Other by-products in the nitration of toluene

Apart from the unsymmetrical isomers of trinitrotoluene, other by-products are formed in the nitration process, owing to oxidation or decomposition processes. 

Oxidation leads to the formation of di- and tri-nitrocresols, impurities frequently met with in TNT. Nitrocresols are mainly formed during the nitration of toluene to mononitrotoluene (Noelting and Forel [11]). If they have not been removed from mononitrotoluene by washing with alkalis, they undergo either further nitration to trinitrocresol, or oxidation to oxalic acid. Trinitrocresol may be removed from TNT by mild alkali washing (e.g. with a NaHC03 or a sodium sulphite solution). 

During the trinitration the methyl group is oxidized to the carboxyl group. The trinitrobenzoic acids behave in different ways in the course of hot washing of crude TNT. Thus 2,4,6-trinitrobenzoic acid is readily decarboxylated and 

It is probable that 2,4-dinitro-3-hydroxybenzoic acid (II) can also be formed in the similar way  

Also hydroxy-2,4,6-trinitrobenzoic acid (III) can be present among the other by-products in the formation of trinitrotoluene. 

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