1. Nomenclature and structure of the carbonyl group
Names: aldehydes -al (methanal, ethanal, 2-methylpropanal, benzaldehyde = benzenecarbaldehyde); ketones -one with locant (propanone, butan-2-one, 1-phenylethan-1-one = acetophenone).
Structure: the carbonyl carbon is sp², trigonal planar (about 120°), with a σ and a π bond to oxygen. The C=O bond is polar (C δ+, O δ−); the carbon is electrophilic and is attacked by nucleophiles from above or below the plane.
Key terms
- Carbonyl group
The C=O group, present in aldehydes, ketones and carboxylic acids and their derivatives.
- Electrophilic carbon
A carbon carrying partial positive charge, open to attack by nucleophiles.
2. Preparation of aldehydes and ketones
- Oxidation of alcohols: 1° → aldehyde (PCC; Cu, 573 K); 2° → ketone (K₂Cr₂O₇/H⁺).
- Ozonolysis of alkenes (O₃, then Zn/H₂O).
- Hydration of alkynes (H₂O, Hg²⁺, H₂SO₄): ethyne → ethanal; propyne → propanone.
- Rosenmund reduction: .
- Stephen reaction: nitrile + SnCl₂/HCl → imine, then H₃O⁺ → aldehyde; DIBAL-H also reduces nitriles and esters to aldehydes.
- Etard reaction: toluene + CrO₂Cl₂ → benzaldehyde; also CrO₃/acetic anhydride, or side-chain chlorination to benzal chloride and hydrolysis.
- Gatterman–Koch: benzene + CO + HCl (anhydrous AlCl₃/CuCl) → benzaldehyde.
- Friedel–Crafts acylation: .
- Ketones from nitriles + Grignard reagent, then hydrolysis; from acid chlorides + dialkylcadmium.
Key terms
- Rosenmund reduction
Hydrogenation of an acid chloride over Pd poisoned with BaSO₄ to give an aldehyde.
- Etard reaction
Oxidation of toluene to benzaldehyde by chromyl chloride.
- Stephen reaction
Reduction of a nitrile by SnCl₂/HCl followed by hydrolysis, giving an aldehyde.
3. Reactions of aldehydes and ketones
Nucleophilic addition (aldehydes more reactive than ketones: steric and +I effects): HCN → cyanohydrin (); NaHSO₃ → bisulfite adduct (purification); RMgX → alcohols; alcohols/dry HCl → hemiacetal, acetal (ketones + ethylene glycol → ketal); NH₂–Z derivatives (hydroxylamine → oxime, hydrazine → hydrazone, 2,4-DNP → orange 2,4-DNP derivative, semicarbazide → semicarbazone).
Reduction: NaBH₄/LiAlH₄/H₂ → alcohols; Clemmensen (Zn–Hg, conc. HCl) and Wolff–Kishner (NH₂NH₂; KOH/ethylene glycol, heat) → CH₂.
Oxidation: aldehydes → acids with mild oxidants. Tollens’ test (silver mirror) for all aldehydes; Fehling’s test (red-brown Cu₂O) for aliphatic aldehydes. Ketones are oxidised only vigorously, with C–C cleavage.
α-Hydrogen reactions: aldol condensation (dilute NaOH): , then dehydration to but-2-enal; crossed aldol gives mixtures. Haloform reaction (CH₃CO– group, I₂/NaOH → yellow CHI₃).
Cannizzaro reaction (no α-H, conc. alkali): ; benzaldehyde → sodium benzoate + phenylmethanol.
Aromatic ring: –CHO and –COR are deactivating and meta-directing.
| Test | Aldehyde (aliphatic) | Aromatic aldehyde | Ketone |
|---|---|---|---|
| Tollens’ | − | ||
| Fehling’s | − | − | |
| 2,4-DNP |
Key terms
- Aldol condensation
Base-catalysed self-addition of carbonyl compounds having α-hydrogens, giving β-hydroxy carbonyl compounds.
