The chemical behaviour of the sulfonic acids of pentamethylbenzene (PMB) and the three tetramethylbenzenes (TeMB's( in concentrated aqueous sulphuric acid has been studied quantitatively using 1H AND 13C NMR and UV spectroscopy.PMB-sulfonic acid yields (protonated) hexamethylbenzene (HMB) and the 2,3,4,5- and 2,3,4,6-TeMB-1-sulfonic acids in an initial ratio of 9/1/8 up to a substrate conversion of 36percent; the ratio of HMB to the TeMB-1-sulfonic acids remains 1/1 and the only subsequent reaction is the relatively slow conversion of the 2,3,4,6- into the 2,3,4,5-TeMB-1-sulfonic acid which is the only stable TeMB-1-sulfonic acid isomer in concentrated sulfuric acid. 2,3,5,6-TeMB-1-sulfonic acid yields, as initial products, 2,3,5- and 2,4,5-trimethylbenzene(TrMB)-1-sulfonic acid together with PMB-1-sulfonic acid in a ratio of ca. 1/3/4.The subsequent products are the 2,3,4,6- and 2,3,4,5-TeMB-1-sulfonic acids, of which the former slowly isomerizes to the latter, together with some HMB. 2,3,4,6-TeMB-1-sulfonic acid yields the stable 2,3,4,5-isomer.It is proposed that the conversion proceeds via inter-molecular trans-methylations involving 1,3,5-TrMB and PMB as intermediates, the concentrations of which, however, are below the limits of NMR detection.Based on (i) the desulfonation-sulfonation equilibrium data of the TrMB-, TeMB- and PMB-sulfonic acids, (ii) the strong rate decrease for the conversion of both 2,3,4,6- and 2,3,5,6-TeMB-1-sulfonic acid in 98.4percent H2SO4 in the presence of added cyclohexane and (iii) the very strong electron-withdrawing and meta-directing effect of the SO3-/SO3H substituent, it is proposed that all the conversions proceed inter-molecularly, that the trans-methylating reagent is the polymethylbenzene (and not the corresponding sulfonate) and that the substrate undergoing methylation is most likely to be once again the polymethylbenzene rather than its sulfonate ion.
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