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Carboxylic Acids | CBSE - Wyatt's Notes

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  • text: Standard textbook reference

Carboxylic Acids

Section titled “Carboxylic Acids”

Carboxylic acids contain the −COOH-\text{COOH} functional group. They are weak acids due to resonance stabilization of the carboxylate ion.

Key Concepts

Section titled “Key Concepts”
  • General formula: RCOOH
  • Acidity: resonance stabilization of RCOO−^- (negative charge delocalized over two oxygens)
  • Electron-withdrawing groups increase acidity (e.g., Cl3_3CCOOH > CH3_3COOH)
  • Electron-donating groups decrease acidity
  • Reactions: esterification, reduction, amide formation, Hell-Volhard-Zelinsky reaction
  • Acid chlorides: RCOCl (from RCOOH + SOCl2_2 or PCl5_5)
  • Acid anhydrides: (RCO)2_2O (from 2 RCOOH with dehydration)
  • Esters: RCOOR’ (from RCOOH + R’OH with acid catalyst)

Worked Example 1 — Acidity Comparison

Section titled “Worked Example 1 — Acidity Comparison”

Problem: Arrange the following in decreasing order of acidity: acetic acid, chloroacetic acid, trichloroacetic acid.

Solution:

Acidity depends on the stability of the conjugate base (carboxylate ion).

  • Trichloroacetic acid (Cl3_3CCOOH): Three electron-withdrawing Cl atoms stabilize the carboxylate through inductive effect. pKa=0.65pK_a = 0.65

  • Chloroacetic acid (ClCH2_2COOH): One Cl atom provides moderate stabilization. pKa=2.86pK_a = 2.86

  • Acetic acid (CH3_3COOH): No electron-withdrawing groups. pKa=4.76pK_a = 4.76

Decreasing acidity: Cl3_3CCOOH > ClCH2_2COOH > CH3_3COOH

Common mistake: Assuming more carbon atoms means higher acidity. Inductive effects depend on electronegativity, not carbon chain length.

Worked Example 2 — Esterification

Section titled “Worked Example 2 — Esterification”

Problem: Write the reaction for the esterification of benzoic acid with ethanol.

Solution:

C6H5COOH+C2H5OH⇌H2SO4ΔC6H5COOC2H5+H2O\text{C}_6\text{H}_5\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightleftharpoons[\text{H}_2\text{SO}_4]{\Delta} \text{C}_6\text{H}_5\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}

Benzoic acid + ethanol ⇌\rightleftharpoons ethyl benzoate + water

The reaction is reversible. To drive it forward:

  1. Use excess alcohol or acid
  2. Remove water (using Dean-Stark trap or molecular sieves)
  3. Use concentrated H2_2SO4_4 as catalyst (absorbs water)

Common mistake: Writing the ester as C6H5OC2H5\text{C}_6\text{H}_5\text{OC}_2\text{H}_5 instead of C6H5COOC2H5\text{C}_6\text{H}_5\text{COOC}_2\text{H}_5. The ester linkage is −COO−-\text{COO}-, not −O−-\text{O}-.

Worked Example 3 — Hell-Volhard-Zelinsky Reaction

Section titled “Worked Example 3 — Hell-Volhard-Zelinsky Reaction”

Problem: What is the product of the reaction of propanoic acid with Br2_2 in the presence of P?

Solution:

The HVZ reaction brominates the alpha-carbon:

CH3CH2COOH+Br2→PCH3CHBrCOOH+HBr\text{CH}_3\text{CH}_2\text{COOH} + \text{Br}_2 \xrightarrow{\text{P}} \text{CH}_3\text{CHBrCOOH} + \text{HBr}

Product: 2-bromopropanoic acid

Mechanism: P reacts with Br2_2 to form PBr3_3, which converts RCOOH to RCOBr (acid bromide). The acid bromide undergoes enolization and bromination at the alpha position, then hydrolysis gives the alpha-bromo acid.

Common mistake: Brominating the beta-carbon or the aromatic ring. The HVZ reaction specifically targets the alpha-carbon.

Practice Problems

Section titled “Practice Problems”
  1. Arrange in order of acidity: formic acid, acetic acid, benzoic acid.
  2. Write the reaction of ethanoic acid with PCl5_5.
  3. How would you convert propanoic acid to propan-1-ol?

Why This Matters

Section titled “Why This Matters”

Carboxylic acids are found in vinegar (acetic acid), citrus fruits (citric acid), and aspirin (acetylsalicylic acid). They are key intermediates in organic synthesis and industrial chemistry.

Common Exam Patterns

Section titled “Common Exam Patterns”
  • Acidity: electron-withdrawing groups increase, electron-donating decrease
  • Esterification is reversible and acid-catalyzed
  • Acid chlorides are the most reactive carboxylic acid derivatives
  • HVZ reaction introduces halogen at the alpha-carbon
  • Decarboxylation: RCOOH + NaOH/CaO →\rightarrow RH (loss of CO2_2)

Intuition

Section titled “Intuition”

Resonance makes the carboxylate ion surprisingly stable: When a carboxylic acid loses its proton, the resulting negative charge doesn’t sit on one oxygen — it delocalizes equally across both oxygens through resonance. This spreading of charge makes carboxylate ions much more stable than alkoxide ions, which is why carboxylic acids are stronger acids than alcohols.

Why it matters: Carboxylic acids are the backbone of organic chemistry — from acetic acid in vinegar to citric acid in citrus fruits to aspirin. Their derivatives (acid chlorides, esters, amides) are the building blocks for pharmaceuticals, polymers, and biological molecules.

The key insight: Electron-withdrawing groups stabilize the conjugate base through the inductive effect, making the acid stronger — this explains why trichloroacetic acid is 10,000 times stronger than acetic acid.

Common Mistakes

Section titled “Common Mistakes”

Assuming more carbon atoms means higher acidity in carboxylic acids. Acidity depends on the stability of the carboxylate conjugate base, which is enhanced by electron-withdrawing groups through the inductive effect. Longer alkyl chains are weakly electron-donating, so formic acid (HCOOH) is more acidic than acetic acid (CH3COOH).

Writing the ester linkage incorrectly. The ester functional group is -COO-, not -O-. In ethyl ethanoate, the structure is CH3COOCH2CH3, where the carbonyl carbon connects to both the oxygen and the ethyl group. Students often write CH3OCH2CH3, which is an ether, not an ester.

Forgetting that the HVZ reaction targets the alpha-carbon. The Hell-Volhard-Zelinsky reaction specifically brominates the carbon adjacent to the carboxyl group. Students sometimes brominate the beta-carbon or the aromatic ring, but the enolisation mechanism ensures only the alpha-position is reactive.

See Also

Section titled “See Also”