Identifying an Unknown Carbonyl Compound
Four unlabeled samples W, X, Y and Z were recovered from a reaction mixture known to contain propanal, propanone, ethanoic acid and aniline. The following tests are recorded. W dissolves in dilute HCl and gives a white precipitate with bromine water. X dissolves in aqueous NaHCO3 with effervescence but gives no precipitate with 2,4-dinitrophenylhydrazine. Y gives an orange-red precipitate with 2,4-dinitrophenylhydrazine and a silver mirror with Tollens reagent, but no yellow precipitate with iodine and NaOH. Z gives an orange-red precipitate with 2,4-dinitrophenylhydrazine and a yellow precipitate with iodine and NaOH, but no silver mirror with Tollens reagent. The student must assign each sample, explain the key test, and separate X and Z from a mixture without using distillation.
1. Assign W, X, Y and Z to the four compounds and justify each assignment using the recorded tests.
Read the worked answer
W dissolves in dilute HCl, so it is basic; aniline is the only basic compound among the four. Bromine water gives a white precipitate of 2,4,6-tribromoaniline, confirming W = aniline.
X dissolves in NaHCO3 with effervescence, so it is acidic; X = ethanoic acid. It gives no 2,4-DNP precipitate because it has no carbonyl group.
Y gives a 2,4-DNP precipitate, so it has a carbonyl group. It also gives a silver mirror with Tollens reagent, so it is an aldehyde; Y = propanal. It gives no iodoform because it does not contain a CH3CO group.
Z gives a 2,4-DNP precipitate, so it is a carbonyl compound, but it does not reduce Tollens reagent. It gives a yellow precipitate with iodine and NaOH, the iodoform test, so it is a methyl ketone; Z = propanone.
2. Explain why 2,4-dinitrophenylhydrazine distinguishes Y and Z from W and X, and why Tollens reagent distinguishes Y from Z.
Read the worked answer
2,4-Dinitrophenylhydrazine reacts with the carbonyl group of aldehydes and ketones to form a coloured hydrazone precipitate. Propanal and propanone both have a carbonyl group, so both give the orange-red precipitate. Ethanoic acid has a carboxyl group, not a carbonyl group of the aldehyde/ketone type, and aniline has an amino group; neither gives the hydrazone precipitate under these conditions. Thus 2,4-DNP separates the carbonyl compounds from the acid and the amine.
Tollens reagent is an mild oxidising agent that oxidises aldehydes to carboxylate ions while Ag⁺ is reduced to metallic silver. Propanal is an aldehyde and gives the silver mirror. Propanone is a ketone and is not oxidised under these conditions, so it gives no silver mirror. This distinguishes Y from Z.
3. Propose a sequence to separate ethanoic acid (X) and propanone (Z) from their mixture without distillation. Include the chemistry of each step.
Read the worked answer
Add excess dilute NaHCO3 to the mixture. Ethanoic acid reacts to form sodium acetate, carbon dioxide and water:
CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O.
Propanone does not react with NaHCO3. Then add saturated sodium bisulfite solution. Propanone forms a crystalline bisulfite addition compound, while sodium acetate remains in solution:
(CH3)2CO + NaHSO3 → (CH3)2C(OH)SO3Na.
Filter off the bisulfite adduct. Acidify the filtrate to convert sodium acetate back to ethanoic acid, which can be recovered by extraction. Decompose the bisulfite adduct with dilute acid or sodium carbonate to regenerate propanone, then separate it. This uses chemical differences rather than distillation.