1. Preparation and physical properties
C₆H₅CHO: –CHO conjugated with the ring; no α-H.
Etard reaction: toluene + CrO₂Cl₂ (CS₂/CCl₄) → brown complex → H₂O → benzaldehyde (oxidation stops at CHO). Also: toluene + 2Cl₂ (hν) → C₆H₅CHCl₂ → (H₂O, 373 K) C₆H₅CHO; Gattermann–Koch: C₆H₆ + CO (HCl, AlCl₃/CuCl).
Properties: colourless oily liquid, bitter-almond smell, b.p. 452 K, density 1.04 g cm⁻³, slightly soluble in water; air-oxidised to benzoic acid.
| Route | Reagent |
|---|---|
| Etard | CrO₂Cl₂, then H₂O |
| Benzal chloride | Cl₂/hν, then H₂O |
| Gattermann–Koch | CO + HCl, AlCl₃/CuCl |
Formulas
- Hydrolysis of benzal chloride
Key terms
- Etard reaction
Oxidation of toluene to benzaldehyde by chromyl chloride.
2. Oxidation, reduction and addition reactions
Oxidation: air, K₂Cr₂O₇/H⁺, KMnO₄ → benzoic acid; Tollens’ → silver mirror; Fehling’s: no reaction. Reduction: H₂/Ni, NaBH₄, LiAlH₄ → benzyl alcohol; Clemmensen or Wolff–Kishner → toluene. Addition: HCN → mandelonitrile C₆H₅CH(OH)CN → (H₃O⁺) mandelic acid; NaHSO₃ → crystalline adduct. Ammonia derivatives: NH₂OH → benzaldoxime; NH₂NH₂ → hydrazone; C₆H₅NHNH₂ → phenylhydrazone. PCl₅ → benzal chloride C₆H₅CHCl₂.
Formulas
- With PCl₅
Key terms
- Mandelonitrile
The cyanohydrin of benzaldehyde, C₆H₅CH(OH)CN.
- Benzal chloride
C₆H₅CHCl₂, (dichloromethyl)benzene.
3. Cannizzaro, benzoin and Perkin reactions
Cannizzaro (conc. NaOH): (hydride transfer). Crossed with HCHO: benzaldehyde → benzyl alcohol, HCHO → HCOONa. Benzoin condensation (KCN in ethanol, catalytic CN⁻): . Perkin ((CH₃CO)₂O + CH₃COONa, 453 K): (cinnamic acid).
| Reaction | Reagent | Product |
|---|---|---|
| Cannizzaro | Conc. NaOH | Benzyl alcohol + benzoate |
| Benzoin | KCN/ethanol | Benzoin |
| Perkin | (CH₃CO)₂O, CH₃COONa | Cinnamic acid |
Formulas
- Cannizzaro
Key terms
- Benzoin
2-Hydroxy-1,2-diphenylethan-1-one, C₆H₅CH(OH)COC₆H₅.
- Cinnamic acid
3-Phenylprop-2-enoic acid, product of the Perkin reaction.
4. Ring substitution, tests and uses
–CHO: −I, −R → deactivating, meta directing. Nitration (cold) → 3-nitrobenzaldehyde; Cl₂/FeCl₃ → 3-chlorobenzaldehyde; fuming H₂SO₄ → 3-formylbenzenesulphonic acid.
Benzaldehyde vs ethanal: Fehling’s (ethanal +, benzaldehyde −); iodoform (ethanal +); dilute alkali (ethanal aldol); conc. alkali (benzaldehyde Cannizzaro); both give Tollens’.
Uses: flavouring (almond), perfumes, dyes (malachite green), cinnamic acid, cinnamaldehyde (cross aldol with ethanal), mandelic acid, benzoin.
| Test | Benzaldehyde | Ethanal |
|---|---|---|
| Fehling’s | − | |
| Iodoform | − | |
| Tollens’ |
Key terms
- Meta director
An electron-withdrawing substituent that directs incoming electrophiles to the meta position.
Exam tips
Common mistakes
- Using KMnO₄ to convert toluene to benzaldehyde (gives benzoic acid).
- Saying benzaldehyde reduces Fehling’s solution.
- Giving o/p products for ring substitution (–CHO is meta directing).
- Confusing benzal chloride (C₆H₅CHCl₂) with benzoyl chloride (C₆H₅COCl).
How marks are usually awarded
- Named reactions (1–2 marks): reagents, conditions, balanced equation and product name.
- Meta direction (2 marks): electron withdrawal and σ-complex argument.
- Distinguishing tests (1–2 marks): reagent and results for both compounds.