Lesson 22 of 34
First detect a carbonyl, then distinguish its class and methyl-carbonyl unit
No single test provides the complete structure.
A-Level 9476 (2026-2027)
I can detect carbonyl compounds with 2,4-DNPH.Syllabus
Syllabus 9476, 11.7(c). Detect carbonyl compounds with 2,4-DNPH.
I can distinguish aldehydes and ketones using test evidence.Syllabus
Syllabus 9476, 11.7(d). Distinguish aldehydes and ketones using test evidence.
I can infer a methyl-carbonyl unit from iodoform.Syllabus
Syllabus 9476, 11.7(e). Infer a methyl-carbonyl unit from iodoform.
First detect a carbonyl, then distinguish its class and methyl-carbonyl unit
No single test provides the complete structure.
| Test and conditions | Positive observation | What it supports |
|---|---|---|
| 2,4-DNPH reagent | Yellow/orange precipitate of a hydrazone derivative. | Aldehyde or ketone carbonyl; ordinary carboxylic acids, esters and amides do not give the same test. |
| Warm with Tollens reagent | Silver mirror or silver deposit. | An aldehyde among the ordinary aldehyde/ketone comparison set. |
| Warm with Fehling reagent | Blue solution gives brick-red Cu2O precipitate. | An ordinary aliphatic aldehyde; aromatic aldehydes such as benzaldehyde usually do not give this test. |
| Acidified oxidant with suitable warming | Dichromate orange to green or acidified manganate(VII) decolourises. | Aldehydes oxidise readily to acids; ordinary ketones resist mild oxidation. |
| Warm alkaline aqueous iodine | Yellow CHI3 precipitate. | A CH3CO- unit in the carbonyl compound, including ethanal where the other substituent is H. |
In acidic conditions, an aldehyde is oxidised to a carboxylic acid: RCHO + [O] → RCOOH. In the alkaline test reagents, the product is the carboxylate ion. Ketones have no H on the carbonyl carbon, so these mild oxidising agents do not oxidise them. Tollens and Fehling results support each other. They are not tests for every organic compound that can act as a reducing agent.
Worked exampleDistinguish the three named carbonyl examples
Compare ethanal, propanone and phenylethanone using 2,4-DNPH, Tollens and alkaline iodine.
Try it first. Then show the worked stepsHide the worked steps
- All three contain aldehyde/ketone carbonyls, so all give a 2,4-DNPH precipitate.
- Ethanal is the aldehyde and gives the Tollens silver result; the two ketones do not under the ordinary test conditions.
- All three have a CH3CO unit: CH3CHO, CH3COCH3 and C6H5COCH3.
All three are iodoform-positive. A positive iodoform test therefore cannot by itself distinguish an aldehyde from a ketone.
The iodoform reaction of a methyl ketone is RCOCH3 + 3I2 + 4OH- → RCOO- + CHI3 + 3I- + 3H2O. The methyl carbon becomes CHI3, and the rest of the molecule becomes a carboxylate with one carbon fewer. Use this carbon count when you work out an unknown.
Check your understanding
Compound X gives an orange precipitate with 2,4-DNPH, no silver mirror with Tollens' reagent, and a yellow precipitate with warm alkaline iodine. What is X most likely to be?
Show the answer
Propanone, a ketone with a CH3CO- group
2,4-DNPH shows a carbonyl. No silver mirror rules out an aldehyde. The yellow CHI3 precipitate shows a CH3CO- unit.