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7.2Advanced Organic Chemistry

Aldehydes and Ketones — AQA A Level Chemistry Revision Notes & Topical Questions

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Aldehydes and Ketones: Reduction and Nucleophilic Addition — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Describe the reactions of carbonyl compounds, including reduction, nucleophilic addition of cyanide, and tests to distinguish them.Aldehydes and ketones contain the carbonyl…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Carbonyl group
  • Nucleophile

Section 1 of 7The carbonyl group

Aldehydes and ketones contain the carbonyl functional group, C=O, which is why they are also known as carbonyl compounds.

The carbonyl group
In aldehydes the carbonyl carbon is at the end of the chain (with at least one H, -CHO); in ketones it is within the chain, bonded to two carbons (-CO-).
The C=O bond is polar: oxygen is more electronegative, so carbon is δ+ and attracts nucleophiles.
Names end in -al (aldehydes, e.g. ethanal) or -one (ketones, e.g. propanone).

Section 2 of 7Reduction of carbonyls

Aldehydes and ketones are reduced by NaBH4 (sodium tetrahydridoborate) in aqueous or methanolic solution, the reducing agent supplying hydride ions (H-) as the nucleophile.

Reduction of carbonyls
Aldehydes give primary alcohols and ketones give secondary alcohols: CH3CHO + 2[H] → CH3CH2OH; CH3COCH3 + 2[H] → CH3CH(OH)CH3.
LiAlH4 in dry ether also works but is far more reactive and hazardous.
The mechanism is nucleophilic addition: H- attacks the δ+ carbon, the C=O π bond breaks to give an alkoxide, and water protonates it to the alcohol.

Section 3 of 7Nucleophilic addition with HCN

Carbonyl compounds react with hydrogen cyanide (generated from KCN with dilute acid, which gives CN- nucleophile) to give hydroxynitriles: CH3CHO + HCN → CH3CH(OH)CN (2-hydroxypropanenitrile).

Nucleophilic addition with HCN
The lone pair on CN- attacks the δ+ carbon of C=O.
The C=O π electrons move onto oxygen, giving a negatively charged intermediate.
The O- picks up H+ from HCN or water to form the OH group, regenerating CN-.

The reaction increases the carbon chain by one, useful in synthesis; the nitrile can be hydrolysed to a carboxylic acid or reduced to an amine. Because the carbonyl group is planar, attack from either side gives a racemic mixture when the product is chiral.

Section 4 of 7Distinguishing aldehydes and ketones

Aldehydes are easily oxidised, ketones are not.

Distinguishing aldehydes and ketones
With Tollens’ reagent ([Ag(NH3)2]+, warmed) an aldehyde gives a silver mirror; with Fehling’s or Benedict’s solution an aldehyde gives a brick-red precipitate of Cu2O; acidified dichromate turns from orange to green.
Ketones give no reaction with these.
Both react with 2,4-dinitrophenylhydrazine (2,4-DNPH) to give an orange/yellow precipitate, whose melting point can identify the compound.

Section 5 of 7Worked examples

  1. Name the product of propanone with NaBH4 and state its type. [2]2 marks

  2. Outline the mechanism for the reaction of ethanal with KCN/H+. [4]4 marks

  3. Describe a test to distinguish propanal from propanone. [3]3 marks

  4. Explain why the product of ethanal with HCN is a racemic mixture. [2]2 marks

Section 6 of 7Key terms

TermMeaning
Carbonyl groupC=O functional group.
NucleophileSpecies donating a lone pair to form a covalent bond.
Nucleophilic additionNucleophile adds across a polar double bond.
HydroxynitrileCompound containing both OH and CN groups.
Tollens’ reagentAmmoniacal silver nitrate used to detect aldehydes.

Section 7 of 7Exam technique

1
Using NaBH4 as an oxidising agent - it is a reducing agent.
2
Drawing the nucleophile attack on oxygen instead of the δ+ carbon.
3
Forgetting the curly arrow from C=O to O.
4
Saying ketones give a silver mirror.

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