Board exam focus — Aldehydes, Ketones and Carboxylic Acids (CBSE & HBSE)

Preparation of aldehydes and ketones, nucleophilic addition reactions (aldol, Cannizzaro, Clemmensen), carboxylic acid preparation and reactions (esterification, Hell-Volhard-Zelinsky).

Aldehydes and Ketones - Preparation

Aldehydes, Ketones and Carboxylic Acids

Nomenclature

Aldehydes: -CHO group (methanal, ethanal, propanal) Ketones: -CO- group (propan-2-one/acetone, pentan-3-one) Carboxylic acids: -COOH group (methanoic, ethanoic, propanoic acid)

Preparation of Aldehydes

1. Oxidation of Primary Alcohols: RCH2OH + [O] → RCHO Using PCC (pyridinium chlorochromate), CrO3, K2Cr2O7/H2SO4 (Careful control to stop at aldehyde; otherwise → carboxylic acid)

2. Reduction of Acid Chlorides: RCOCl + H2 (Pd-BaSO4) → RCHO + HCl (Rosenmund reduction)

3. From Gattermann-Koch synthesis: C6H6 + CO + HCl (AlCl3/CuCl) → C6H5CHO (benzaldehyde)

4. Ozonolysis: RCH=CHR + O3 → 2 RCHO (with Zn/H2O workup)

5. Etard's Reaction: C6H5CH3 + CrO2Cl2 → C6H5CHO (benzaldehyde from toluene)

6. From Grignard reagent + formate: RMgX + HCHO → RCH2OH (not aldehyde) RMgX + HC(OC2H5)2 → RCHO + 2C2H5OH (orthoformate synthesis)

Preparation of Ketones

1. Oxidation of Secondary Alcohols: R2CHOH + [O] → R2C=O Using K2Cr2O7/H2SO4 or KMnO4/H2SO4

2. From Acid Chlorides (Friedel-Crafts acylation): ArH + RCOCl (AlCl3) → ArCOR + HCl

3. From Grignard with acid chloride: RMgX + R'COCl → RCOR' (may give tertiary alcohol by 2nd addition) Better: RMgX + R'CN → imine → RCOR' (after hydrolysis)

4. Dry distillation of calcium salts: (RCOO)2Ca → RCOR + CaCO3 (CH3COO)2Ca → CH3COCH3 (acetone) + CaCO3

Physical Properties

• Polar molecules (C=O dipole)
• Higher bp than alkanes but lower than alcohols (no H-bonding)
• C=O can H-bond with water (as acceptor)
• Lower aldehydes (formaldehyde, acetaldehyde) miscible with water

Reactions of Aldehydes and Ketones

Reactions of Aldehydes and Ketones

Nucleophilic Addition to C=O

Carbon of C=O is electrophilic (partial +ve). Nucleophile attacks C.

1. Addition of HCN (Cyanohydrin formation): RCHO + HCN → RCH(OH)CN (alpha-hydroxynitrile/cyanohydrin) Catalyzed by base (generates CN- nucleophile) Aldehydes more reactive than ketones (less steric hindrance)

2. Addition of Sodium Bisulfite: RCHO + NaHSO3 → RCH(OH)SO3Na (sodium bisulfite addition product) Crystalline solid; used to purify/identify aldehydes and methyl ketones. Aldehydes and methyl ketones react; not bulky ketones.

3. Addition of Grignard Reagent: RMgX + HCHO → primary alcohol (RCH2OH) RMgX + RCHO → secondary alcohol RMgX + R2CO → tertiary alcohol

Condensation Reactions

4. Formation of Semicarbazone (with NH2NHCONH2): RCHO + NH2NHCONH2 → RCH=NNHCONH2 (semicarbazone, solid; used to identify carbonyl)

5. Aldol Condensation (with dilute NaOH): Aldehydes/ketones with alpha-H → self condensation: 2CH3CHO (dil. NaOH, cold) → CH3CH(OH)CH2CHO (3-hydroxybutanal, beta-hydroxyaldehyde) On heating: lose H2O → CH3CH=CHCHO (crotonaldehyde, conjugated)

Mixed Aldol (Cross Aldol): Between two different carbonyls; gives mixture (not preferred unless one lacks alpha-H)

6. Cannizzaro Reaction (no alpha-H): Aldehydes without alpha-H undergo disproportionation in conc. NaOH. 2HCHO + NaOH → HCOOH (formate) + CH3OH (methanol) 2C6H5CHO + NaOH → C6H5COOH (benzoate) + C6H5CH2OH (benzyl alcohol) One molecule oxidized (→ acid), one reduced (→ alcohol).

