Aromatic sulfonation 20: The sulfonation of some chlorobenzenes with sulfur trioxide in nitromethane as solvent

The sulfonation of chlorobenzene and p-dichlorobenzene with sulfur trioxide in nitromethane as solvent has been studied. The main reaction products are arenesulfonic anhydride and probably arenepyrosulfonic acid. With p-dichlorobenzene as substrate the amount of diaryl sulfone is undetectably small; with chlorobenzene it is < 1-mole % relative to the converted substrate. The sulfonation is of the first order with respect to the substrate and of the second order with respect to sulfur trioxide. For each mole of converted substrate two moles of sulfur trioxide are consumed. The sulfonation of p-dichlorobenzene proceeds without a primary kinetic isotope effect of hydrogen. It is suggested that the sulfonation proceeds via 1-arenonium-1-sulfonate and 1-arenonium-1-pyrosulfonate as the subsequent reactive intermediates, with the formation of the latter intermediate as the rate-limiting step. The kinetics of the formation of 4,4′-dichlorodiphenyl sulfone are compatible with a mechanism in which the species p-ClC6H4S3O9H is the effective sulfonylating entity. The literature on aromatic sulfonation with sulfur trioxide in aprotic solvents is critically reviewed.