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A cyclic compound containing a benzene ring with six carboxylic acid (-COOH) groups attached to all six carbon atoms (mellitic acid, C6(COOH)6) is heated strongly. Each pair of adjacent -COOH groups undergoes decarboxylation (loses CO2) to give a carboxylic anhydride unit. How many moles of CO2 are evolved per mole of mellitic acid on strong heating?

1. 3
2. 4
3. 5
4. 6

Correct answer: 3

Solution

Mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) on strong heating undergoes decarboxylation. Each pair of adjacent COOH groups loses CO2 and condenses to form a cyclic anhydride linkage (-CO-O-CO-). There are 3 such adjacent pairs in mellitic acid, so 3 moles of CO2 are evolved per mole of substrate, giving mellitic trianhydride. Note: the original ASCII structure in the source was garbled; this reconstruction is based on the standard JEE mellitic acid decarboxylation problem.

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