Identifying an Unknown Carbonyl Compound
An unknown compound A (C₄H₈O) gives a positive 2,4-dinitrophenylhydrazine test but does not reduce Tollens' reagent. It forms a yellow precipitate with iodine in alkali. On mild oxidation it gives a carboxylic acid B that is optically inactive. A second compound C, an isomer of A, reduces Tollens' reagent and gives a silver mirror, but does not react with iodine in alkali. Both A and C decolourise bromine water slowly only under UV light.
1. Deduce the structures of A and C, showing how each test result constrains the structure.
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Molecular formula C₄H₈O has one degree of unsaturation, consistent with a carbonyl and no ring or C=C.
A: positive DNPH confirms a carbonyl. Negative Tollens' rules out an aldehyde, so A is a ketone. Yellow precipitate with I₂/NaOH is a positive iodoform test, which requires a CH₃CO– group. The only C₄ ketone with CH₃CO– is butan-2-one, CH₃COCH₂CH₃. Mild oxidation gives a mixture of acids; the stated optically inactive acid is consistent with cleavage products such as acetic acid and propanoic acid, neither chiral.
C: reduces Tollens', so it is an aldehyde. Negative iodoform rules out CH₃CHO, leaving butanal, CH₃CH₂CH₂CHO. Oxidation of butanal gives butanoic acid, which is achiral.
Judgement: the iodoform test distinguishes a methyl ketone from other ketones, and Tollens' distinguishes aldehyde from ketone; both are needed.
2. Explain why neither A nor C decolourises bromine water rapidly in the dark, and what this tells you about the unsaturation.
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Bromine water decolourisation in the dark is a test for C=C or C≡C, proceeding by electrophilic addition. A and C contain no carbon–carbon multiple bond: their single degree of unsaturation is the carbonyl group.
The carbonyl does react with bromine, but by a different mechanism: acid- or base-catalysed α-halogenation via the enol form. This requires catalysis and is slow in the dark, and it consumes bromine without the rapid disappearance seen for alkenes.
Under UV light, radical substitution at the α or alkyl positions can occur, which explains the slow decolourisation noted.
Judgement: a slow or light-dependent bromine result is not evidence of an alkene; the rate and conditions of decolourisation matter as much as the colour change.
3. Predict what happens if A is treated with excess bromine water in the presence of acid, and how this could be used to confirm the structure.
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Acid-catalysed α-bromination of butan-2-one gives substitution at the CH₂ group adjacent to the carbonyl, since enolisation occurs on that side: CH₃COCH₂CH₃ → CH₃COCHBrCH₃, and with excess bromine further substitution can occur.
The monobromo product CH₃COCHBrCH₃ has a chiral centre at the carbon bearing Br, so it would be formed as a racemic mixture, which is optically inactive overall.
Confirmation: the number of bromine atoms incorporated and the position of substitution can be checked by the product's properties, and the reaction rate is acid-dependent, unlike alkene addition.
Judgement: α-halogenation is regioselective for the more substituted enol and is a mechanistic probe, not a simple unsaturation test; interpreting the product requires enol chemistry rather than addition chemistry.