AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
AQA® A-Level
YouTube Channel
YouTube Channel
@chemistry.student
*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

3.8 Aledhydes and Ketones (A-level only)

3.8 Aldehydes and Ketones

Aldehydes and Ketones

Specification Reference Organic chemistry, Aldehydes and ketones 3.3.8

Quick Notes

  • Aldehydes can be oxidised to carboxylic acids using oxidising agents such as acidified potassium dichromate (K2Cr2O7/H2SO4).
    AQA A-Level Chemistry diagram showing oxidation of an aldehyde to a carboxylic acid with acidified potassium dichromate
  • Aldehydes can be identified using:
    • Fehling’s solution (blue solution to brick red precipitate)
    • Tollens’ reagent (silver precipitate forms - silver mirror).
    • Ketones don’t give a result with either.
  • Aldehydes reduce to primary alcohols, and ketones reduce to secondary alcohols using NaBH4 (nucleophilic addition).
  • Carbonyl compounds react with KCN followed by dilute acid to form hydroxynitriles, potentially forming enantiomers.

Full Notes

Oxidation of Aldehydes

Aldehydes can be oxidised to carboxylic acids by an oxidising agent such as acidified potassium dichromate (K2Cr2O7/H2SO4).

Ketones do not undergo oxidation under normal conditions.

Observation: Orange K2Cr2O7 turns green (Cr2O72- ions are reduced to green Cr3+ ions).

Equation for aldehyde oxidation:
CH3CHO + [O] → CH3COOH

AQA A-Level Chemistry scheme: aldehyde CH3CHO oxidised to carboxylic acid CH3COOH with acidified potassium dichromate

Distinguishing Aldehydes and Ketones

Aldehydes can be further oxidised to carboxylic acids.

Ketones cannot be further oxidised.

Fehling’s Test:

Reagent: Fehling’s solution (contains Cu2+ ions).

AQA A-Level Chemistry illustration of Fehling’s test where a blue solution forms a brick-red Cu2O precipitate with aldehydes

Aldehyde result: Brick-red precipitate (Cu2O formed).
Ketone result: No visible change.

Tollens’ Test:

Reagent: Tollens’ reagent ([Ag(NH3)2]+ complex).

AQA A-Level Chemistry illustration of Tollens’ reagent producing a silver mirror with aldehydes

Aldehyde result: Silver mirror (Ag precipitate forms).
Ketone result: No visible change.

Reduction of Aldehydes and Ketones (Nucleophilic Addition)

Photo of Matt
Matt’s exam tip

Remember in organic chemistry that oxidation is the gaining of a bond to a more electronegative element (such as oxygen or nitrogen) and, or the loss of a bond to a less electronegative element (hydrogen).

Reduction is the opposite of this - the losing of a bond to a more electronegative element (such as oxygen or nitrogen) and the gaining of a bond to hydrogen.

[H] in reactions represents the addition of hydrogen, indicating a reduction reaction. It is often used as shorthand for a reducing agent, such as sodium borohydride (NaBH4) when reducing carbonyls.

Aldehydes can be reduced to primary alcohols.

Reagent: NaBH4 in aqueous solution.

AQA A-Level Chemistry scheme showing an aldehyde reduced by NaBH4(aq) to a primary alcohol

Equation for aldehyde reduction:
CH3CHO + 2[H] → CH3CH2OH

Ketones can be reduced to secondary alcohols.

Reagent: NaBH4 in aqueous solution.

AQA A-Level Chemistry scheme showing a ketone reduced by NaBH4(aq) to a secondary alcohol

Equation for ketone reduction:
CH3COCH3 + 2[H] → CH3CH(OH)CH3

Mechanism

The reduction reactions for both aldehydes and ketones follow a nucleophilic addition mechanism, with the nucleophile a hydride ion (H−) coming from the NaBH4.

AQA A-Level Chemistry nucleophilic addition mechanism showing hydride attack on the carbonyl carbon of an aldehyde followed by protonation to form an alcohol

Reaction with KCN to Form Hydroxynitriles (Nucleophilic Addition)

Aldehydes and ketones react with KCN followed by dilute acid to form hydroxynitriles.

Reagent: KCN followed by H+.

Mechanism: Nucleophilic addition of CN− to the carbonyl carbon.

AQA A-Level Chemistry mechanism: CN− adds to a carbonyl carbon to form a cyanohydrin intermediate that is protonated to give a hydroxynitrile

The cyanide (CN−) ion comes from KCN which dissociates to release K+ and CN−.

AQA A-Level Chemistry diagram showing KCN dissociating to K+ and CN− in solution

KCN is highly toxic, so safety precautions are required.

Formation of Enantiomers

Aldehydes and unsymmetrical ketones can form mixtures of enantiomers when reacting with KCN. See Optical Isomerism for more detail.

Reason: The planar carbonyl group can be attacked from either side, forming two mirror-image products.

AQA A-Level Chemistry diagram showing attack of CN− on either side of a planar carbonyl to form two enantiomeric hydroxynitriles

There is an equal chance of the carbonyl group being attacked from either side (above or below the plane), meaning a racemic product mixture is made.

Summary