Aldehydes and Ketones (Cambridge AS/A‑Level – Topic 18)
Learning outcomes (syllabus mapping)
- Identify and name aldehydes and ketones (IUPAC, common names, skeletal formulas).
- Explain the electronic structure of the carbonyl group (resonance, polarity, partial charges).
- Predict physical properties (polarity, solubility, boiling‑point trends) from structure.
- Describe the general reactivity of carbonyl compounds – nucleophilic addition, oxidation, reduction, and protection.
- Write mechanisms for the key reactions: nucleophilic addition, Grignard addition, acetal formation, enolate chemistry, aldol condensation, Wittig olefination.
- Carry out and interpret the classical qualitative tests (Fehling, Tollens, Schiff, 2,4‑DNP, Jones).
- Use oxidation‑state concepts to rationalise oxidation and oxidative cleavage.
- Apply practical exam tips for quick, accurate answers.
1. Naming & basic formulas
| Common name |
IUPAC name |
Structural formula |
Skeletal formula |
| Formaldehyde |
Methanal |
H‑C(=O)‑H |
H‑C=O |
| Acetaldehyde |
Ethanal |
CH₃‑C(=O)‑H |
CH₃‑C=O |
| Acetone |
Propan‑2‑one |
CH₃‑C(=O)‑CH₃ |
CH₃‑C=O‑CH₃ |
| Cyclohexanone |
Cyclohexan‑one |
C₆H₁₀‑C(=O)‑ |
⟳‑C=O |
2. Electronic structure of the carbonyl group
- Dominant resonance form:
R‑C(=O)‑R′ ⇌ R‑C⁺‑O⁻‑R′
- Resulting partial charges: δ⁺ on carbonyl carbon (≈ +0.5 – +1.0 e) and δ⁻ on oxygen (≈ ‑0.5 e).
- Consequences:
- Carbonyl carbon is electrophilic → susceptible to nucleophilic attack.
- In aldehydes the carbonyl carbon is less hindered, giving a larger δ⁺ and higher reactivity than in ketones.
3. Physical properties
| Property |
Aldehyde (R‑CHO) |
Ketone (R‑CO‑R′) |
| Polarity |
Carbonyl C=O is strongly polar; the –CHO hydrogen can act as a H‑bond donor as well as the oxygen as an acceptor. |
Carbonyl C=O is polar; no H‑bond donor – only H‑bond acceptance. |
| Solubility in water |
≤ C₄ aldehydes (formaldehyde, acetaldehyde, propionaldehyde) are miscible; solubility falls with chain length. |
Generally less soluble than comparable aldehydes because of the missing H‑bond donor. |
| Boiling‑point trend |
- ≈ 5 °C (≈ 2 kJ mol⁻¹) rise per added –CH₂–.
- Lower than ketones of the same molecular weight: less steric bulk → weaker dipole‑dipole interactions; very small aldehydes benefit from H‑bond donation.
|
- ≈ 5 °C increase per –CH₂–.
- Higher than aldehydes of the same formula because two alkyl groups increase the dipole moment and steric hindrance.
|
| Reactivity toward nucleophiles |
More electrophilic (δ⁺ ≈ +1); carbonyl carbon is less hindered. |
Less electrophilic (δ⁺ ≈ +0.5) but still undergoes addition. |
4. General reactivity overview
- Nucleophilic addition – the principal reaction mode for both families.
- Oxidation – aldehydes → carboxylic acids (mild oxidants). Ketones require strong oxidisers for C–C cleavage.
- Reduction – both are reduced to alcohols; aldehydes give primary, ketones give secondary alcohols.
- Acid‑catalysed hydration – reversible formation of gem‑diols (hydrates), illustrating the addition mechanism.
- Protection – conversion to acetals or ketals under acidic conditions.
5. Key mechanisms
5.1. General nucleophilic addition
R‑C(=O)‑R′ + Nu⁻ → R‑C(–O⁻)(‑Nu)‑R′ (tetrahedral alkoxide)
R‑C(–O⁻)(‑Nu)‑R′ + H⁺ → R‑C(=O‑H)(‑Nu)‑R′
- Curly‑arrow steps:
- Nu⁻ donates a lone pair to the carbonyl carbon.
- The π‑bond electrons move onto oxygen, giving O⁻.
- O⁻ is protonated by solvent or added acid.
5.2. Reduction
5.3. Grignard addition
R″MgX + R‑C(=O)‑R′ → R‑C(–OMgX)(‑R″)‑R′ → (H⁺) R‑C(OH)(‑R″)‑R′
- From aldehydes → secondary alcohols; from ketones → tertiary alcohols.
5.4. Acetal (ketal) formation – protection of carbonyls
R‑C(=O)‑R′ + 2 R″OH ⇌ R‑C(OR″)₂‑R′ + H₂O (acid catalyst)
- Acetals are stable to bases and nucleophiles; they are removed by aqueous acid.
5.5. Acid‑catalysed hydration (gem‑diol)
R‑CHO + H₂O ⇌ R‑CH(OH)₂
R‑CO‑R′ + H₂O ⇌ R‑C(OH)₂‑R′
- Equilibrium lies left for most carbonyls; electron‑withdrawing substituents (Cl, CN) shift it right.
