Learning Objectives
- Understand the carbonyl group and its reactivity
- Learn nucleophilic addition reactions of aldehydes and ketones
- Study named reactions: Aldol, Cannizzaro, Wittig, Clemmensen, Wolff-Kishner
- Understand carboxylic acid preparation, acidity, and reactions
- Learn tests to distinguish aldehydes from ketones
Key Concepts
Aldehydes (RCHO) and Ketones (RCOR')
Both contain the carbonyl group (C=O). Aldehydes have at least one H on the carbonyl carbon; ketones have two carbon groups.
Preparation:
- Oxidation of alcohols: 1° → aldehyde (PCC/mild oxidant), 2° → ketone (CrO₃/KMnO₄).
- Ozonolysis of alkenes: gives aldehydes/ketones depending on substitution.
- Rosenmund reduction: RCOCl + H₂/Pd-BaSO₄ → RCHO + HCl.
- Stephen's reaction: RCN + SnCl₂/HCl → RCHO.
- Gattermann-Koch: ArH + CO + HCl → ArCHO (AlCl₃, CuCl catalyst).
- Friedel-Crafts acylation: ArH + RCOCl → ArCOR (AlCl₃).
Nucleophilic Addition Reactions
Mechanism: Nu⁻ attacks electrophilic carbonyl carbon → tetrahedral intermediate → product.
Reactivity: HCHO > other aldehydes > ketones (steric and electronic effects).
- HCN: R₂C=O + HCN → R₂C(OH)(CN) (cyanohydrin). Used to extend carbon chain.
- NaHSO₃: R₂C=O + NaHSO₃ → R₂C(OH)(SO₃Na). Used for purification.
- NH₃ derivatives: R₂C=O + NH₂-G → R₂C=N-G + H₂O (condensation).
- With NH₂OH → oxime; NH₂NH₂ → hydrazone; C₆H₅NHNH₂ → phenylhydrazone; 2,4-DNP → 2,4-dinitrophenylhydrazone (yellow/orange ppt, test for C=O).
- Grignard reagent: RMgX → secondary alcohol (from aldehyde), tertiary alcohol (from ketone).
Other Important Reactions
Aldol condensation: Aldehydes/ketones with α-H undergo base-catalysed self-condensation: 2CH₃CHO → CH₃CH(OH)CH₂CHO (aldol). On heating: → CH₃CH=CHCHO + H₂O (crotonaldehyde).
Cannizzaro reaction: Aldehydes without α-H (HCHO, C₆H₅CHO) undergo disproportionation in strong base: 2HCHO + NaOH → HCOONa + CH₃OH.
Clemmensen reduction: R₂C=O + Zn-Hg/conc. HCl → R₂CH₂ (reduces C=O to CH₂).
Wolff-Kishner reduction: R₂C=O + NH₂NH₂/KOH/glycol → R₂CH₂ (basic conditions).
Wittig reaction: R₂C=O + Ph₃P=CR'₂ → R₂C=CR'₂ + Ph₃P=O (forms alkene).
Distinction Between Aldehydes and Ketones
- Tollen's test (Silver mirror): RCHO + [Ag(NH₃)₂]⁺ → RCOO⁻ + Ag mirror. Ketones do not react.
- Fehling's test: RCHO + Cu²⁺ (tartrate) → RCOO⁻ + Cu₂O (red ppt). Ketones do not react.
- Iodoform test: CH₃CO-R + I₂/NaOH → CHI₃ (yellow ppt) + RCOONa. Given by methyl ketones and acetaldehyde.
Carboxylic Acids (RCOOH)
Preparation: Oxidation of primary alcohols/aldehydes. Hydrolysis of nitriles: RCN + H₃O⁺ → RCOOH. Grignard + CO₂: RMgX + CO₂ → RCOOH.
Acidity: RCOOH > H₂CO₃ > ArOH > ROH > H₂O.
Electron-withdrawing groups (EWG) increase acidity: CF₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
Electron-donating groups decrease acidity.
Reactions:
- Esterification: RCOOH + R'OH ⇌ RCOOR' + H₂O (H⁺ catalyst).
- Hell-Volhard-Zelinsky: RCOOH + X₂/P → α-haloacid (RCHXCOOHl).
- Decarboxylation: RCOONa + NaOH/CaO → RH + Na₂CO₃ (soda lime).
- Kolbe electrolysis: 2RCOO⁻ → R-R + 2CO₂ (at anode).
Summary
Aldehydes and ketones undergo nucleophilic addition at the C=O group. Aldehydes are more reactive than ketones. Named reactions (Aldol, Cannizzaro, Clemmensen, Wolff-Kishner) are important for JEE/NEET. Tollen's and Fehling's tests distinguish aldehydes from ketones. Carboxylic acids are the most acidic organic compounds, and their acidity is influenced by substituent effects.
Important Terms
- Nucleophilic Addition: Attack of nucleophile on electrophilic carbonyl carbon
- Aldol Condensation: Self-condensation of aldehydes/ketones with α-H
- Cannizzaro Reaction: Disproportionation of aldehydes without α-H
- Tollen's Test: Silver mirror test for aldehydes
- Iodoform Test: Test for CH₃CO- group (yellow CHI₃ precipitate)
- Hell-Volhard-Zelinsky: α-halogenation of carboxylic acids
Quick Revision
- Reactivity: HCHO > RCHO > RCOR' (nucleophilic addition)
- Tollen's: Ag mirror (aldehyde); Fehling's: red Cu₂O ppt (aldehyde)
- Iodoform: CH₃COR + I₂/NaOH → CHI₃ (yellow)
- Aldol: α-H required; Cannizzaro: no α-H required
- Clemmensen: Zn-Hg/HCl; Wolff-Kishner: NH₂NH₂/KOH
- Acidity: EWG increases; EDG decreases; RCOOH > ArOH > ROH