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

Abstract

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 ΔE∗ of 10kcal/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+2SO3, 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 π-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 1–5kcal/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.