Identifying Propanal, Propanone and Ethanoic Acid with Limited Tests
Three reagent bottles in a school laboratory have lost their labels. The teacher says the contents are propanal, propanone, and ethanoic acid, one in each bottle. On the available bench are 2,4-dinitrophenylhydrazine (2,4-DNP) solution, Tollens’ reagent made freshly from AgNO3 and dilute ammonia, Fehling’s solution, solid sodium bicarbonate, and iodine in sodium hydroxide. Each sample may be subjected to at most two tests, because the stocks are small. One student proposes adding 2,4-DNP to all three and identifying the two carbonyls by the precipitates, then using Fehling’s to separate propanal from propanone. Another student warns that 2,4-DNP does not distinguish the carbonyls and that Fehling’s results can be spoilt by a dirty test tube. The teacher asks for a reliable, minimal decision sequence with observations, and for the chemistry behind each observation.
1. Design a two-test-per-sample scheme that identifies all three compounds. Give the observation and the functional group responsible in each case.
Read the worked answer
A reliable sequence is:
Test 1 - add a small spatula of NaHCO3 to each sample.
- The bottle that effervesces gives CO2 and is ethanoic acid. The responsible group is the carboxylic acid –COOH; it is acidic enough to liberate CO2 from hydrogen carbonate.
- The other two bottles show no effervescence. They are the carbonyl compounds, propanal and propanone.
Test 2 - to the two non-effervescing samples, add freshly prepared Tollens’ reagent and warm gently in a clean water bath.
- The sample that gives a silver mirror is propanal. The aldehyde –CHO group is oxidised to carboxylate while Ag+ is reduced to Ag.
- The sample that gives no silver mirror is propanone. The ketone carbonyl is not oxidised by Tollens’ reagent under these conditions.
This uses one test on all three samples and a second test only on the two that did not react with bicarbonate, so no sample exceeds two tests. 2,4-DNP is not needed. If Tollens’ is unavailable or unreliable, Fehling’s solution can replace it: propanal gives a brick-red Cu2O precipitate on warming, propanone does not. Tollens’ is preferred because it is more specific and gives a clearer visual result.
2. Explain why 2,4-DNP gives a precipitate with propanal and propanone but not with ethanoic acid, and why it cannot distinguish the two carbonyls.
Read the worked answer
2,4-DNP reacts with the carbonyl group of aldehydes and ketones in an acid-catalysed condensation. The carbonyl oxygen is protonated, the amine attacks the carbonyl carbon, and water is eliminated to form a yellow-orange 2,4-dinitrophenylhydrazone:
R2C=O + H2N–NH–C6H3(NO2)2 → R2C=N–NH–C6H3(NO2)2 + H2O.
The product is sparingly soluble and appears as a yellow, orange, or red precipitate. Both propanal (an aldehyde) and propanone (a ketone) have a reactive carbonyl group, so both give a precipitate.
Ethanoic acid does not give this precipitate. Its carbonyl group is part of the carboxyl function and is delocalised with the –OH oxygen; nucleophilic attack at that carbon is much less favourable, and the acid functional group does not form a stable 2,4-dinitrophenylhydrazone under these conditions. Ethanoic acid therefore gives no 2,4-DNP precipitate.
2,4-DNP cannot distinguish propanal from propanone because both contain the same reactive C=O unit and both form analogous hydrazones. The test identifies the presence of a carbonyl group but not whether it is aldehydic or ketonic. A second test that depends on oxidation of –CHO, such as Tollens’ or Fehling’s, is required.
3. A student heats propanone with Fehling’s solution for ten minutes and reports a brick-red precipitate, concluding that the sample is propanal. Give two plausible explanations for this false positive and state how Tollens’ reagent would settle the question.
Read the worked answer
Fehling’s solution contains Cu2+ in alkaline tartrate. Aldehydes reduce it to brick-red Cu2O on warming; ketones such as propanone should not do so under normal conditions. A brick-red result from a sample believed to be propanone therefore points to a problem in the test rather than to the ketone itself.
Two plausible explanations:
- Contamination of the test tube or reagent. A tube previously used for an aldehyde may retain traces of aldehyde or reduced copper, or the Fehling’s bottle may have been contaminated with an aldehyde. Even a small amount of aldehyde can give a visible red precipitate.
- The sample is not pure propanone. The bottle may contain propanal as an impurity, or the samples may have been mislabelled. Propanone itself is not oxidised by Fehling’s, so a genuine red precipitate indicates a reducing impurity.
A clean Tollens’ test is more decisive. Propanal gives a silver mirror; pure propanone gives no mirror even on warming. The tube must be cleaned with hot nitric acid or dichromate/sulfuric acid, rinsed with distilled water, and the Tollens’ reagent freshly prepared. If a mirror still appears with the suspected propanone, contamination or mislabelling is confirmed. If no mirror appears, the Fehling’s result was a false positive.