The carbonyl group (C=O) unifies this chapter. Aldehydes (–CHO) have at least one H on the carbonyl carbon; ketones (R–CO–R') have two carbon groups; carboxylic acids (–COOH) have carbonyl bonded to –OH.
Structure and Bonding
The carbonyl carbon is sp2 hybridised and trigonal planar. The C=O bond is polar (O is delta–, C is delta+), so the carbonyl carbon is electrophilic → susceptible to nucleophilic addition.
Preparation
Aldehydes: (1) Oxidation of 1° alcohols with PCC (stops at aldehyde). (2) Rosenmund: RCOCl + H2 -(Pd/BaSO4)→ RCHO. (3) Stephen reaction: RCN + SnCl2/HCl → RCHO.
Ketones: (1) Oxidation of 2° alcohols. (2) Friedel-Crafts acylation: ArH + RCOCl -(AlCl3)→ Ar–CO–R.
Carboxylic acids: (1) Oxidation of 1° alcohols or aldehydes with KMnO4. (2) Hydrolysis of nitriles, esters, or amides. (3) Grignard + CO2: RMgX + CO2 → RCOOH.
Reactions of Aldehydes and Ketones
- Nucleophilic addition:
- With HCN → cyanohydrin (used for chain extension).
- With NaHSO3 → bisulphite addition product (purification of aldehydes).
- With Grignard (RMgX) → secondary or tertiary alcohol after H3O^+ workup.
- With NH2OH → oxime; 2,4-DNP reagent → orange precipitate (identification test).
Aldol condensation: 2 CH3CHO -(dil. NaOH)→ 3-hydroxybutanal (aldol) → heat → but-2-enal. Requires alpha-H.
Cannizzaro reaction: Aldehydes with NO alpha-H (HCHO, C6H5CHO) disproportionate with conc. NaOH: one oxidised to acid, one reduced to alcohol.
Clemmensen reduction: C=O → CH2 (Zn-Hg / HCl, acidic).
Wolff-Kishner reduction: C=O → CH2 (NH2NH2 / KOH / ethylene glycol, basic).
- Distinguishing tests:
- Tollens: Aldehydes → silver mirror; ketones do not react.
- Fehling: Aliphatic aldehydes → red Cu2O precipitate; ketones and aromatic aldehydes do not.
- Iodoform test: CH3CO– group → yellow CHI3 precipitate with I2/NaOH. Positive for CH3CHO, CH3COCH3, C2H5OH, propan-2-ol.
Carboxylic Acids
Acidity: Carboxylate anion stabilised by resonance (both C–O bonds equivalent). EWG near –COOH increases acidity; EDG decreases it. Order: Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH.
- 1.Reactions:
- 2.Esterification: RCOOH + R'OH -(H^+)→ RCOOR' + H2O (reversible).
- 3.Acid chloride: RCOOH + SOCl2 → RCOCl + SO2 + HCl.
- 4.Decarboxylation: RCOONa + NaOH -(sodalime)→ R–H + Na2CO3.
- 5.HVZ reaction: RCOOH + Br2 -(P)→ alpha-bromoacid.
- 6.Reduction: RCOOH + LiAlH4 → RCH2OH (NaBH4 does NOT reduce –COOH).
Worked Examples
Which gives silver mirror with Tollens reagent — CH3COCH3 or CH3CHO?
Tollens reagent is reduced only by aldehydes.
Answer: CH3CHO gives silver mirror; CH3COCH3 (ketone) does not.
Which undergoes Cannizzaro reaction — CH3CHO or HCHO?
Cannizzaro requires no alpha-H. CH3CHO has alpha-H (aldol); HCHO has none.
Answer: HCHO: 2HCHO + NaOH → HCOONa + CH3OH.
Increasing acidity: acetic acid, chloroacetic acid, dichloroacetic acid.
More Cl = greater electron withdrawal = more stable COO^- = stronger acid.
Answer: CH3COOH < ClCH2COOH < Cl2CHCOOH.
Clemmensen reduction of acetophenone.
C6H5COCH3 + Zn-Hg/HCl → C6H5CH2CH3.
Answer: Ethylbenzene.
Aldol condensation of acetaldehyde.
Dil. NaOH → enolate attacks second CH3CHO → 3-hydroxybutanal; heating → but-2-enal.
Answer: But-2-enal (crotonaldehyde) on heating; 3-hydroxybutanal without heating.
Prepare propanoic acid from ethyl cyanide.
CH3CH2CN + H2O -(H^+)→ CH3CH2COOH.
Answer: Propanoic acid.
Iodoform test on propan-2-ol?
Propan-2-ol is first oxidised to CH3COCH3 (methyl ketone), which has CH3CO– group → yellow CHI3 precipitate.
Answer: Positive iodoform test — yellow CHI3 forms.
Common mistakes
> Watch out: Fehling solution does NOT react with ketones or aromatic aldehydes. NaBH4 does NOT reduce carboxylic acids (use LiAlH4). Iodoform test is positive for CH3CO– compounds AND their precursors (ethanol, propan-2-ol) — not just ketones.
Summary
Aldehydes are more reactive than ketones (less steric hindrance and electron donation). Key tests: Tollens (silver mirror for aldehydes), Fehling (aliphatic aldehydes only), iodoform (CH3CO– group). Carboxylic acids are acidic due to resonance-stabilised –COO^-; EWG increases acidity. Key reactions: esterification, HVZ, LiAlH4 reduction.