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Which of the following compounds give a positive test with Fehling's solution? (I) CH3CHO (II) CH3COCH3 (III) C6H5CHO (IV) A six-membered cyclic hemiacetal (glucopyranose-type) with a free anomeric OH group (V) A six-membered cyclic acetal with an adjacent OCH3 group

1. (I) and (IV)
2. (I), (III) and (IV)
3. (II) and (V)
4. (III) and (V)

Correct answer: (I) and (IV)

Solution

Fehling's solution (Cu²+ complex) is reduced to Cu2O (brick-red precipitate) by aldehydes that can donate electrons, specifically aliphatic aldehydes and hemiacetals (reducing sugars). CH3CHO (I) is an aliphatic aldehyde — positive. CH3COCH3 (II) is a ketone — negative. C6H5CHO (III) is an aromatic aldehyde — negative (cannot reduce Fehling's). Compound (IV) is a cyclic hemiacetal (like glucose in ring form): it equilibrates with the open-chain aldehyde form, giving a positive Fehling test. Compound (V) is a cyclic acetal (with OCH3, fully protected): it cannot open to give a free aldehyde, so negative.

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