Aromatic sulfonation. Part XVII: Kinetics and mechanism of the sulfonation of chlorobenzene in aqueous sulfuric acid†

Chlorobenzene has been sulfonated homogeneously in aqueous sulfuric acid varying from 83.4 to 99.6 wt-%, at 25°. The isomer distribution-98.8% para, 0.8% ortho, 0.4% meta - is constant over the acid range studied. The substrate isotope effect has been determined under conditions of rapid hydrogen exchange; kD/kH varies from 0.8 in 95% sulfuric acid to 0.4 in 97%. The dependence of the reaction rate and the isotope effect on the acid concentration is explained by an SE2-mechanism involving attack by H2S2O7 followed by proton removal from the σ-complex by HSO4−; the latter process is partly rate-limiting at high acid concentrations.