Benzenepentacarboxylate

Neither benzenepentacarboxylic acid nor mellitic acid are manufactured commercially, but synthetic mellitic acid can be purchased as a laboratory chemical (99). Both can be synthesized by oxidizing the corresponding methylbenzenes or other substituted benzenes, and both are present in trace amounts after oxidation of coal or coal-like substances. 

Products from the direct oxidation of lignin that give information about its structure include acetic, oxalic, and succinic acids, vanillin, vanillic acid, and dehydrodivanillin. Regarding lignin constitution, they are of only minor interest, but the yields in which they are obtained are significant. For vanillin and its derivatives the total yield is about 33% of spruce lignin (24, 36, 37, 38). Two extraordinary products of this direct oxidation deserve special attention—i.e., benzenepentacarboxylic (45) and tricarballylic acids (47), (XXIX) and (XXX). They will be mentioned later in connection with lignenolide (XXVII). 

Figure 14. Typical gas chromatogram of oxidation product. Key a,by solvent c, benzenedicarboxylic acid d, benzenemonohydroxydicarboxylic acid e, benzenetricarboxylic acid /, unknown g, benzenetetracarboxylic acid h, unknown i, benzenepentacarboxylic acid benzenehexacarbox-...

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