Experimental and molecular modelling studies on aromatic sulfonation

Abstract

The mechanism of the sulfonation of toluene has been explored both experimentally and theoretically using molecular orbital methods. Sulfonation with sulfur trioxide is proposed to proceed initially via the formation of a toluene-S₂O₆ π-complex (3) which rearranges to form a Wheland pyrosulfonate intermediate (5) which in turn undergoes a facile prototropic rearrangement involving the transfer of the ring hydrogen at the sp³ carbon to the sulfonate oxygen atom to form toluenepyrosulfonic acid (7). Once formed, this acid is thought to attack toluene to form two equivalents of toluenesulfonic acid (6) which preferentially react with sulfur trioxide to re-form the pyrosulfonic acid (7). Experimentally, sulfonation using either acetylsulfonic acid (9), trifluoroacetylsulfonic acid (10), or trimethylacetylsulfonic acid (11), as models for pyrosulfonic acid (7), appears to show second order kinetics at room temperature. The reaction with acetylsulfonic acid (9) shows no significant kinetic isotope effect when 4-deuterotoluene is used as the substrate, suggesting that sulfonation proceeds via attack of the π-electrons of the toluene ring at the sulfur atom, S8, of acetylsulfonic acid or toluenepyrosulfonic acid with simultaneous cleavage of the O7–S8 bond, where the displaced acetate or toluenesulfonate anion respectively can facilitate the removal of the ring proton at the sp³ carbon.