Four Unlabelled Organic Bottles: Identification and Waste Handling
A student is given four unlabelled bottles from a project shelf: benzaldehyde, acetophenone, benzoic acid and phenol. The supervisor provides only the following test results, obtained under standard conditions. Sample W dissolves in aqueous NaHCO3 with brisk effervescence. Sample X gives a violet colour with neutral FeCl3 solution. Sample Y gives an orange-yellow precipitate with 2,4-dinitrophenylhydrazine, but no silver mirror with Tollens' reagent even after warming. Sample Z gives a silver mirror with Tollens' reagent and also gives an orange-yellow precipitate with 2,4-dinitrophenylhydrazine. All four are liquids or low-melting solids and are soluble in ether. The student must identify each sample, choose one further test that would distinguish Y from Z if the 2,4-DNP result were ambiguous, and propose a safe way to handle the Tollens' waste. The laboratory has dilute NaOH, iodine in KI, and ammoniacal AgNO3 available. The student should not rely on smell alone.
1. Identify W, X, Y and Z. Explain the diagnostic feature of each positive test.
Read the worked answer
W is benzoic acid. Effervescence with NaHCO3 indicates a carboxylic acid reacting to release CO2; benzoic acid is the only carboxylic acid in the list.
X is phenol. A violet colour with neutral FeCl3 is characteristic of a phenolic -OH group able to form a coloured iron-phenoxide complex.
Y is acetophenone. The orange-yellow precipitate with 2,4-DNP shows a carbonyl compound, but the negative Tollens test shows it is not an aldehyde; acetophenone is the ketone in the list.
Z is benzaldehyde. It gives both the 2,4-DNP precipitate and the silver mirror, so it is an aldehyde.
The tests are consistent because 2,4-DNP detects aldehydes and ketones, while Tollens' reagent distinguishes aldehydes from ketones under the stated conditions.
2. Describe one further test using the available reagents that would distinguish Y from Z if both gave similar 2,4-DNP precipitates. Give the expected observation for each.
Read the worked answer
Use the iodoform test with iodine in KI and dilute NaOH. Warm a small portion of each sample with I2/KI and NaOH.
Y, acetophenone, contains the CH3CO- group and gives a yellow precipitate of iodoform, CHI3, together with the characteristic smell.
Z, benzaldehyde, does not contain a CH3CO- group and does not give iodoform under these conditions; it may be oxidised to benzoate, but no yellow CHI3 precipitate is expected.
The test is useful because it depends on a methyl ketone structural feature, not merely on the presence of a carbonyl group. The result should be confirmed alongside the Tollens test rather than interpreted in isolation.
3. Explain why Tollens' reagent should not be stored after use and describe a safe procedure for dealing with the silver-containing waste.
Read the worked answer
Tollens' reagent contains the diamminesilver(I) complex, [Ag(NH3)2]^+. On standing, especially if the ammonia evaporates or the solution becomes concentrated, silver can deposit and potentially form explosive fulminating silver or silver nitride. The reagent should therefore be prepared fresh and used promptly.
After use, do not pour it down the sink. Acidify the waste carefully with dilute nitric acid in a fume cupboard to destroy the ammoniacal silver complex, then add chloride ion to precipitate silver chloride. Filter the AgCl, place it in a labelled heavy-metal waste container, and dispose of it according to local regulations. Neutralise and handle the filtrate appropriately. The key safety points are to avoid storing the reagent and to convert soluble silver to an insoluble, labelled waste.