Class 12 Chemistry Carboxylic Acid and its Derivatives Notes

Class 12 Chemistry Carboxylic Acid and its Derivatives Notes
Unit 14
Organic Chemistry
Class 12 Chemistry

Carboxylic Acid and its Derivatives

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NEB/CDC syllabus scope: Unit 14 is a 9-teaching-hour chapter. It includes nomenclature and isomerism of aliphatic and aromatic carboxylic acids; preparation of monocarboxylic acids from aldehydes, nitriles, dicarboxylic acids, sodium alkoxide and trihaloalkanes; preparation of benzoic acid from alkylbenzene; physical and chemical properties; Hell–Volhard–Zelinsky reaction; electrophilic substitution of benzoic acid; substituent effects on acidity; abnormal behaviour of methanoic acid; and the preparation, comparative properties and reactivity of acid halides, amides, esters and anhydrides, including Claisen condensation, Hofmann bromamide reaction and amphoteric nature of amides.

1. Introduction to Carboxylic Acids

Definition Carboxylic acids are organic compounds containing the carboxyl group, –COOH. The group combines a carbonyl unit (C=O) and hydroxyl unit (–OH) on the same carbon.
General formula: R–COOH

Examples include methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH) and benzoic acid (C₆H₅COOH).

Structure of the Carboxyl Group C O OH R R–C(=O)–OH = R–COOH

Diagram 1: The carboxyl functional group

2. Nomenclature and Isomerism

2.1 IUPAC Nomenclature

For open-chain monocarboxylic acids, choose the longest chain containing –COOH and replace the final “e” of the parent alkane by –oic acid. The carboxyl carbon is carbon 1.

FormulaIUPAC nameCommon name
HCOOHMethanoic acidFormic acid
CH₃COOHEthanoic acidAcetic acid
C₂H₅COOHPropanoic acidPropionic acid
CH₃CH₂CH₂COOHButanoic acidButyric acid
C₆H₅COOHBenzoic acidBenzoic acid

2.2 Isomerism

  • Chain isomerism: different carbon skeletons.
  • Position isomerism: seen in substituted aromatic carboxylic acids.
  • Functional isomerism: carboxylic acids and esters can have the same molecular formula.
Functional isomers: C₂H₄O₂ CH₃COOH = ethanoic acid HCOOCH₃ = methyl methanoate

3. Preparation of Monocarboxylic Acids

3.1 From Aldehydes

RCHO + [O] → RCOOH CH₃CHO + [O] → CH₃COOH

3.2 From Nitriles

Acidic or alkaline hydrolysis of nitriles gives carboxylic acids or carboxylates.

RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺

3.3 From Dicarboxylic Acids by Decarboxylation

Suitable dicarboxylic acids can lose CO₂ on heating to give monocarboxylic acids.

HOOC–CH₂–COOH → CH₃COOH + CO₂

3.4 From Sodium Alkoxide / Related Carboxylation Route

In the school syllabus, carboxylation routes involving strongly nucleophilic carbon species are grouped under methods that ultimately produce a carboxylate followed by acidification. The key idea is formation of RCOO⁻ followed by H⁺ to give RCOOH.

3.5 From Trihaloalkanes

Compounds containing –CX₃ can undergo alkaline hydrolysis to carboxylates, which give acids after acidification.

RCCl₃ + 4NaOH → RCOONa + 3NaCl + 2H₂O RCOONa + HCl → RCOOH + NaCl
Preparation Routes to R–COOH R–COOH carboxylic acid Aldehyde oxidation Nitrile hydrolysis Dicarboxylic acid decarboxylation Trihaloalkane alkaline hydrolysis Carboxylate acidification

Diagram 2: Syllabus preparation routes of monocarboxylic acids

4. Preparation of Benzoic Acid from Alkylbenzene

Any alkylbenzene containing at least one benzylic hydrogen can be strongly oxidized at the side chain to benzoic acid.

C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O

Common oxidant: hot alkaline KMnO₄ followed by acidification.

Side-Chain Oxidation C₆H₅CH₃ toluene C₆H₅COOH benzoic acid hot KMnO₄ then H⁺ The entire oxidizable side chain becomes –COOH.

Diagram 3: Preparation of benzoic acid from toluene

5. Physical Properties of Monocarboxylic Acids

Carboxylic acids form strong hydrogen bonds and often associate as cyclic dimers.

