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A teaching laboratory tests ten compounds with four reagents: 2,4-dinitrophenylhydrazine, Tollens'…
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A teaching laboratory tests ten compounds with four reagents: 2,4-dinitrophenylhydrazine, Tollens' reagent, Fehling's solution and iodine with sodium hydroxide. The compounds are methanal, ethanal, propanone, pentan-3-one, benzaldehyde, acetophenone, ethanol, propan-2-ol, propan-1-ol and phenol. Methanal and ethanal give a red precipitate with Fehling's solution, but benzaldehyde does not. Assume that each test is positive wherever the lesson says it is. Count five things. First, the compounds that give the iodoform test. Second, the ketones among them. Third, the compounds that give Tollens' but not Fehling's. Fourth, the compounds with exactly one positive test. Fifth, the compounds with no positive test.
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Step 1. Iodoform. Ethanal, propanone, acetophenone, ethanol and propan-2-ol: 5.
Step 2. Ketones with iodoform. Propanone and acetophenone: 2. Pentan-3-one has no methyl group on the carbonyl carbon.
Step 3. Tollens' but not Fehling's. Methanal and ethanal give both. Only benzaldehyde gives the silver mirror alone: 1.
Step 4. Exactly one positive test. Pentan-3-one (2,4-DNP only), ethanol (iodoform only) and propan-2-ol (iodoform only): 3.
Step 5. No positive test. Propan-1-ol and phenol: 2.
Answer: 5, 2, 1, 3 and 2.
Check. Positive tests per compound: 3, 4, 2, 1, 2, 2, 1, 1, 0, 0. The total is 16, the same as from the columns.
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