Chapter 8: Aldehydes, Ketones & Carboxylic Acids

Rosenmund Reduction (Only Aldehydes)

Acyl chlorides (acid chlorides) are hydrogenated over catalyst, palladium on barium sulphate. BaSO₄ acts as a poison to prevent further reduction to alcohol.

Stephen Reaction (From Nitriles)

Nitriles are reduced to corresponding imines with stannous chloride (SnCl₂) in the presence of HCl, which on hydrolysis give corresponding aldehyde.

Etard Reaction

Chromyl chloride (CrO₂Cl₂) oxidizes the methyl group of Toluene to a chromium complex, which on hydrolysis gives Benzaldehyde.

Gattermann-Koch Reaction

Benzene is treated with carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of anhydrous AlCl₃/CuCl to give benzaldehyde.

Preparation of Ketones

• From Acyl Chlorides: 2R-MgX + CdCl₂ → R₂Cd + Mg(X)Cl. Then, 2R'-COCl + R₂Cd → 2R'-CO-R + CdCl₂.
• Friedel-Crafts Acylation: Benzene + R-COCl (Anhyd. AlCl₃) → Aryl Ketone.

Why are Aldehydes more reactive than Ketones?

Steric Factor: Two alkyl groups in ketones hinder the approach of nucleophile.
Electronic Factor: Alkyl groups show +I effect, which reduces the electrophilicity of the carbonyl carbon in ketones.

Addition Reactions

• Addition of HCN: Forms cyanohydrins. Catalyst: base (to generate stronger nucleophile CN⁻).
• Addition of NaHSO₃: Forms bisulphite addition compound (white crystalline solid). Used for purification and separation of aldehydes/ketones.
• Addition of Alcohols: Aldehydes react with monohydric alcohol to form hemiacetals, then Acetals. Ketones form Ketals with ethylene glycol.

Addition of Ammonia Derivatives

Reacts with derivatives (NH₂-Z) to form >C=N-Z. Examples:

• Hydroxylamine (NH₂OH) → Oxime
• Hydrazine (NH₂NH₂) → Hydrazone
• 2,4-Dinitrophenylhydrazine (2,4-DNP) → 2,4-DNP derivative (Orange-Red precipitate - used as a test for carbonyl group).

Reduction to Hydrocarbons (Very Important)

• Clemmensen Reduction: Carbonyl group is reduced to CH₂ group using Zinc amalgam and conc. HCl.
  >C=O + Zn-Hg / conc. HCl → >CH₂ + H₂O
• Wolff-Kishner Reduction: Carbonyl group is reduced using Hydrazine (NH₂NH₂) followed by heating with KOH in ethylene glycol.
  >C=O + NH₂NH₂ → Hydrazone (KOH/Glycol, heat) → >CH₂ + N₂

Distinction Tests (Oxidation)

• Tollens' Test (Silver Mirror Test): Aldehydes (aliphatic & aromatic) reduce Tollens' reagent (ammoniacal silver nitrate) to a bright silver mirror. Ketones do not.
• Fehling's Test: Aliphatic aldehydes reduce Fehling's solution (Cu²⁺) to red-brown precipitate of Cu₂O. (Aromatic aldehydes and Ketones do NOT give this test).
• Haloform (Iodoform) Test: Compounds having CH₃-CO- group (or CH₃-CH(OH)- group) react with NaOI (I₂ + NaOH) to give yellow precipitate of Iodoform (CHI₃).

Aldol Condensation

Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali (dil. NaOH) to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol). On heating, they lose water to form α,β-unsaturated carbonyl compounds.

Cross-Aldol Condensation

When Aldol condensation is carried out between two different aldehydes/ketones. If both contain α-hydrogen, it gives a mixture of 4 products.

Cannizzaro Reaction (No Alpha-Hydrogen)

Aldehydes which DO NOT have an α-hydrogen atom (like Formaldehyde HCHO, Benzaldehyde C₆H₅CHO) undergo self oxidation-reduction (disproportionation) when heated with concentrated alkali (conc. KOH/NaOH).

Preparation

• From 1° Alcohols & Aldehydes: Oxidation using alkaline KMnO₄ or acidic K₂Cr₂O₇.
• From Nitriles & Amides: Hydrolysis with H⁺ or OH⁻ with heat.
• From Grignard Reagents: RMgX + Solid CO₂ (Dry ice) → R-COOMgX (H₃O⁺) → R-COOH.

Acidic Strength

Carboxylic acids are stronger acids than phenols because carboxylate ion is stabilized by two equivalent resonance structures (negative charge on more electronegative oxygen atoms).

• Electron Withdrawing Groups (EWG) like -NO₂, -Cl increase acidic strength. (Cl₃C-COOH is strongest).
• Electron Donating Groups (EDG) like -CH₃ decrease acidic strength.

Hell-Volhard-Zelinsky (HVZ) Reaction

Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus.

Decarboxylation

Sodium salts of carboxylic acids heated with soda lime (NaOH + CaO in 3:1 ratio) lose carbon dioxide to form hydrocarbons containing one carbon less.