Lesson 13 of 13 · 17 min
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Must-know facts
23 facts
- 01Carbonyl carbon: sp², trigonal planar, ~120°; C=O polar (C electrophilic, O nucleophilic).
- 02Aldehyde from nitrile: Stephen (SnCl₂ + HCl, then hydrolysis) or DIBAL-H; DIBAL-H also turns esters into aldehydes.
- 03Rosenmund: RCOCl + H₂ over Pd–BaSO₄ → RCHO.
- 04Benzaldehyde from toluene: Etard (CrO₂Cl₂), CrO₃ in acetic anhydride, or side-chain chlorination then hydrolysis; from benzene: Gatterman-Koch (CO + HCl, anhydrous AlCl₃ or CuCl).
- 05Ketones: dialkylcadmium + acyl chloride; nitrile + Grignard then hydrolysis; Friedel-Crafts acylation.
- 06b.p. (K) at mass 58–60: n-butane 273 < methoxyethane 281 < propanal 322 < acetone 329 < propan-1-ol 370.
- 07Nucleophilic addition: attack perpendicular to the plane, sp² → sp³ tetrahedral alkoxide, then H⁺. Aldehydes > ketones (steric + electronic).
- 08HCN addition needs base (CN⁻); NaHSO₃ adducts purify aldehydes; dry HCl + alcohol → hemiacetal → acetal; ketone + ethylene glycol → ketal.
- 09H₂N–Z products: oxime (NH₂OH), hydrazone (N₂H₄), phenylhydrazone, semicarbazone, 2,4-DNP derivative (yellow/orange/red), Schiff's base (RNH₂).
- 10C=O → CH₂: Clemmensen (Zn–Hg, conc. HCl) or Wolff-Kishner (NH₂NH₂, then KOH/NaOH in ethylene glycol, heat).
- 11Tollens' (silver mirror) and Fehling's (red-brown Cu₂O) are given by aldehydes, not ketones; aromatic aldehydes fail Fehling's.
- 12Iodoform (yellow CHI₃) = CH₃CO– or CH₃CH(OH)–; haloform gives an acid salt with one C fewer.
- 13Aldol: needs α-H, dilute alkali → β-hydroxy carbonyl → α,β-unsaturated on losing water. Cannizzaro: no α-H, concentrated alkali, disproportionation.
- 14Formalin = 40% formaldehyde.
- 15Acids from Grignard + CO₂ and from nitrile hydrolysis add one carbon to the alkyl halide.
- 16Side-chain oxidation turns any 1° or 2° alkyl group on a ring into –COOH; a tertiary group is not oxidised.
- 17Carboxylic acids boil above alcohols of similar mass; dimers in vapour; up to 4 C miscible with water.
- 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.
- 19Acids release CO₂ from NaHCO₃; phenols do not.
- 20SOCl₂ is preferred for RCOCl because SO₂ and HCl escape as gases.
- 21LiAlH₄ or B₂H₆ reduce –COOH to –CH₂OH; NaBH₄ does not.
- 22Sodalime = NaOH : CaO = 3 : 1; decarboxylation loses one carbon. HVZ: X₂ + red P → α-halo acid.
- 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
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.