Aldehydes, Ketones and Carboxylic Acids

1. Naming, Structure of Carbonyl and Carboxyl

The carbonyl group C=O is the star of the chapter. In an aldehyde it is bonded to H (R–CHO), in a ketone to two carbons (R–CO–R′), in a carboxylic acid to OH (R–COOH).

Numbering: the aldehyde and acid carbon is always C-1; in a ketone number from the end nearest C=O. Greek letters α, β, γ in common names count away from the carbonyl carbon.

Structure. The carbonyl carbon is sp² hybridised (trigonal planar, about 120°). The C=O bond is polarised (C is δ⁺, O is δ⁻), so the carbon is an electrophile and the oxygen a nucleophilic centre. In a carboxylic acid the lone pair on the OH oxygen is delocalised into C=O, so the acid carbon is less electrophilic than the carbon of an aldehyde or ketone. That is why acids do not give the addition reactions of carbonyl compounds.

2. Preparation of Aldehydes and Ketones

Common to both: oxidation of 1° alcohol (aldehyde) or 2° alcohol (ketone); dehydrogenation over Cu at 573 K; ozonolysis of alkenes (O₃, then Zn/H₂O); hydration of alkynes (Hg²⁺/H₂SO₄): ethyne gives ethanal, every other alkyne gives a ketone.

Aldehydes only

• Rosenmund reduction: RCOCl + H₂ over Pd–BaSO₄. The barium sulphate poisons the catalyst so the reduction stops at the aldehyde.
• Stephen reaction: RCN + SnCl₂/HCl gives an imine, then H₃O⁺ gives RCHO. DIBAL-H reduces nitriles and esters to aldehydes.
• Aromatic aldehydes: Etard reaction (CrO₂Cl₂ on toluene, then H₃O⁺); CrO₃ in acetic anhydride; side-chain chlorination then hydrolysis; Gattermann–Koch (benzene + CO/HCl, anhydrous AlCl₃/CuCl).

Ketones only: acyl chloride + dialkylcadmium (R₂Cd); nitrile + Grignard reagent then H₃O⁺; Friedel–Crafts acylation (arene + RCOCl, anhydrous AlCl₃).

3. Physical Properties

Aldehydes and ketones are polar, so their b.p. is higher than hydrocarbons and ethers of similar mass (dipole–dipole forces) but lower than alcohols (no H-bonding between their own molecules). Carboxylic acids boil highest because they form H-bonded dimers, and the bonds survive even in the vapour.

Order: alkane < ether < aldehyde ≈ ketone < alcohol < carboxylic acid (butane 273 K < methoxyethane 281 K < propanal 322 K < acetone 329 K < propan-1-ol 370 K).

Lower aldehydes and ketones (methanal, ethanal, propanone) are miscible with water because they H-bond with water. Acids up to four carbons are also miscible. Solubility falls as the alkyl chain lengthens.

4. Nucleophilic Addition Reactions

Alkenes undergo electrophilic addition, but the polar C=O undergoes nucleophilic addition: Nu⁻ attacks the δ⁺ carbon (slow, sp² to sp³), and the alkoxide picks up H⁺ (fast).

Reactivity: HCHO > other aldehydes > ketones. Reasons: (i) steric, two alkyl groups hinder the approach in ketones; (ii) electronic, alkyl groups (+I) reduce the δ⁺ on carbon.

• HCN: gives a cyanohydrin. Pure HCN is slow, so a trace of base creates CN⁻, the real nucleophile.
• NaHSO₃: gives a crystalline, water-soluble bisulphite adduct (most aldehydes, few ketones because of sterics). Dilute acid or alkali regenerates the carbonyl compound, so this is used to purify aldehydes.
• Alcohols (dry HCl): aldehyde → hemiacetal → acetal (gem-dialkoxy). Ketone + ethylene glycol gives a cyclic ethylene glycol ketal.
• Ammonia derivatives (H₂N–Z): product is >C=N–Z after loss of water. Z = OH oxime, NH₂ hydrazone, NHC₆H₅ phenylhydrazone, NHCONH₂ semicarbazone, 2,4-dinitrophenyl (2,4-DNP: orange-red ppt, used to identify aldehydes and ketones).

5. Redox, Aldol, Cannizzaro, Tests

Reduction. NaBH₄, LiAlH₄ or H₂/Ni give 1° and 2° alcohols. To convert C=O to CH₂: Clemmensen (Zn–Hg/conc. HCl, acidic) or Wolff–Kishner (NH₂NH₂, then KOH in ethylene glycol, heat, N₂ released, basic).

