6.2.2Organic Chemistry and Analysis
Testing for Carbonyl Compounds
5 min read 4 parts Lesson 8 of 33 Virtual lab
Part 1 of 4Detecting the C=O Group
A two-stage approach first confirms a carbonyl group and then identifies which one it is.
2,4-DNP reagent
- A solution of 2,4-dinitrophenylhydrazine gives an orange or yellow precipitate with any aldehyde or ketone.
Reaction type
- The reagent undergoes a condensation reaction, losing water, to form a 2,4-dinitrophenylhydrazone derivative.
Limitation
- The test shows only that a carbonyl is present; it does not separate aldehydes from ketones.
Not positive
- Carboxylic acids and esters contain C=O but do not give a precipitate with this reagent.
Safety
- 2,4-DNP solution is toxic and stains skin, so it is handled with gloves and eye protection.
Part 2 of 4Identifying the Compound
The solid hydrazone can be used to name the original carbonyl.
1
Add the unknown to 2,4-DNP solution and filter off the coloured precipitate that forms.
2
Purify the crystals by recrystallisation so that impurities do not lower or broaden the melting point.
3
Measure the melting point of the dry crystals using a melting point apparatus.
4
Compare the value with a data table of derivatives to identify the carbonyl compound.
5
A sharp value close to the data book figure indicates a pure sample and a reliable identification.
6
Each carbonyl compound forms a hydrazone with its own characteristic melting point, so the value acts like a fingerprint for the original compound.
Part 3 of 4Distinguishing Aldehydes from Ketones
Distinguishing Aldehydes from Ketones
Tollens' reagent
- This is ammoniacal silver nitrate, [Ag(NH₃)₂]⁺, which is warmed gently with the unknown in a clean test tube.
Aldehyde result
- The aldehyde is oxidised to a carboxylate, and Ag⁺ is reduced to silver, giving a silver mirror on the glass.
Ketone result
- Ketones cannot be oxidised easily, so the solution stays colourless and no mirror forms.
Alternative
- Warming with acidified K₂Cr₂O₇ turns orange to green for an aldehyde but gives no change with a ketone.
Equation
- CH₃CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → CH₃COO⁻ + 2Ag + 4NH₃ + 2H₂O shows silver ions being reduced as the aldehyde is oxidised.
| Test | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNP | Orange precipitate | Orange precipitate |
| Tollens' reagent | Silver mirror | No change |
| Acidified K₂Cr₂O₇ | Orange to green | No change |
Part 4 of 4Exam Checklist & Tips
6.2.2 – Key Points for the Exam
- 2,4-DNP 2,4-dinitrophenylhydrazine, the reagent that forms a coloured precipitate with carbonyls.
- Tollens' reagent Ammoniacal silver nitrate used to detect aldehydes.
- Recrystallisation Purifying a solid by dissolving in hot solvent and cooling to form crystals.
- Derivative A new compound made from the unknown to help identify it.
Common Mistakes
- Claiming a positive 2,4-DNP test proves an aldehyde; ketones also give a precipitate.
- Writing that the silver mirror is formed by a ketone, or that Ag⁺ is oxidised.
- Skipping recrystallisation, so the melting point is low and the identification unreliable.
- Forgetting that carboxylic acids and esters do not react with 2,4-DNP.