title: Electrophilic aromatic sulfonation with SO3: concerted or classic S(E)Ar mechanism?
abstract: The electrophilic sulfonation of several arenes with SO(3) was examined by electronic structure computations at the M06-2X/6-311+G(2d,2p) and SCS-MP2/6-311+G(2d,2p) levels of theory. In contrast to the usual interpretations, the results provide clear evidence that in nonpolar media and in the absence of catalysts the mechanism of aromatic sulfonation with a single SO(3) is concerted and does not involve the conventionally depicted 1:1 σ complex (Wheland) intermediate. Moreover, the computed activation energy for the 1:1 process is unrealistically high; barriers for alternative 2:1 mechanisms involving attack by two SO(3) molecules are 12-20 kcal/mol lower! A direct 2:1 sulfonation mechanism, involving a single essential transition state, but no Wheland type intermediate, is preferred generally at MP2 as well as at M06-2X in isolation (gas phase) or in noncomplexing solvents (such as CCl(4) or CFCl(3)). However, in polar, higher dielectric SO(3)-complexing media, M06-2X favors an S(E)Ar mechanism for the 2:1 reaction involving a Wheland-type arene-(SO(3))(2) dimer intermediate. The reaction is slower in complexing solvents, since the association energy, e.g., with nitromethane, must be overcome. But, in accord with the experimental kinetics (second-order in SO(3)), attack by two sulfur trioxide molecules is still favored energetically over reaction with a single SO(3) in CH(3)NO(2). The theoretical results also reproduce the experimental reactivity and regioselectivity trends for benzene, toluene, and naphthalene sulfonation accurately.
authors: Department of Chemistry, University of Sofia, Sofia 1164, Bulgaria.
authors: Gergana
authors: G
authors: Koleva
authors: Boris
authors: B
authors: Galabov
authors: Jing
authors: J
authors: Kong
authors: Henry F
authors: HF
authors: Schaefer
authors: Paul von R
authors: Pv
authors: Schleyer
mesh_terms: D002722:Chlorobenzenes / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory; D013451:Sulfonic Acids / Q000138:chemical synthesis* / Q000737:chemistry; D013461:Sulfur Oxides / Q000737:chemistry*
chemical_list: D002722:Chlorobenzenes; D013451:Sulfonic Acids; D013461:Sulfur Oxides; C018511:4-dichlorobenzene; C011118:sulfur trioxide