🔹 1. Preparation of Aldehydes and Ketones
(a) Oxidation of Primary Alcohols → Aldehydes
- Reagent: Acidified potassium dichromate(VI) (K₂Cr₂O₇/H⁺) or acidified potassium manganate(VII) (KMnO₄/H⁺)
- Conditions: Distillation (to prevent further oxidation to carboxylic acid)
- Reaction: (Ethanol → Ethanal)
(b) Oxidation of Secondary Alcohols → Ketones
- Reagent: Acidified K₂Cr₂O₇ or KMnO₄
- Conditions: Reflux or Distillation
- Reaction: (Propan-2-ol → Propanone)
🔹 2. Reactions of Aldehydes and Ketones
(a) Reduction to Alcohols
- Reagents:
- NaBH₄ (Sodium borohydride): Milder, aqueous/alcoholic solution
- LiAlH₄ (Lithium aluminium hydride): Stronger, in dry ether
- Reaction:
- Aldehyde → Primary alcohol
- Ketone → Secondary alcohol
(b) Reaction with HCN (Nucleophilic Addition)
- Reagents: HCN (generated in situ), KCN as a catalyst, heat gently
- Products: Hydroxynitriles
- Example:
- Ethanal + HCN → 2-Hydroxypropanenitrile
- Propanone + HCN → 2-Hydroxy-2-methylpropanenitrile
🔹 3. Mechanism: Nucleophilic Addition of HCN
Steps:
- Nucleophilic attack: CN⁻ ion attacks the partially positive carbon of the C=O group.
- Tetrahedral intermediate: A negatively charged intermediate is formed.
- Protonation: The intermediate gains a proton (H⁺) to form the hydroxynitrile.
General Mechanism:
🔹 4. Detection Using 2,4-Dinitrophenylhydrazine (2,4-DNPH)
- Test: Add 2,4-DNPH solution to the compound.
- Observation: Formation of a yellow/orange precipitate indicates the presence of a carbonyl group (C=O) in aldehydes or ketones.
- No reaction with carboxylic acids or esters.
🔹 5. Distinguishing Aldehydes and Ketones
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens’ reagent (Ag⁺/NH₃) | Silver mirror formed | No reaction |
| Fehling’s solution | Brick-red ppt. (Cu₂O) | No reaction |
| Oxidation | Easily oxidized to carboxylic acid | Resistant to mild oxidation |
🔹 6. Iodoform Test (Triiodomethane Test)
- Reagent: Iodine (I₂) in alkaline solution (NaOH)
- Positive result: Formation of yellow precipitate of CHI₃ (iodoform) with antiseptic smell.
- Condition: Presence of a CH₃CO– group (methyl ketone) or ethanol.
Examples:
- Propanone (CH₃COCH₃) → Positive
- Ethanol (CH₃CH₂OH) → Positive (oxidized to ethanal first)
- CH₃COCH₂CH₃ → Positive
- Butanone → Positive
✅ Summary Table of Key Reactions
| Reaction Type | Reagent(s) | Product |
|---|---|---|
| Oxidation (1° alcohol) | Acidified K₂Cr₂O₇ (distil) | Aldehyde |
| Oxidation (2° alcohol) | Acidified K₂Cr₂O₇ | Ketone |
| Reduction | NaBH₄ or LiAlH₄ | Alcohol |
| HCN Addition | HCN, KCN, heat | Hydroxynitrile |
| Carbonyl Detection | 2,4-DNPH | Yellow/orange ppt. |
| Aldehyde test | Fehling’s/Tollens’ | Red ppt. / Silver mirror |
| CH₃CO– group | I₂ / OH⁻ | CHI₃ yellow ppt. |
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