PropertyTrend / explanation
StateLower acids are liquids; higher homologues may be solids.
Boiling pointHigh because of strong intermolecular hydrogen bonding and dimerization.
Water solubilityLower members are highly soluble; solubility decreases as hydrocarbon chain length increases.
OdourLower aliphatic acids often have sharp or pungent odours.
PolarityHigh due to two electronegative oxygen atoms and the polar O–H bond.
Hydrogen-Bonded Carboxylic Acid Dimer R–C(=O)–OH HO–C(=O)–R H-bond H-bond Two hydrogen bonds stabilize a cyclic dimer. This contributes to unusually high boiling points.

Diagram 4: Cyclic dimer of a carboxylic acid

6. Acidic Nature of Carboxylic Acids

RCOOH ⇌ RCOO⁻ + H⁺

Carboxylic acids are significantly more acidic than alcohols and phenols because the resulting carboxylate ion is strongly stabilized by resonance.

Resonance Stabilization of RCOO⁻ R–C(=O)–O⁻ ↔ R–C(–O⁻)=O The negative charge is delocalized equally over two oxygen atoms. Greater conjugate-base stability increases acidity.

Diagram 5: Resonance of the carboxylate ion

7. Chemical Properties of Carboxylic Acids

7.1 With Alkalis

RCOOH + NaOH → RCOONa + H₂O

7.2 With Metal Oxides

2RCOOH + MgO → (RCOO)₂Mg + H₂O

7.3 With Carbonates

2RCOOH + Na₂CO₃ → 2RCOONa + CO₂↑ + H₂O

7.4 With Bicarbonates

RCOOH + NaHCO₃ → RCOONa + CO₂↑ + H₂O
Important Test Effervescence of CO₂ with sodium bicarbonate is a useful qualitative indication of a carboxylic acid.

7.5 With PCl₃

3RCOOH + PCl₃ → 3RCOCl + H₃PO₃

7.6 Reduction with LiAlH₄

RCOOH + 4[H] → RCH₂OH + H₂O

7.7 Dehydration

Two carboxylic-acid molecules can lose water under suitable dehydrating conditions to form an acid anhydride.

2RCOOH → (RCO)₂O + H₂O

8. Hell–Volhard–Zelinsky (HVZ) Reaction

HVZ reaction Carboxylic acids containing an α-hydrogen undergo halogenation at the α-carbon when treated with halogen in the presence of red phosphorus or phosphorus halide.
RCH₂COOH + Br₂ → RCHBrCOOH + HBr   (red P)

Example

CH₃CH₂COOH + Br₂ → CH₃CHBrCOOH + HBr
Hell–Volhard–Zelinsky Reaction R–CH₂–COOH α-H present Br₂ red P R–CHBr–COOH α-bromo acid + HBr Halogen replaces an α-hydrogen next to –COOH.

Diagram 6: α-Bromination by HVZ reaction

9. Electrophilic Substitution of Benzoic Acid

The –COOH group withdraws electron density from the benzene ring. It is deactivating and meta directing.

Nitration

C₆H₅COOH + HNO₃ → m-NO₂C₆H₄COOH + H₂O   (H₂SO₄)

Bromination

C₆H₅COOH + Br₂ → m-BrC₆H₄COOH + HBr   (FeBr₃)

Sulphonation

C₆H₅COOH + H₂SO₄(fuming) → m-HO₃SC₆H₄COOH + H₂O
–COOH Is Deactivating and Meta Directing COOH meta meta Nitration, bromination and sulphonation occur mainly at meta positions.

Diagram 7: Meta orientation in benzoic acid

10. Effect of Substituents on Acidic Strength

Anything that stabilizes the carboxylate ion increases acidity; anything that destabilizes it decreases acidity.

Substituent effectExampleEffect on acidity
Electron-withdrawing (−I)Cl, F, NO₂Increases acidity
Electron-donating (+I)Alkyl groupsDecreases acidity
Closer EWG to –COOHClCH₂COOH vs CH₃CHClCH₂COOHCloser group has stronger effect
More EWG groupsCCl₃COOH vs CH₂ClCOOHMore withdrawal → stronger acid
Typical order CCl₃COOH > CH₂ClCOOH > CH₃COOH

11. Abnormal Behaviour of Methanoic Acid

Methanoic acid, HCOOH, is unusual because its carboxyl carbon is directly bonded to hydrogen. It therefore shows some reducing behaviour resembling aldehydes.

