Aldehydes, Ketones and Carboxylic Acids

Chemistry · Class 12

Lesson 13 of 13 · 17 min

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Must-know facts

23 facts

  1. 1Carbonyl carbon: sp², trigonal planar, ~120°; C=O polar (C electrophilic, O nucleophilic).
  2. 2Aldehyde from nitrile: Stephen (SnCl₂ + HCl, then hydrolysis) or DIBAL-H; DIBAL-H also turns esters into aldehydes.
  3. 3Rosenmund: RCOCl + H₂ over Pd–BaSO₄ → RCHO.
  4. 4Benzaldehyde from toluene: Etard (CrO₂Cl₂), CrO₃ in acetic anhydride, or side-chain chlorination then hydrolysis; from benzene: Gatterman-Koch (CO + HCl, anhydrous AlCl₃ or CuCl).
  5. 5Ketones: dialkylcadmium + acyl chloride; nitrile + Grignard then hydrolysis; Friedel-Crafts acylation.
  6. 6b.p. (K) at mass 58–60: n-butane 273 < methoxyethane 281 < propanal 322 < acetone 329 < propan-1-ol 370.
  7. 7Nucleophilic addition: attack perpendicular to the plane, sp² → sp³ tetrahedral alkoxide, then H⁺. Aldehydes > ketones (steric + electronic).
  8. 8HCN addition needs base (CN⁻); NaHSO₃ adducts purify aldehydes; dry HCl + alcohol → hemiacetal → acetal; ketone + ethylene glycol → ketal.
  9. 9H₂N–Z products: oxime (NH₂OH), hydrazone (N₂H₄), phenylhydrazone, semicarbazone, 2,4-DNP derivative (yellow/orange/red), Schiff's base (RNH₂).
  10. 10C=O → CH₂: Clemmensen (Zn–Hg, conc. HCl) or Wolff-Kishner (NH₂NH₂, then KOH/NaOH in ethylene glycol, heat).
  11. 11Tollens' (silver mirror) and Fehling's (red-brown Cu₂O) are given by aldehydes, not ketones; aromatic aldehydes fail Fehling's.
  12. 12Iodoform (yellow CHI₃) = CH₃CO– or CH₃CH(OH)–; haloform gives an acid salt with one C fewer.
  13. 13Aldol: needs α-H, dilute alkali → β-hydroxy carbonyl → α,β-unsaturated on losing water. Cannizzaro: no α-H, concentrated alkali, disproportionation.
  14. 14Formalin = 40% formaldehyde.
  15. 15Acids from Grignard + CO₂ and from nitrile hydrolysis add one carbon to the alkyl halide.
  16. 16Side-chain oxidation turns any 1° or 2° alkyl group on a ring into –COOH; a tertiary group is not oxidised.
  17. 17Carboxylic acids boil above alcohols of similar mass; dimers in vapour; up to 4 C miscible with water.
  18. 18pKa: HCl −7.0; CF₃COOH 0.23; 4-nitrobenzoic 3.41; benzoic 4.19; 4-methoxybenzoic 4.46; acetic 4.76; phenol 10; ethanol ~16.
  19. 19Acids release CO₂ from NaHCO₃; phenols do not.
  20. 20SOCl₂ is preferred for RCOCl because SO₂ and HCl escape as gases.
  21. 21LiAlH₄ or B₂H₆ reduce –COOH to –CH₂OH; NaBH₄ does not.
  22. 22Sodalime = NaOH : CaO = 3 : 1; decarboxylation loses one carbon. HVZ: X₂ + red P → α-halo acid.
  23. 23–CHO, –COR and –COOH on a ring are deactivating and meta-directing; benzoic acid gives no Friedel-Crafts reaction.

Common traps

Where marks are lost

Refusing to call methanal an aldehyde because its C=O carbon has no carbon group.

Methanal, HCHO, carries two hydrogens on the carbonyl carbon and is the simplest aldehyde; it behaves as one (Tollens' positive, Cannizzaro).

Using KMnO₄ to make benzaldehyde from toluene.

KMnO₄ goes on to benzoic acid. The Etard reaction (CrO₂Cl₂) or CrO₃ in acetic anhydride stops at the aldehyde.