7. Clemmensen Reduction (Zn(Hg)/HCl): RCOR' → RCH2R' (reduce C=O to CH2) Used for acid-sensitive compounds (alternative to Wolff-Kishner).

8. Wolff-Kishner Reduction (NH2NH2 + KOH): RCOR' → RCH2R' (same as Clemmensen but for base-tolerant compounds)

9. Tollen's Test (Silver Mirror Test): RCHO + 2[Ag(NH3)2]+ + 2OH- → RCOO- + 2Ag↓ + 4NH3 + H2O Silver mirror on inner wall of clean test tube. Aldehydes positive; ketones negative (except cyclic ketones).

10. Fehling's Test: RCHO + 2Cu2+(Fehling) → RCOO- + Cu2O↓ (red precipitate) Aliphatic aldehydes positive; benzaldehyde and ketones negative.

Carboxylic Acids

Carboxylic Acids

Preparation

1. Oxidation: RCH2OH → RCHO → RCOOH (primary alcohol + KMnO4 or K2Cr2O7/H2SO4) RCHO + [O] → RCOOH (aldehydes easily oxidized) ArCH3 + KMnO4 → ArCOOH (ArCH3 toluene → C6H5COOH benzoic acid)

2. Carboxylation of Grignard Reagent: RMgX + CO2 → RCOOMgX → RCOOH + MgX(OH) (Dry CO2 passed through Grignard reagent, then hydrolysis)

3. Carbonylation of Methanol (Industrial Acetic Acid): CH3OH + CO → CH3COOH (Monsanto process, Rh catalyst)

4. Hydrolysis of Nitrile: RCN + H2O (H+) → RCONH2 → RCOOH (or conc. acid/base) RCN + H2O (KOH, then H+) → RCOOK → RCOOH

5. Hydrolysis of Ester/Amide: RCOOR' + NaOH → RCOONa → RCOOH

Physical Properties

• High bp (H-bonding and dimer formation)
• Acetic acid bp 118 C; formic acid bp 101 C
• Lower carboxylic acids miscible with water; higher ones insoluble
• Acidity: RCOOH (pKa 4-5) much stronger than ROH (pKa 16)

Acidic Character

Acidity: HCOOH > CH3COOH (methyl donates electrons, destabilizes carboxylate) Electron-withdrawing groups increase acidity: Cl3CCOOH > CCl2HCOOH > CClH2COOH > CH3COOH

Chemical Reactions

1. Salt formation: RCOOH + NaOH → RCOONa + H2O RCOOH + Na2CO3 → RCOONa + CO2 + H2O RCOOH + NaHCO3 → RCOONa + CO2 + H2O (phenols do NOT react with NaHCO3)

2. Esterification: RCOOH + R'OH ⇌ RCOOR' + H2O (H+ catalyst, reversible) (Fischer esterification)

3. Formation of Acid Chloride: RCOOH + SOCl2 → RCOCl + SO2 + HCl RCOOH + PCl5 → RCOCl + POCl3 + HCl

4. Decarboxylation: RCOONa + NaOH (CaO, heat) → RH + Na2CO3 (Soda lime) E.g., CH3COONa + NaOH → CH4 + Na2CO3

5. Hell-Volhard-Zelinsky (HVZ) Reaction: RCH2COOH + Br2 + PBr3 → RCH(Br)COOH + HBr Alpha-halogenation of carboxylic acid at alpha-carbon. P (phosphorus) activates alpha-C by converting RCOOH to RCOBr (acid bromide).

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