Oxidation. Aldehydes are easily oxidised to acids, even by mild reagents; ketones need vigorous conditions and the C–C bond breaks, giving acids with fewer carbons. Haloform reaction: methyl ketones (CH₃CO–) with NaOX give RCOONa + CHX₃.

α-Hydrogen reactions. The α-H is acidic because C=O is strongly electron-withdrawing and the conjugate base (enolate) is resonance stabilised.

• Aldol condensation: dil. NaOH, aldehyde or ketone with α-H gives a β-hydroxy carbonyl compound (aldol or ketol); on heating it loses water to give an α,β-unsaturated product. Cross-aldol between two different carbonyl compounds each with α-H gives four products.
• Cannizzaro reaction: aldehydes with no α-H, on heating with conc. alkali, disproportionate: alcohol + carboxylate salt.
• Ring substitution: C=O is deactivating and meta-directing (nitration of benzaldehyde gives m-nitrobenzaldehyde).

6. Preparation of Carboxylic Acids

• Oxidation of 1° alcohols or aldehydes: alkaline KMnO₄, K₂Cr₂O₇/H⁺, or CrO₃–H₂SO₄ (Jones reagent).
• Alkylbenzenes + KMnO₄–KOH, heat: the entire side chain becomes –COOH (benzoic acid) whatever its length, provided it has a benzylic H (tertiary groups are not oxidised).
• Nitriles and amides: hydrolysis (H⁺ or OH⁻): RCN → RCONH₂ → RCOOH.
• Grignard reagent + CO₂ (dry ice), then H₃O⁺: gives an acid with one more carbon than the alkyl halide.
• Acyl halides, anhydrides, esters: hydrolysis gives the acid (acid hydrolysis of an ester gives the acid; base hydrolysis gives the salt).

7. Acidity and Reactions of Carboxylic Acids

Carboxylic acids react with Na, NaOH, and (unlike phenol) with NaHCO₃, giving CO₂, which is the test for –COOH. They are stronger than phenol and alcohols because the carboxylate ion has two equivalent resonance structures with the negative charge on the electronegative oxygen atoms; in phenoxide the charge sits partly on the less electronegative carbon and the structures are not equivalent.

Substituent effect: electron-withdrawing groups (–F, –Cl, –NO₂, –CF₃) stabilise the anion and increase acidity; the effect weakens with distance from COOH and grows with the number of EWG (CF₃COOH > CCl₃COOH > CHCl₂COOH > CH₂ClCOOH). Electron-donating groups (alkyl, –OCH₃) decrease acidity. Benzoic acid (4.19) is stronger than acetic acid (4.76), as the sp² carbon is more electronegative than sp³ carbon.

• Esterification: RCOOH + R′OH ⇌ RCOOR′ + H₂O (conc. H₂SO₄, reversible). It is a nucleophilic acyl substitution: protonation of C=O, attack by alcohol, proton transfer, loss of H₂O. Remove water (or ester) to push the equilibrium.
• Acid chloride: PCl₅, PCl₃ or SOCl₂ (best: SO₂ and HCl are gases). With NH₃ the acid gives ammonium salt, which on heating gives the amide.
• Reduction: LiAlH₄ or B₂H₆ give RCH₂OH; NaBH₄ does not reduce COOH.
• Decarboxylation: sodium salt + soda lime (NaOH + CaO), heat gives R–H (one carbon less). Kolbe electrolysis of the sodium salt gives R–R.
• Hell–Volhard–Zelinsky: RCH₂COOH + X₂/red P, then H₂O gives the α-halo acid.
• Ring: COOH is deactivating and meta-directing, and benzoic acid does not undergo Friedel–Crafts (AlCl₃ complexes with COOH).

8. Quick Sheet & Last-Minute Checklist

Before the exam, check you can:

• Write the nucleophilic addition mechanism and rank the carbonyl compounds by reactivity.
• Pick Aldol or Cannizzaro from the α-hydrogen, and write the products.
• Choose the right reduction (Clemmensen or Wolff–Kishner) and oxidation reagent for a given step.
• Distinguish pairs: propanal / propanone, pentan-2-one / pentan-3-one, acetophenone / benzophenone, ethanal / ethanoic acid.
• Arrange acids by pKa and explain the substituent effect.
• Solve the "identify A to F" chains by working backwards from the last compound.