Which of the following compounds produces a yellow crystalline precipitate when treated with iodine and aqueous sodium hydroxide (positive iodoform test)?
Answer and explanation
Correct answer: B. Acetophenone
- Step 1: The iodoform test is specific for compounds containing a methyl carbonyl group () or a methyl carbinol group ().
- Step 2: In acetophenone (), the carbonyl group is directly adjacent to a methyl group.
- Step 3: In alkaline iodine, the -methyl hydrogens are progressively replaced by iodine to form a tri-iodomethyl ketone (), which undergoes nucleophilic hydroxide cleavage to yield sodium benzoate and yellow iodoform precipitate ().
Answer
Acetophenone ().
Why not the others: Benzophenone has two phenyl rings and lacks a methyl group; Pentan-3-one has ethyl groups on both sides with -methylene carbons rather than methyl; Benzaldehyde has only a single aldehydic hydrogen and no methyl group.
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