Sulfonation by Halosulfonic Acids

Less mechanistic studies have been carried out with chlorosulfonic acid than with sulfuric acid and generally the precise nature of the electrophilic species involved remains uncertain and appears to vary with the nature of the substrate and the reaction conditions. Experimental data and thermodynmnic studies indicated that when an aromatic substrate reacts wifii an equimolar quantity of chlorosulfonic acid, the first step )delds the sulfonic acid (Equation 21). In the presence of an excess of the reagent, the initially formed sulfonic acid is slowly converted into the sulfonyl chloride with liberation of sulfuric acid (Equation 22). 

The first stage may be driven to completion by removal of the hydrogen chloride gas and the progress of the reaction can be monitored by measurement of the volume of gas evolved. Kinetic studies demonstrated that the rate of the second stage (Equation 22) was slower than the observed overall rate of formation of the sulfonyl chloride and therefore the earlier proposed mechanism shown by Equations 21 and 22 must be incorrect.  

Studies of the reaction of benzene with chlorosulfonic acid (one molar equivalent) showed that the major product was benzenesulfonic acid (Equation 21) with a little diphenyl sulfone. When an excess of chlorosulfonic acid was used benzenesulfonyl chloride was obtained (Equation 22). Spryskov and Kuz mina demonstrated the reversibility of Equation 22 and measured the equilibrium constants for several different aromatic substrates. In the benzene-chlorosulfonic acid reaction, the quantity of diphenyl sulfone produced was increased by addition of anhydrous benzenesulfonic acid, but not by benzenesulfonyl chloride. The sulfone therefore apparently derived from reaction of benzenesulfonic acid and benzene under the influence of chlorosulfonic acid. Sulfone formation appeared to be relatively favoured at low temperatures and this may be due to the formation of an intermediate pyrosulfuric acid (Equations 23,24). 

The sulfonating power of pyrosulfuryl chloride or chlorosulfonic anhydride (CI2S2O5) is less than that of chlorosulfonic acid however, addition of the anhydride to the reagent enhances the yield of the arylsulfonyl chloride. The anhydride probably acts by converting the sulfonic acid and sulfuric acid into the sulfonyl chloride and chlorosulfonic acid respectively (Equations 26, 27). 

The relative rates of sulfonation of sulfonyl chlorides, sulfonic acids and sulfonate anions with chlorosulfonic acid follow the order  

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