- Cannizzaro reaction
Disproportionation of an aldehyde without α-hydrogen in concentrated alkali to an alcohol and a carboxylate.
- Tollens’ reagent
Ammoniacal silver nitrate; gives a silver mirror with aldehydes.
4. Carboxylic acids: preparation
- Oxidation of 1° alcohols and aldehydes (KMnO₄, K₂Cr₂O₇, CrO₃).
- Oxidation of alkylbenzenes with hot alkaline KMnO₄ (any side chain → –COOH on the ring).
- Hydrolysis of nitriles and amides: (one carbon more than the alkyl halide used to make the nitrile).
- Grignard reagent + CO₂ (dry ice), then H₃O⁺: → .
- Hydrolysis of acid chlorides, anhydrides and esters.
| From | Reagent | Change in carbon count |
|---|---|---|
| RCH₂OH | KMnO₄ | Same |
| C₆H₅R | KMnO₄, heat | Side chain → COOH |
| RX → RCN | KCN, then H₃O⁺ | +1 |
| RX → RMgX | CO₂, then H₃O⁺ | +1 |
Key terms
- Dry ice
Solid carbon dioxide, used with Grignard reagents to make carboxylic acids.
5. Carboxylic acids: properties, acidity, reactions and uses
Physical: hydrogen-bonded dimers → higher bp than alcohols of similar mass (ethanoic acid 391 K, propan-1-ol 370 K); C1–C4 acids water-miscible.
Acidity: stronger than alcohols and phenols (carboxylate resonance, charge on two O atoms); liberate CO₂ from NaHCO₃: . −I groups (F, Cl, NO₂, CN) increase acidity (more groups, closer → stronger); +I alkyl groups decrease it. pKa: CF₃COOH 0.23, CCl₃COOH 0.65, CHCl₂COOH 1.26, FCH₂COOH 2.66, ClCH₂COOH 2.86, HCOOH 3.75, C₆H₅COOH 4.19, CH₃COOH 4.76, CH₃CH₂COOH 4.88.
Reactions: acid chlorides (PCl₅, PCl₃, SOCl₂); esters (alcohol, H⁺); amides (NH₃, heat); anhydrides (P₂O₅ or heat); reduction to 1° alcohols (LiAlH₄ or B₂H₆, not NaBH₄); decarboxylation (one C fewer), Kolbe electrolysis; HVZ ; ring substitution of benzoic acid at meta (no Friedel–Crafts).
Uses: methanoic acid (rubber, textiles, leather, electroplating); ethanoic acid (vinegar, solvent); hexanedioic acid (nylon-6,6); sodium benzoate (preservative); esters of benzoic acid (perfumes); higher fatty acids (soaps).
| Acid | pKa |
|---|---|
| CCl₃COOH | 0.65 |
| ClCH₂COOH | 2.86 |
| HCOOH | 3.75 |
| C₆H₅COOH | 4.19 |
| CH₃COOH | 4.76 |
Key terms
- Decarboxylation
Loss of CO₂ from a carboxylic acid or its salt, e.g. with soda lime, giving a hydrocarbon with one carbon fewer.
- HVZ reaction
α-Halogenation of a carboxylic acid with Cl₂ or Br₂ and red phosphorus.
Exam tips
Common mistakes
- Claiming ketones give Tollens’ or Fehling’s tests; or that benzaldehyde gives Fehling’s test.
- Choosing aldol for compounds without α-H (they give Cannizzaro with conc. alkali).
- Forgetting that soda lime removes one carbon and that nitrile/Grignard routes add one.
- Using NaBH₄ to reduce a carboxylic acid.
- Ranking acidity without considering the number and position of −I groups.
How marks are usually awarded
- 1-mark items: product, reagent, name, test result.
- Named reactions (2 marks): equation (1) + conditions/reagent (1).
- Tests (2 marks): reagent (½–1) + observation for each compound.
- Acidity explanations: conjugate-base stability with the effect named (−I/+I, resonance).