The sulfonation of chlorobenzene, chlorobenzene-benzene, and toluene-benzene mixtures by sulfur trioxide in liquid sulfur dioxide at -12.5° C. was studied to obtain isomer distribution and relative rate data. The isomer distribution for the reaction on chlorobenzene was determined by an isotope dillution technique. Relative rate experiments were conducted by sulfonating C14 labeled benzene in competition with nonradioactive chlorobenzene and toluene and the count rate of the products was used to determine product composition. Ultraviolet spectrophotometry was also used to investigate variations in isomer distributions as a function of sulfur trioxide concentration. The apparent relative rates of sulfonation (kX/kB) as calculated by Ingold's equation, vary with both the initial ratio of arenes and the concentration of sulfur trioxide. The ortho/para ratio decreases with increasing sulfur trioxide concentration. These observations are explained in terms of secondary sulfonation reactions which produce significant amounts of product sulfonic acids by reaction of the arenes with reagents other than sulfur trioxide. The secondary sulfonating agents are presumed to be sulfonic acid anhydrides and/or pyrosulfonic acids. The secondary reactions contribute proportionately more to the total product of that species formed in the smaller amounts by the primary reaction. Criteria for the applicability of Ingold's equation are discussed in connection with the possibility of secondary reactions. Relative rate constants for competitive chlorobenzene:benzene and toluene:benzene sulfonation with sulfur trioxide, corrected for secondary reaction effects, are 0.087±0.002 and 27.0±1.0 respectively. The observed isomer distribution for the sulfonation of chlorobenzene is o = 0.95 ± 0.03%, m = 0.09 ± 0.02%, and p = 98.96 ± 0.12%. Partial rate factors calculated for the sulfonation of chlorobenzene and toluene are pf = 0.517, of = 0.0025, mf = 0.00024 and pf = 144.6, of = 81.2, and mf = 0.59 respectively. Neither sulfonation fits the selectivity relationship predicted for it, although correction for secondary reaction effects greatly improves the fit for the toluene data. The deviation of the toluene data from the selectivity relationship probably results from a low percentage of meta isomer in the reported isomer distribution for toluene.



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



Date Submitted


Document Type





Sulfonation, Chlorobenzene, Toluene



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Chemistry Commons