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-S 2 O 6 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 sp 3 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 4deuterotoluene 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 sp 3 carbon.

The Electrophilic Substitution of Arenes: Is the π Complex a Key Intermediate and What is Its Nature?

“…The corresponding CT and σ complexes have also been investigated experimentally and theoretically for the reaction of arenes with other electrophiles, such as H + ,10 secondary and tertiary alkyl cations,11 and SO 3 12. The existence of a symmetrical over‐ring structure can be excluded in these cases as energetically it lies much higher than the other configurations.…”

“…In comparison to the tert ‐butyl cation the 2‐propyl cation lies, according to the results of Pezacki et al,15 about five units of the electrophile parameter ( E ) higher, and thus lies in the “yellow‐red” region of the scale: a stable CT complex is therefore kinetically less likely. The reaction of toluene with SO 3 in solution has been investigated experimentally and calculated theoretically with less rigorous basis sets (4‐31G/S*) 12. Here corner‐coordinated (atom‐coordinated) π and σ complexes with arene–SO 3 –SO 3 structures are found, the experimentally determined reactivities of which can be predicted well theoretically.…”

Neue Ergebnisse zur elektrophilen Substitution von Arenen: Ist der π‐Komplex eine wesentliche Zwischenstufe und was ist seine Natur?

“…Auch für die Reaktion von Arenen mit anderen Elektrophilen wie H + ,10 sekundären und tertiären Alkylkationen11 sowie SO 3 12 wurden jetzt die entsprechenden CT‐ bzw. σ‐Komplexe experimentell und theoretisch untersucht.…”

“…Die Reaktion von Toluol mit SO 3 wurde in Lösung experimentell untersucht und theoretisch mit weniger aufwendigen Basissätzen (4‐31G/S*) berechnet 12. Man findet hier eckenkoordinierte π‐ und σ‐Komplexe mit Aren‐SO 3 ‐SO 3 ‐Strukturen, deren gefundene Reaktivität sich theoretisch gut vorhersagen lässt.…”

Aromatic sulfonation with sulfur trioxide: mechanism and kinetic model

“…The mechanism proposed by Cerfontain and coworkers involving a σ-complex intermediate with one SO 3 has been challenged by recent theoretical studies. Morley et al 20 , 21 performed quantum chemical calculations at the Hartree–Fock level on the sulfonation of toluene with SO 3 in gas phase, and concluded that formation of a toluene–SO 3 σ-complex was unlikely due to the high energy change required. Instead, they proposed the initial formation of a toluene···SO 3 ···SO 3 π-complex followed by a toluene–S 2 O 6 σ-complex with almost the same energy, and a cyclic proton rearrangement yielding toluenepyrosulfonic acid.…”