Skip to main content
WalesChemistrySyllabus dot point

How do carbonyl compounds react and how do we tell them apart?

The carbonyl group, nucleophilic addition, reduction and oxidation of aldehydes and ketones, and tests to identify and distinguish them.

A focused answer to WJEC A-Level Chemistry Unit 4, covering the carbonyl group, nucleophilic addition of cyanide and reduction, the oxidation of aldehydes, and the chemical tests that identify carbonyls and distinguish aldehydes from ketones.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

WJEC wants you to describe the carbonyl group, the nucleophilic addition mechanism, the reduction of carbonyls and oxidation of aldehydes, and the chemical tests that identify carbonyls and tell aldehydes from ketones.

The answer

The carbonyl group

Nucleophilic addition

Tests for carbonyls

Why aldehydes oxidise but ketones resist

The difference between the two carbonyls comes down to the hydrogen atom on the carbonyl carbon. In an aldehyde, that carbon carries a hydrogen, so a mild oxidant can remove it and form a carboxylic acid: RCHO+[O]→RCOOH\text{RCHO} + [\text{O}] \rightarrow \text{RCOOH}. In a ketone the carbonyl carbon is bonded to two other carbon atoms and has no such hydrogen, so oxidation would require breaking a strong C−C\text{C}-\text{C} bond, which mild reagents cannot do. This single structural fact explains why Tollens' and Fehling's, both gentle oxidising agents, react only with aldehydes, and it is the reasoning markers look for when a question asks you to justify the test result rather than just state it.

Summary of the carbonyl tests

Reagent Aldehyde Ketone
2,4-DNP (Brady's) Orange precipitate Orange precipitate
Tollens' (silver mirror) Silver mirror No change
Fehling's solution Brick-red precipitate No change
Acidified dichromate Orange to green No change

Read together, these tests first confirm a carbonyl is present, then separate aldehyde from ketone, then (with dichromate) confirm the aldehyde is oxidisable to an acid.

Examples in context

Identifying carbonyls in synthesis. Brady's reagent gives a precipitate whose melting point identifies the exact carbonyl, a classic structure-determination tool before spectroscopy was routine. Blood-sugar testing. Fehling's and Benedict's solutions detect aldehyde (reducing) sugars such as glucose, the historical basis of urine tests for diabetes. Hydroxynitriles in synthesis. The cyanide addition that lengthens a chain by one carbon is a key step in building larger molecules and introduces a chiral centre, linking this topic directly to the stereoisomerism that follows.

Try this

Q1. Name the type of mechanism by which HCN\text{HCN} reacts with propanone. [1 mark]

  • Cue. Nucleophilic addition.

Q2. State the observation when an aldehyde is warmed with Fehling's solution. [1 mark]

  • Cue. A brick-red precipitate forms.

Q3. Name the product when butanone is reduced by NaBH4\text{NaBH}_4. [1 mark]

  • Cue. Butan-2-ol (a secondary alcohol).

Q4. Explain, in terms of structure, why ethanal can be oxidised to an acid but propanone cannot. [2 marks]

  • Cue. Ethanal has a hydrogen on the carbonyl carbon that can be replaced by OH\text{OH}; propanone's carbonyl carbon is bonded only to carbons, so oxidation would need to break a strong C−C\text{C}-\text{C} bond.

Q5. Name the product of the reaction between ethanal and HCN\text{HCN}. [1 mark]

  • Cue. 2-hydroxypropanenitrile.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20194 marksDescribe the mechanism of the reaction between propanal and hydrogen cyanide, naming the type of mechanism and the role of the cyanide ion.
Show worked answer →

The mechanism is nucleophilic addition. The carbonyl carbon is δ+\delta+ because oxygen is more electronegative, so it attracts a nucleophile.

The cyanide ion CN−\text{CN}^- uses its lone pair to attack the carbonyl carbon; a curly arrow goes from CN−\text{CN}^- to the carbon and another from the C=O\text{C}=\text{O} bond to the oxygen, forming an alkoxide.

The alkoxide is then protonated (by HCN or water) to give the product 2-hydroxybutanenitrile.

Markers reward naming nucleophilic addition, attack by CN−\text{CN}^- on the δ+\delta+ carbon, formation and protonation of the alkoxide, and the hydroxynitrile product.

WJEC 20213 marksDescribe a chemical test, with reagent and observation, to distinguish between propanal and propanone.
Show worked answer →

Warm each with Tollens' reagent (ammoniacal silver nitrate).

Propanal, an aldehyde, reduces the silver(I) ions to silver, depositing a silver mirror on the tube.

Propanone, a ketone, does not react, so no silver mirror forms.

Markers reward Tollens' reagent, the silver mirror with the aldehyde, and no reaction with the ketone (Fehling's solution, brick-red precipitate, is an equally valid answer).

Related dot points

Sources & how we know this