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Citric acid digested at a temperature below 40�C. with concentrated sulphuric acid gives off carbon monoxide and forms acetone dicarboxylic acid.

Thus ortho-phenylene diamine yields the following products:— In some cases oxidation of condensed benzenoid-heterocyclic nuclei results in the rupture of the heterocyclic ring with the formation of a benzene dicarboxylic acid; but if the aromatic nucleus be weakened by the introduction of an amino group, then it is the benzenoid nucleus which is destroyed and a dicarboxylic acid of the heterocyclic ring system obtained.