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Tamil Nadu Class 12 Chemistry Carbonyl Compounds and Carboxylic Acids — Samacheer Kalvi

Plus Two Chemistry notes on aldehydes, ketones, nucleophilic addition, Tollens test, aldol, Cannizzaro reaction, carboxylic acids, and acidity.

Updated 6 June 20263 min read

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Quick Summary

Carbonyl chemistry is about the polar C=O bond. Aldehydes and ketones undergo nucleophilic addition, while carboxylic acids show acidity and substitution reactions. Named reactions and conversion chains are especially important.

Aldehydes and Ketones

  • Carbonyl carbon is electrophilic because oxygen withdraws electron density.
  • Aldehydes are generally more reactive than ketones because of lower steric hindrance and lower +I effect.

Diagnostic Tests

TestAldehydeKetone
Tollens reagentsilver mirrorusually no reaction
Fehling solutionred precipitate for aliphatic aldehydesno reaction
2,4-DNPorange precipitateorange precipitate

Named Reactions

  • Aldol condensation: carbonyl compound with alpha hydrogen gives beta-hydroxy carbonyl product
  • Cannizzaro reaction: aldehyde without alpha hydrogen undergoes disproportionation
  • Clemmensen reduction: carbonyl -> hydrocarbon using Zn-Hg/HCl
  • Wolff-Kishner reduction: carbonyl -> hydrocarbon using hydrazine and base

Carboxylic Acids

  • Acidic because carboxylate ion is resonance-stabilised
  • Electron-withdrawing groups increase acidity
  • Form salts, esters, acid chlorides, amides, and undergo decarboxylation

Important Reactions

  • Esterification with alcohols
  • Hell-Volhard-Zelinsky reaction for alpha halogenation
  • Decarboxylation using soda lime

Key Reaction Equations

Aldol condensation (needs α-H): 2CH₃CHO →(dil. NaOH) CH₃CH(OH)CH₂CHO →(Δ, −H₂O) CH₃CH=CHCHO Cannizzaro (no α-H, e.g. HCHO): 2HCHO + NaOH → CH₃OH + HCOONa (one reduced, one oxidised) Tollens test (silver mirror): RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O Clemmensen reduction: R₂C=O + 4[H] →(Zn-Hg/HCl) R₂CH₂ + H₂O Esterification: CH₃COOH + C₂H₅OH ⇌(conc. H₂SO₄) CH₃COOC₂H₅ + H₂O HVZ reaction (α-halogenation): CH₃COOH + Cl₂ →(red P) ClCH₂COOH + HCl

Acidity Order (with reasoning)

HCOOH > CH₃COOH (alkyl +I effect reduces acidity) Cl-substituted acids are stronger: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH (electron-withdrawing Cl stabilises the carboxylate ion) Carboxylic acid > phenol > alcohol (carboxylate has stronger resonance)

Important Questions

One Mark

1. Name the reagent used in Tollens test. 2. Which carbonyl compounds undergo Cannizzaro reaction? 3. Why are aldehydes more reactive than ketones? 4. What is the product of esterification? 5. Name one reducing reagent for aldehydes or ketones.

Short Answer

1. Distinguish aldol and Cannizzaro reactions. 2. Explain why carboxylic acids are stronger acids than phenols. 3. Write tests to distinguish acetaldehyde and acetone. 4. Explain the effect of electron-withdrawing groups on acidity.

Long Answer

1. Discuss nucleophilic addition reactions of aldehydes and ketones. 2. Explain preparation and reactions of carboxylic acids. 3. Complete an organic conversion chain involving alcohol, aldehyde, acid, and ester.

Common Mistakes

  • Giving Fehling test to aromatic aldehydes without checking applicability
  • Forgetting alpha hydrogen requirement in aldol condensation
  • Writing Cannizzaro reaction for aldehydes that contain alpha hydrogen
  • Explaining acid strength without resonance

Revision Checklist

  • [ ] Carbonyl reactivity order
  • [ ] Tollens, Fehling, and 2,4-DNP tests
  • [ ] Aldol and Cannizzaro reactions
  • [ ] Carboxylic-acid acidity factors
  • [ ] Esterification and HVZ reaction

Frequently Asked Questions

Aldehydes usually give Tollens and Fehling tests because they are readily oxidised, while ordinary ketones do not.