5.6. Enolisation & enolate chemistry
R‑CH₂‑C(=O)‑R′ + B⁻ → R‑CH⁻‑C(=O)‑R′ (enolate) + HB
- Base removes an α‑hydrogen, generating an enolate (nucleophilic at carbon).
- Enol (R‑CH=C(OH)‑R′) is the protonated tautomer; resonance between enolate and enol stabilises the intermediate.
5.7. Aldol condensation (base‑catalysed)
Enolate + R″‑CHO → β‑hydroxy carbonyl (aldol)
β‑hydroxy carbonyl →(Δ) α,β‑unsaturated carbonyl + H₂O
- Requires at least one α‑hydrogen on the carbonyl that forms the enolate.
- Typical exam example: acetone + benzaldehyde → benzalacetone (α,β‑unsaturated ketone).
5.8. Wittig olefination
Ph₃P=CH₂ + R‑C(=O)‑R′ → R‑CH=CH₂ + Ph₃P=O
- Phosphonium ylide attacks the carbonyl, forming a betaine that collapses to an alkene and triphenylphosphine oxide.
- Choice of ylide (stabilised vs non‑stabilised) controls E/Z geometry.
6. Classical qualitative tests
| Test |
Reagents / Conditions |
Observed result |
Interpretation |
| Fehling’s test |
Fehling A (CuSO₄) + Fehling B (KNaC₄H₄O₆), alkaline (NaOH) |
Brick‑red precipitate of Cu₂O |
Positive for aldehydes (including aromatic). No precipitate with ketones. |
| Tollens’ test |
AgNO₃ + NH₃ (ammoniacal Ag⁺), alkaline |
Silver mirror on glass (Ag⁰) |
Positive for aldehydes that possess an α‑hydrogen (most aliphatic aldehydes). α‑Keto‑aldehydes also give a positive test. |
| Schiff’s test |
Schiff’s reagent (de‑colourised fuchsin sulphurous acid) |
Pink to magenta colour |
Positive for aldehydes (including aromatic). Requires an α‑hydrogen for imine formation. |
| 2,4‑DNP test |
2,4‑Dinitrophenylhydrazine in acidic ethanol |
Yellow precipitate (aldehydes) or orange/red precipitate (ketones) |
Both give a hydrazone; colour helps distinguish the two. |
| Jones oxidation (chromic acid test) |
CrO₃ in aqueous H₂SO₄ (orange solution) |
Colour change orange → green (Cr³⁺) for aldehydes; no change for most ketones. |
Aldehyde is oxidised to the corresponding carboxylic acid; ketones are generally unchanged. |
Why the α‑hydrogen matters: In Tollens, Schiff and aldol reactions the carbonyl must be able to form an imine or an enolate. The presence of an α‑hydrogen allows deprotonation → enolate → nucleophilic attack, giving a positive test or condensation product.
7. Oxidation‑state perspective & oxidative cleavage
| Functional group |
Oxidation state of carbonyl carbon |
Typical oxidation |
Product(s) |
| Aldehyde (R‑CHO) |
+1 |
Fehling, Tollens, Jones (mild) |
Carboxylic acid (C = +3) |
| Ketone (R‑CO‑R′) |
+2 |
Strong oxidisers (KMnO₄, CrO₃) – oxidative C–C cleavage |
Two carboxylic acids (each C = +3) |
Example: Oxidative cleavage of a symmetric ketone
R‑CO‑R + [O] → 2 R‑COOH
8. Practical exam tips
- Check for an α‑hydrogen first. Its presence tells you:
- If the compound can undergo enolate chemistry (aldol, alkylation).
- Whether Tollens or Schiff tests will be positive.
- Boiling‑point comparisons
- Count –CH₂– groups (≈ 5 °C per group).
- Remember: very small aldehydes can H‑bond donate → slightly higher b.p. than expected.
- Ketones are usually higher than aldehydes of the same molecular weight because of greater steric bulk around the carbonyl.
- 2,4‑DNP colour cue
- Yellow → aldehyde.
- Orange/red → ketone.
- Reduction questions
- NaBH₄ = selective for aldehydes & ketones only.
- LiAlH₄ = reduces a broader range (esters, acids, acid chlorides, etc.).
- Protecting a carbonyl – write the acetal formation using a diol (e.g., ethylene glycol → cyclic acetal).
- Jones oxidation colour change – orange (Cr(VI)) → green (Cr(III)) indicates oxidation of an aldehyde; ketones remain orange.
- Mechanism drawing – in the exam booklet, sketch the nucleophilic addition curly‑arrow sequence (attack, alkoxide, protonation) for full credit.
9. Suggested diagrams for the exam booklet
- Resonance structures of the carbonyl group showing δ⁺ on carbon and δ⁻ on oxygen.
- Curly‑arrow mechanism of a generic nucleophilic addition (including protonation).
- Enolate formation from a carbonyl with an α‑hydrogen (base‑catalysed).
- Aldol condensation (enolate attack → β‑hydroxy carbonyl → dehydration).
- Acetal formation from a ketone and ethylene glycol (showing acid catalyst and water removal).