Aromatic sulfonation. Part 24†: Kinetics and mechanism of the sulfonation of o- and m-xylene in aqueous sulfuric acid‡

The sulfonation of o- and m-xylene in concentrated aqueous sulfuric acid has been investigated. Rates and isomer distributions have been measured. The results are discussed in terms of two sulfonating mechanisms. The isomer distributions at 25.0° for the mechanism with H2S2O7 as sulfonating entity are 45.1 ± 0.4% 3- and 54.9 ± 0.4% 4-substitution for o-xylene, and 14.5 ± 0.6% 2-, 84.3 ± 0.6% 4- and 1.2 ± 0.2% 5-substitution for m-xylene. For the mechanism with H3SO4+ as sulfonating entity these data are 6.5 ± 0.9%, 3- and 93.5 ± 0.9% 4-substitution with o-xylene, and 0.5 ± 0.2% 2-, 98.9 ± 0.4% 4- and 0.6 ± 0.2% 5-substitution with m-xylene. The smaller degree of 3-substitution with o-xylene for the H2S2O7 - as compared with the H3SO4+ - mechanism is mainly due to a difference in entropy of activation. The former mechanism encounters more steric hindrance than the latter on substitution ortho to a methyl group. The experimental partial rate factors for the three isomeric xylenes for the two sulfonation mechanisms differ by less than a factor of 4 from those calculated on the basis of additivity of substituent effects with the exception of the 2-position of m-xylene, for sulfonation by the entity H3SO4+, and of the 2- and 4-positions of m-xylene for sulfonation by the entity H2S2O7.