11.1 Tollens’ Reagent

Methanoic acid can reduce Tollens’ reagent to metallic silver while itself being oxidized to CO₂.

11.2 Oxidation

HCOOH + [O] → CO₂ + H₂O

11.3 Dehydration

HCOOH → CO + H₂O   (conc. H₂SO₄)
Safety Carbon monoxide is acutely toxic. The dehydration equation is a theoretical/syllabus reaction and should not be attempted outside a properly controlled laboratory.

12. Derivatives of Carboxylic Acids

Important derivatives are obtained by replacing the –OH portion of –COOH with another group.

DerivativeGeneral formulaExample
Acid halideRCOXCH₃COCl
Acid anhydride(RCO)₂O(CH₃CO)₂O
EsterRCOOR′CH₃COOC₂H₅
AmideRCONH₂CH₃CONH₂
Carboxylic Acid Derivative Family R–COOH carboxylic acid RCOX acid halide (RCO)₂O anhydride RCOOR′ ester RCONH₂ amide All contain an acyl group, R–C(=O)–, bonded to a heteroatom-containing leaving group.

Diagram 8: Main carboxylic-acid derivatives

13. Preparation of Acid Derivatives from Carboxylic Acid

13.1 Acid Chloride

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Other phosphorus chlorides may also be used.

13.2 Acid Anhydride

2RCOOH → (RCO)₂O + H₂O   (dehydrating conditions)

13.3 Ester

RCOOH + R′OH ⇌ RCOOR′ + H₂O   (H⁺)

13.4 Amide

Carboxylic acid reacts with ammonia to form an ammonium carboxylate, which yields an amide on heating.

RCOOH + NH₃ → RCOO⁻NH₄⁺ → RCONH₂ + H₂O
Preparation of Acid Derivatives RCOOH carboxylic acid RCOCl SOCl₂ (RCO)₂O dehydration RCOOR′ R′OH / H⁺ RCONH₂ NH₃ / heat

Diagram 9: Conversion of RCOOH into its major derivatives

14. Comparative Physical and Chemical Properties of Acid Derivatives

14.1 Comparative Physical Properties

DerivativeHydrogen-bond abilityGeneral physical tendency
Acid halideNo O–H/N–H donorOften volatile, reactive liquids; lower boiling than amides
AnhydrideNo donorPolar; often liquids/solids with characteristic reactivity
EsterNo donor, but oxygen accepts H-bondsOften volatile with characteristic odours; lower bp than acids/amides of similar size
AmideStrong H-bonding when N–H presentUsually higher melting/boiling; often strongly polar

14.2 Hydrolysis

RCOCl + H₂O → RCOOH + HCl (RCO)₂O + H₂O → 2RCOOH RCOOR′ + H₂O ⇌ RCOOH + R′OH RCONH₂ + H₂O → RCOOH + NH₃   (acid/base, heat)

14.3 Ammonolysis

RCOCl + 2NH₃ → RCONH₂ + NH₄Cl RCOOR′ + NH₃ → RCONH₂ + R′OH

14.4 Reaction with Amines

RCOCl + 2R′NH₂ → RCONHR′ + R′NH₃Cl

14.5 Alcoholysis

RCOCl + R′OH → RCOOR′ + HCl (RCO)₂O + R′OH → RCOOR′ + RCOOH

14.6 Reduction

Strong hydride reduction commonly converts acid derivatives to alcohols; amides can be reduced to amines.

RCOOR′ → RCH₂OH + R′OH   (LiAlH₄) RCONH₂ → RCH₂NH₂   (LiAlH₄)

15. Relative Reactivity of Acid Derivatives

Acid halide > Acid anhydride > Ester > Amide

The trend mainly reflects the ability of the attached group to leave during nucleophilic acyl substitution and how strongly it donates electron density by resonance.

Relative Reactivity toward Nucleophilic Acyl Substitution RCOCl highest (RCO)₂O anhydride RCOOR′ ester RCONH₂ lowest RCOCl > (RCO)₂O > RCOOR′ > RCONH₂ Better leaving group + weaker resonance donation → greater acyl reactivity.

Diagram 10: Relative reactivity of acid derivatives

16. Claisen Condensation

Claisen condensation Esters containing an α-hydrogen undergo base-promoted condensation to form a β-keto ester.