Ranking ketones as more reactive than aldehydes towards nucleophiles.

Two alkyl groups crowd the carbonyl carbon and lower its positive character, so ketones react more slowly: butanone < propanone < propanal < ethanal.

Expecting benzaldehyde to give Fehling's test because it gives Tollens' test.

Tollens' reagent oxidises aromatic aldehydes too, but aromatic aldehydes do not respond to Fehling's test.

Marking propanal positive in the iodoform test because it is a carbonyl compound.

The iodoform test needs CH₃CO– (or CH₃CH(OH)–). Propanal is CH₃CH₂CHO and fails; ethanal and propanone pass.

Running an aldol reaction on methanal or benzaldehyde.

Neither has an α-hydrogen, so no enolate forms. With concentrated alkali they undergo the Cannizzaro reaction instead.

Using concentrated alkali for aldol and dilute alkali for Cannizzaro.

It is the other way round: aldol with dilute alkali, Cannizzaro with concentrated alkali and heat.

Writing α-hydroxy aldehyde as the aldol product.

The –OH ends up on the β-carbon: ethanal gives 3-hydroxybutanal, a β-hydroxy aldehyde.

Arguing that phenol is more acidic than a carboxylic acid because phenoxide has more resonance forms.

Carboxylate's two forms are equivalent and put the charge on two oxygens; phenoxide's forms put charge on ring carbons. The carboxylate is more stable, so the acid is stronger.

Expecting a phenyl group on –COOH to weaken the acid by resonance donation.

Benzoic acid (4.19) is stronger than acetic acid (4.76): the sp² ring carbon is more electronegative than an sp³ carbon.

Reducing a carboxylic acid with NaBH₄.

NaBH₄ does not reduce –COOH. Use LiAlH₄ or, more selectively, diborane.

Formulas

6 to know

pKa and Ka

pKa = −log Ka; Ka ratio = 10^(ΔpKa)

Benzoic (4.19) vs acetic (4.76): 10^0.57 ≈ 3.7.

Tollens' test

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃

Two silver atoms per aldehyde group.

Fehling's test

RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O

Red-brown Cu₂O; aromatic aldehydes do not react.

Haloform reaction

R–CO–CH₃ + NaOX → R–COONa + CHX₃

X = Cl, Br, I; CHI₃ is the yellow iodoform.

Cannizzaro reaction

2HCHO + conc. KOH → CH₃OH + HCOOK

No α-H; one molecule reduced, one oxidised.

Decarboxylation

RCOONa + NaOH (CaO, heat) → R–H + Na₂CO₃

Sodalime NaOH : CaO = 3 : 1.

Key terms

13 terms

Carbonyl group
>C=O, a carbon double-bonded to oxygen.
Cyanohydrin
The product of HCN adding to C=O: a carbon carrying both –OH and –CN.
Hemiacetal
An alkoxyalcohol from an aldehyde and one molecule of alcohol.
Acetal / ketal
A gem-dialkoxy compound from an aldehyde (or ketone) and two alcohol groups.
Schiff's base
A substituted imine, >C=N–R, from a carbonyl compound and a primary amine.
2,4-DNP derivative
Coloured solid formed with 2,4-dinitrophenylhydrazine; identifies aldehydes and ketones.
α-Hydrogen
A hydrogen on the carbon next to the carbonyl group; acidic.
Aldol
A β-hydroxy aldehyde (or ketol, β-hydroxy ketone) from the aldol reaction.
Disproportionation
One species both oxidised and reduced, as in the Cannizzaro reaction.
Fatty acids
Aliphatic carboxylic acids with C₁₂–C₁₈ chains, found as glycerol esters in fats.
Jones reagent
CrO₃ in aqueous acid, an oxidant for alcohols.
Sodalime
NaOH and CaO in the ratio 3 : 1, used for decarboxylation.
Hell-Volhard-Zelinsky reaction
α-Halogenation of a carboxylic acid with Cl₂ or Br₂ and red phosphorus.
Test yourself: 10 questionsExam-style questions on Aldehydes, Ketones and Carboxylic Acids, with full solutions.Start
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