Sulfonation mechanism of benzene with SO3 in sulfuric acid or oleum or aprotic solvent: Obeying the transition state theory via a trimolecular electrophilic substitution clarified by density functional theory calculation

It is found that the benzene sulfonation in sulfuric acid is a brief two-step reaction by DFT calculation at the B3lyp/6-311++G(d,p) level. First, a H2SO4 molecule generates a SO3 and a H2O molecule through a hydroxyl-oxygen protonation completed by another H2SO4 molecule. The protonation requires to cross a barrier Delta E* of 10 kcal/mol. Then, the SO3 molecule accomplishes the sulfonation through a trimolecular electrophilic substitution. The trimolecular system is C6H6+ SO3 + H2SO4. The H2SO4 is a catalyst; it is not only a proton acceptor, but also a donor. In aprotic solvents the trimolecular system is C6H6 + 2SO(3), but in liquid SO2 the system is C6H6 + SO3 + SO2. The trimolecular sulfonations all follow the transition state theory. SO3 is the electrophile. The three molecules first form spontaneously a trimolecular it -complex, and then across a transition state (TS) and complete the sulfonation. The calculation shows that in addition to SO2 as solvent, in other polar solvents the barriers are 1-5 kcal/mol, thus the sulfonations are actually a spontaneous reaction. These calculation results are in agreement with the kinetic and experimental results. Based on this study, the popular benzene sulfonation mechanism in textbooks needs to be corrected. (C) 2017 Elsevier B.V. All rights reserved.