Dimethylbenzene sulphonic acid

An early study reported the use of barium peroxide for the preparation of 3,4-dimethylbenzene sulphonic acid from the sulphinic acid276. However, the synthetic utility of this reaction has not been reported to date. 

Aerial oxidation of this affords anthraquinone in 37% yield. Both 1-naphthyl and 2,4-dimethylbenzene sulphonates follow the same path but yield only traces of the corresponding quinone. In the last case, 2,4-dimethylbenzenesulphonate, the quinone is accompanied by 0.8% of 2,4-dimethylphenol. The second path involves loss of sulphur trioxide to yield aryl radicals which afford the products, the arene and/or the biaryl, shown in Scheme 17212,213. Other studies have shown that anthraquinone-1-sulphonic acid (256)214,215 and anthraquinone-2-sulphonate216 are also photochemically labile. A study of the photochemical reactivity of azulene sulphonic acids has also been reported217. Photochromism has been studied with respect to the stilbene derivative 257218. 

In 195932, the heats of sulphonation of the isomeric ethylbenzene and the three dimethylbenzenes (xylenes) were reported by Leitman and Pevzner. After correcting for heats of solvation and dilution, these authors found the following exothermicities for ethylbenzene (37), 5.1 0.2 o-xylene (38a), 5.5 0.2 m-xylene (38b), 3.9 0.1 p-xylene (38c), 4.1 0.2 kcal mol-1. No product analysis was reported in this study so that we cannot ascertain the site(s) of sulphonation or even the possibility of rearrangement and/or transalkylation reactions. However, that 37,38a-38c have comparable heats of formation [AHf(lq) = — 2.9, — 5.8, — 6.1, — 5.8 kcal mol - A], and that their heats of sulphonation are comparable, suggests that all of these results are consistent with each other. The heat of formation of any of the solid isomeric ethyl- or dimethylbenzenesulphonic acids are thus ca — 136 kcal mol-1 with an anticipated few kcal mol-1 spread of values33. 

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