Example: Ethyl Ethanoate

2CH₃COOC₂H₅ → CH₃COCH₂COOC₂H₅ + C₂H₅OH   (C₂H₅ONa)

The product is ethyl 3-oxobutanoate (ethyl acetoacetate).

Claisen Condensation 2 CH₃COOC₂H₅ ethyl ethanoate C₂H₅ONa base CH₃COCH₂COOC₂H₅ β-keto ester + C₂H₅OH Key requirement: an ester with α-hydrogen.

Diagram 11: Claisen condensation of ethyl ethanoate

17. Hofmann Bromamide Reaction

Hofmann bromamide degradation A primary amide reacts with bromine and strong alkali to form a primary amine containing one carbon atom fewer than the original amide.
RCONH₂ + Br₂ + 4NaOH → RNH₂ + 2NaBr + Na₂CO₃ + 2H₂O

Example

CH₃CONH₂ → CH₃NH₂
Hofmann Bromamide: One Carbon Is Lost R–CONH₂ primary amide Br₂ NaOH R–NH₂ primary amine one fewer carbon The carbonyl carbon is removed as carbonate/CO₂-derived inorganic carbon.

Diagram 12: Hofmann bromamide degradation

18. Amphoteric Nature of Amides

Amides have both very weak basic and weak acidic character. The oxygen can be protonated in strong acid, while an N–H proton can be removed only by sufficiently strong bases.

As a Weak Base

RCONH₂ + H⁺ ⇌ [RCONH₂H]⁺

As a Weak Acid

Amides containing N–H can react with very strong bases to form an amide anion.

Why amides are weak bases The nitrogen lone pair is delocalized into the carbonyl group by resonance, making it much less available to bind H⁺ than the lone pair of an ordinary amine.
Amide Resonance and Weak Basicity R–C(=O)–NH₂ ↔ R–C(–O⁻)=NH₂⁺ Nitrogen lone-pair delocalization reduces its basicity. This resonance also contributes to the stability and low reactivity of amides.

Diagram 13: Resonance explanation of amide properties

19. Common Exam Mistakes

  • Forgetting that the carboxyl carbon is carbon 1 in IUPAC numbering.
  • Confusing carboxylic acids with esters when both have two oxygen atoms.
  • Writing carboxylate resonance with unequal oxygen atoms; the two major resonance contributors are equivalent for an unsubstituted carboxylate group.
  • Forgetting CO₂ evolution with NaHCO₃.
  • Applying HVZ to a carboxylic acid with no α-hydrogen.
  • Writing –COOH as ortho/para directing. It is deactivating and meta directing.
  • Ignoring the unusual reducing behaviour of methanoic acid.
  • Confusing acid chloride and alkyl chloride reactivity; acyl chlorides undergo nucleophilic acyl substitution readily.
  • Using the wrong reactivity order. Remember: acid halide > anhydride > ester > amide.
  • Forgetting that hydrolysis of an ester is generally slower than hydrolysis of an acid chloride.
  • Calling Claisen condensation an aldol reaction; Claisen involves esters and gives a β-keto ester.
  • Forgetting the one-carbon loss in Hofmann bromamide reaction.
  • Saying amides are strongly basic like amines. Resonance makes amides much weaker bases.

20. Important Exam Questions

Short-Answer Questions

  1. Define carboxylic acid and write its general formula.
  2. Give IUPAC names of HCOOH, CH₃COOH and C₆H₅COOH.
  3. What is functional isomerism between acid and ester?
  4. How is a carboxylic acid prepared from an aldehyde?
  5. How is a carboxylic acid prepared from a nitrile?
  6. How is benzoic acid prepared from toluene?
  7. Why do carboxylic acids have high boiling points?
  8. Why are carboxylic acids more acidic than alcohols?
  9. Write the reaction of ethanoic acid with NaHCO₃.
  10. What is the HVZ reaction?
  11. Why is –COOH meta directing?
  12. How do electron-withdrawing groups affect acidity?
  13. Why is methanoic acid abnormal?
  14. Name the four major derivatives of carboxylic acid.
  15. How is an acid chloride prepared from a carboxylic acid?
  16. How is an ester prepared from carboxylic acid?
  17. Write the relative reactivity order of acid derivatives.
  18. What is Claisen condensation?
  19. What is Hofmann bromamide reaction?
  20. Explain the amphoteric nature of amide.

