Hydroxybenzenedicarboxylic acids

Hydroxybenzenedicarboxylic acid derivatives (IV) also are considered to be typical lignin degradation products (17, 24). 

Two of the six possible isomers of hydroxybenzenedicarboxylic acids (4-OH-l,2-di-COOH and 4-OH-l,3-di-COOH) were identified in large amounts in the oxidation products of Samples 1, 2, and 4. These dicar-boxylic acids also are considered to be typical lignin degradation products (17, 24), Individual identification of other isomers has not been completed positively at present because of lack of authentic standards or published GC data. 

It is interesting that a bituminous coal (Sample 4) gave organic acids qualitatively similar to those of lignite coal see Figure Id). Major identified compounds were p-hydroxybenzoic acid and two isomers of hydroxybenzenedicarboxylic acid, benzene di- and tricarboxylic acids. No ortho or meta isomer of hydroxybenzoic acid was detected. We have found that solvent-extractable hydrocarbons obtained from this raw coal consist mainly of n-alkanes (Cjj to 3 ). This is quite different from other results which showed that aromatic hydrocarbons were the major solvent-extractable material of several bituminous and anthracite coals (21). Indeed, petrographic analysis shows that this coal has a high content of sporinite (14.3 wt %) and a low content of vitrinite (30.2 wt %) (33). 

Although one cannot rule out the possibility that plant phenols (3, 22, 35) could be one of the important precursors for coal formation, the present work shows that lignin-like polymers have been incorporated into the macromolecules of coals, and are still identifiable in lower-rank coals. Evidence for this is the identification of p-hydroxy- and 3,4-dihydroxy-benzoic acids, and hydroxybenzenedicarboxylic acids which are known lignin oxidation products. 

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