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Phthalaldehyde (benzene-1,2-dicarbaldehyde) is treated with concentrated KOH solution. Identify the organic product formed.

1. A lactone (cyclic ester) fused with the benzene ring
2. A benzene ring bearing one -CH2OH group and one -COO^-K⁺ group at the 1,2-positions
3. Benzene-1,2-dimethanol (both CHO groups reduced to CH2OH)
4. Dipotassium benzene-1,2-dicarboxylate (both CHO groups oxidised to COO^-K⁺)

Correct answer: A benzene ring bearing one -CH2OH group and one -COO^-K⁺ group at the 1,2-positions

Solution

Phthalaldehyde has no alpha-H atoms, so both aldehyde groups undergo the Cannizzaro reaction with KOH. In the intramolecular version, one -CHO is oxidised to -COO^-K⁺ and the other is reduced to -CH2OH, giving potassium 2-(hydroxymethyl)benzoate.

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