estimated in connection, respectively, with the stoichiometric (rate = k2obs[Aromatic]st [HNO3]st) and with the effective concentration of solutes (rate = k2-degrees [Aromatic][NO2+]). In the range 40-90 wt% H2SO4, the k2-degrees 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.