1. Nomenclature, structure and physical properties
Aldehydes -al (CHO = C-1): 3-methylbutanal; ketones -one: pentan-3-one, 3-methylbutan-2-one; dialdehydes -dial. Carbonyl C is sp², planar, ≈ 120°; C=O polar (C δ+).
B.p.: alkanes/ethers < aldehydes/ketones (dipole–dipole) < alcohols (H-bonding); lower members water-soluble (water H-bonds to C=O).
Key terms
- Carbonyl group
The C=O group; polar, with an electrophilic carbon.
2. Preparation
- Alcohols: 1° → RCHO (PCC; Cu, 573 K); 2° → ketone.
- Alkenes: ozonolysis (O₃; Zn/H₂O). Alkynes: H₂O/HgSO₄/H₂SO₄ — ethyne → ethanal; others → ketones.
- Acid chlorides: Rosenmund (H₂, Pd/BaSO₄) → RCHO; → ketones.
- Calcium salts: ; with calcium methanoate → aldehyde.
- Nitriles: Stephen (SnCl₂/HCl; H₂O) → RCHO; R′MgX then H₃O⁺ → ketone; DIBAL-H → RCHO. Esters: DIBAL-H → RCHO.
| Method | Product |
|---|---|
| Rosenmund | Aldehyde |
| Stephen | Aldehyde |
| R′₂Cd + RCOCl | Ketone |
| RCN + R′MgX | Ketone |
| (RCOO)₂Ca, heat | Ketone |
Formulas
- Rosenmund
Key terms
- Rosenmund reduction
Reduction of an acid chloride to an aldehyde with H₂ over Pd poisoned with BaSO₄.
- Stephen reaction
Reduction of a nitrile with SnCl₂/HCl followed by hydrolysis to an aldehyde.
3. Nucleophilic addition
Reactivity HCHO > RCHO > R₂CO (+I and steric effects). HCN (trace base → CN⁻): CN⁻ attacks C=O (slow) → alkoxide → protonated → cyanohydrin. NaHSO₃ → crystalline adduct (purification). Grignard: HCHO → 1°, RCHO → 2°, R₂CO → 3° alcohols. Ammonia derivatives (addition–elimination): NH₂OH → oxime; NH₂NH₂ → hydrazone; C₆H₅NHNH₂ → phenylhydrazone; NH₂NHCONH₂ → semicarbazone; 2,4-DNP → orange 2,4-dinitrophenylhydrazone.
Formulas
- Cyanohydrin formation
Key terms
- Cyanohydrin
Product of HCN addition to a carbonyl compound: R₂C(OH)CN.
- Oxime
C=N–OH compound formed from a carbonyl compound and hydroxylamine.
4. Oxidation, iodoform and reduction
Aldehydes oxidised easily (K₂Cr₂O₇/H⁺, Tollens’, Fehling’s); ketones only by strong oxidants with C–C cleavage. Iodoform: CH₃CO– (or CH₃CH(OH)–) + I₂/NaOH → yellow CHI₃ + carboxylate (one C fewer).
Reduction: NaBH₄/LiAlH₄/H₂–Ni → alcohols (1° from aldehydes, 2° from ketones). C=O → CH₂: Clemmensen (Zn–Hg, conc. HCl; acid); Wolff–Kishner (NH₂NH₂, then KOH/ethylene glycol, heat; base); red P + HI.
| Reagent | Ketone gives |
|---|---|
| NaBH₄ | 2° alcohol |
| Zn–Hg/HCl | Alkane |
| NH₂NH₂/KOH | Alkane |
Formulas
- Red P / HI reduction
Key terms
- Clemmensen reduction
Reduction of C=O to CH₂ with zinc amalgam and conc. HCl.
- Wolff–Kishner reduction
Reduction of C=O to CH₂ via the hydrazone with KOH in ethylene glycol.
5. Aldol, cross aldol and Cannizzaro
α-H acidic (enolate resonance). Aldol (dilute OH⁻): enolate attacks C=O of another molecule → β-hydroxy carbonyl → (heat) α,β-unsaturated carbonyl. → but-2-enal. Cross aldol: best with one partner lacking α-H (HCHO + CH₃CHO → 3-hydroxypropanal).
Cannizzaro (no α-H, conc. OH⁻): ; crossed with HCHO: HCHO oxidised, the other aldehyde reduced.
| Condition | Reaction |
|---|---|
| α-H, dilute OH⁻ | Aldol |
| No α-H, conc. OH⁻ | Cannizzaro |
Formulas
- Cannizzaro
Key terms
- Aldol
A β-hydroxy aldehyde or ketone formed by base-catalysed self-addition of a carbonyl compound with α-H.
- Cannizzaro reaction
Disproportionation of an aldehyde without α-H in concentrated alkali to an alcohol and a carboxylate.
6. Tests and uses
Aldehyde vs ketone: Tollens’ (silver mirror), Fehling’s (red-brown Cu₂O, aliphatic aldehydes), Schiff’s reagent (pink). Methanal vs ethanal: iodoform (ethanal +). Methyl ketones: iodoform. 2,4-DNP: orange ppt for any C=O.
Uses: methanal — formalin, Bakelite and urea–formaldehyde resins, urotropine ((CH₂)₆N₄); ethanal — ethanoic acid, ethyl ethanoate; propanone — solvent, nail-polish remover; butanone — solvent.
| Test | HCHO | CH₃CHO | CH₃COCH₃ |
|---|---|---|---|
| Tollens’ | − | ||
| Fehling’s | − | ||
| Iodoform | − |
Key terms
- Tollens’ reagent
Ammoniacal silver nitrate; reduced by aldehydes to a silver mirror.
- Fehling’s solution
Alkaline copper(II) tartrate; reduced by aliphatic aldehydes to red-brown Cu₂O.
Exam tips
Common mistakes
- Propyne hydration giving propanal (it gives propanone).
- Aldol with benzaldehyde or methanal alone (no α-H: Cannizzaro).
- Methanal giving iodoform (it does not).
- Confusing Clemmensen (acid) and Wolff–Kishner (base).
How marks are usually awarded
- Mechanisms (2–3 marks): nucleophile formation, attack, protonation.
- Named preparations (1 mark each): reagent, condition and product.
- Tests (1–2 marks): reagent and observations for both compounds.