Molecular Modeling Studies on Aromatic Sulfonation. 1. Intermediates Formed in the Sulfonation of Toluene.

Molecular modeling studies suggest that the mechanism of the sulfonation of toluene with sulfur trioxide proceeds via the formation of a pi-complex (4b) which rearranges to form a Wheland intermediate (5). This structure is unable to form toluenesulfonic acid (8) directly and prefers to react with a further molecule of sulfur trioxide to form a pyrosulfonate intermediate (6a) which undergoes a facile prototropic rearrangement involving the transfer of the ring hydrogen at the sp(3) carbon of 6a to the sulfonate oxygen atom to form the corresponding acid (7). The formation of toluenesulfonic acid (8) appears to arise from an exothermic reaction of between the pyrosulfonic acid (7) and toluene. The overall calculated thermodynamic change in moving from the reaction of one molecule of sulfur trioxide with toluene to the sulfonic acid (8b) is fully consistent with an estimated experimental value of -33.5 kcal mol(-1) for the same reaction using simple alkylbenzenes.