Long-Answer Questions

  1. Describe preparation of monocarboxylic acids from aldehydes, nitriles, dicarboxylic acids and trihaloalkanes.
  2. Explain the physical properties of carboxylic acids using hydrogen-bonded dimer formation.
  3. Explain the acidic nature of carboxylic acids using resonance of carboxylate ion.
  4. Describe reactions of carboxylic acids with alkalis, metal oxides, carbonates, bicarbonates, PCl₃ and LiAlH₄.
  5. Explain HVZ reaction with an example.
  6. Describe nitration, bromination and sulphonation of benzoic acid.
  7. Explain substituent effects on carboxylic-acid strength.
  8. Describe abnormal behaviour of methanoic acid.
  9. Explain preparation of acid halides, anhydrides, esters and amides from carboxylic acid.
  10. Compare hydrolysis, ammonolysis, alcoholysis and reduction of acid derivatives.
  11. Explain the relative reactivity of acid derivatives.
  12. Explain Claisen condensation and Hofmann bromamide reaction.
  13. Explain why amides are amphoteric and weakly basic.

Conversion Questions

  1. Ethanal → ethanoic acid.
  2. Ethanenitrile → ethanoic acid.
  3. Toluene → benzoic acid.
  4. Ethanoic acid → ethanoyl chloride.
  5. Ethanoic acid → ethanoic anhydride.
  6. Ethanoic acid → ethyl ethanoate.
  7. Ethanoic acid → ethanamide.
  8. Ethanoic acid → ethanol.
  9. Ethanamide → methylamine.
  10. Ethyl ethanoate → ethyl 3-oxobutanoate.

Diagram Practice Questions

  1. Draw the carboxyl group.
  2. Draw preparation routes of carboxylic acids.
  3. Draw toluene → benzoic acid oxidation.
  4. Draw the hydrogen-bonded carboxylic-acid dimer.
  5. Draw resonance structures of carboxylate ion.
  6. Draw HVZ reaction.
  7. Draw meta-directing orientation of –COOH.
  8. Draw the family tree of acid derivatives.
  9. Draw derivative-preparation map.
  10. Draw relative reactivity order.
  11. Draw Claisen condensation.
  12. Draw Hofmann bromamide reaction.
  13. Draw amide resonance.
Exam Strategy For this chapter, divide revision into two blocks: (A) carboxylic acids and (B) acid derivatives. Learn each named reaction with reagent, condition and product, then memorize the reactivity order and acidity explanations.

21. One-Minute Revision

  • Carboxylic acid functional group = –COOH.
  • General form = RCOOH.
  • Carboxylate ion is resonance stabilized, explaining acidity.
  • Carboxylic acids form hydrogen-bonded dimers and have high boiling points.
  • Aldehydes oxidize to carboxylic acids.
  • Nitriles hydrolyze to carboxylic acids.
  • Alkylbenzene side-chain oxidation gives benzoic acid if a benzylic H is present.
  • RCOOH + NaHCO₃ gives CO₂ effervescence.
  • LiAlH₄ reduces RCOOH to RCH₂OH.
  • HVZ gives α-halocarboxylic acid and requires α-H.
  • –COOH is deactivating and meta directing.
  • Electron-withdrawing groups increase acidity.
  • Methanoic acid shows reducing behaviour and can be oxidized to CO₂.
  • Main derivatives: acid halide, anhydride, ester, amide.
  • Acid halide is the most reactive common derivative.
  • Reactivity: RCOX > anhydride > ester > amide.
  • Acid chloride hydrolyses rapidly to acid.
  • Ammonolysis can form amides.
  • Alcoholysis forms esters.
  • Claisen condensation of an ester with α-H gives a β-keto ester.
  • Hofmann bromamide converts RCONH₂ to RNH₂ with one fewer carbon.
  • Amides are weakly amphoteric and resonance stabilized.

22. Diagram Practice

Practice these labelled diagrams:

  1. Carboxyl group structure.
  2. Preparation routes of monocarboxylic acids.
  3. Oxidation of alkylbenzene to benzoic acid.
  4. Hydrogen-bonded acid dimer.
  5. Carboxylate resonance.
  6. HVZ α-halogenation.
  7. Meta-directing effect of –COOH.
  8. Acid-derivative family tree.
  9. Preparation of acid derivatives.
  10. Relative reactivity order of derivatives.
  11. Claisen condensation.
  12. Hofmann bromamide degradation.
  13. Amide resonance and amphoteric character.
Source handling: The original Nepal eNotes PDF remains embedded above. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.
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