Why are carboxylic acids acidic?

The carboxylate ion formed after deprotonation is stabilised by resonance over two oxygen atoms.

Class 12

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

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

Test	Aldehyde	Ketone
Tollens reagent	silver mirror	usually no reaction
Fehling solution	red precipitate for aliphatic aldehydes	no reaction
2,4-DNP	orange precipitate	orange 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.