Aldehydes and ketones: properties, oxidation and reduction

Naming and structure (spec 15.6)

• Aldehydes: C=O at the end of the chain, -CHO, suffix -al: ethanal CH3CHO, propanal CH3CH2CHO.
• Ketones: C=O within the chain, suffix -one with a number where needed: propanone CH3COCH3, butanone CH3COCH2CH3, pentan-2-one.

Physical properties (spec 15.7)

C=O is polar, so there are permanent dipole-dipole forces as well as London forces. There is no O-H, so carbonyl molecules cannot hydrogen bond with each other, and their boiling points are lower than those of alcohols of similar Mr. They can accept hydrogen bonds from water (through the lone pairs on O), so small aldehydes and ketones are soluble in water.

Oxidation (spec 15.8 i)

Aldehydes are oxidised to carboxylic acids; ketones are not oxidised by these reagents.

• Fehling's or Benedict's solution, warm: blue solution gives a red-brown (brick-red) precipitate of copper(I) oxide with aliphatic aldehydes.
• Tollens' reagent (ammoniacal silver nitrate), warm: a silver mirror forms with aldehydes.
• Acidified potassium dichromate(VI), warm: orange to green with aldehydes.

CH3CHO + [O] -> CH3COOH

Reduction (spec 15.8 ii)

Lithium tetrahydridoaluminate(III), LiAlH4, in dry ether (ethoxyethane), then dilute acid:

• aldehyde -> primary alcohol: CH3CH2CHO + 2[H] -> CH3CH2CH2OH
• ketone -> secondary alcohol: CH3COCH3 + 2[H] -> CH3CH(OH)CH3

The ether must be dry because LiAlH4 reacts violently with water.

Worked example

Describe a test to distinguish propanal from propanone, with results.

1. Warm each with Tollens' reagent (or Fehling's solution).
2. Propanal is an aldehyde and is oxidised; propanone is a ketone and is not.

Answer: Propanal gives a silver mirror (or red-brown precipitate with Fehling's); propanone gives no change.

Check yourself

1. What is seen when an aldehyde is warmed with Tollens' reagent?
   • A red-brown precipitate forms
   • The solution turns orange
   • A silver mirror forms
   • A yellow precipitate forms
   Show answer

   A silver mirror forms. Ag+ is reduced to silver metal.

2. What is the positive result for Fehling's solution?
   • Blue solution forms a red-brown precipitate
   • Orange solution turns green
   • Colourless solution forms a silver mirror
   • Purple solution turns colourless
   Show answer

   Blue solution forms a red-brown precipitate. Cu2+ (blue) is reduced to Cu2O (red-brown).

3. What is the organic product when butanone is reduced by LiAlH4 in dry ether?
   • Butan-1-ol
   • Butanal
   • Butan-2-ol
   • Butanoic acid
   Show answer

   Butan-2-ol. Ketones are reduced to secondary alcohols.

4. Why must the reduction with LiAlH4 be carried out in dry ether?
   • Water oxidises the carbonyl compound
   • LiAlH4 reacts violently with water
   • Ether is the reducing agent
   • Water would make the product racemic
   Show answer

   LiAlH4 reacts violently with water. Water destroys the reducing agent, releasing hydrogen.

5. Why does propanal have a lower boiling point than propan-1-ol?
   • Propanal has fewer electrons
   • Propanal is non-polar
   • Propanal has a higher Mr
   • Propanal molecules cannot hydrogen bond with each other
   Show answer

   Propanal molecules cannot hydrogen bond with each other. Propanal has no O-H group; propan-1-ol does.

6. Why is propanone soluble in water?
   • Propanone molecules hydrogen bond to each other
   • Water molecules form hydrogen bonds to the oxygen atom of propanone
   • Propanone is ionic
   • Propanone reacts with water
   Show answer

   Water molecules form hydrogen bonds to the oxygen atom of propanone. The lone pairs on the carbonyl O accept hydrogen bonds from water.

7. What is the name of CH3CH2COCH2CH3?
   • Pentan-2-one
   • Pentanal
   • Butanone
   • Pentan-3-one
   Show answer

   Pentan-3-one. Five carbons, C=O on carbon 3.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.