Introduction
Aldehydes (–CHO) and ketones (C=O flanked by two carbon groups) are carbonyl compounds. This chapter is very important for NEET — typically 2–3 questions on named reactions and tests.
Preparation
- Aldehydes: Oxidation of primary alcohols; Rosenmund reduction (acid chloride + H₂/Pd-BaSO₄)
- Ketones: Oxidation of secondary alcohols; Dry distillation of calcium salts of fatty acids
- Both: Ozonolysis of alkenes
Nucleophilic Addition Reactions
The carbonyl carbon is electrophilic. Nu⁻ attacks → addition product.
- Addition of HCN → cyanohydrin
- Addition of NaHSO₃ → bisulphite addition (only methyl ketones and aldehydes)
- Addition of NH₂OH → oxime (–C=N–OH)
- Addition of NH₂–NH₂ → hydrazone
- Addition of Grignard reagent (RMgX) → secondary/tertiary alcohol
Tests to Distinguish Aldehyde from Ketone
| Test | Aldehydes | Ketones |
|---|---|---|
| Tollens' (silver mirror test) | +ve (silver mirror) | −ve |
| Fehling's test | +ve (brick red ppt) | −ve (except HCHO) |
| Schiff's reagent | Pink/red colour | −ve |
Named Reactions
- Aldol Condensation: Aldehydes/ketones with α-H undergo self-condensation in dilute base → β-hydroxy carbonyl compound
- Cannizzaro Reaction: Aldehydes without α-H (HCHO, C₆H₅CHO) undergo self-disproportionation in conc. NaOH → one oxidised (carboxylate) + one reduced (alcohol)
- Clemmensen Reduction: C=O → CH₂ using Zn/Hg + conc. HCl (acidic medium)
- Wolff–Kishner Reduction: C=O → CH₂ using NH₂–NH₂ then KOH/Δ (basic medium)
- Cross Aldol: Between two different aldehydes
Iodoform Test
Positive for methyl ketones (CH₃COR), acetaldehyde (CH₃CHO), and secondary alcohols with –CH(OH)CH₃ group. Yellow precipitate of CHI₃ formed.