CUET UG Chemistry Organic Chemistry Carboxylic Acids and Derivatives Acidity Order Reactions

Must-Know (15–20 detailed bullets)

• Carboxylic acids are more acidic than alcohols and phenols due to resonance stabilization of the carboxylate ion; e.g., acetic acid (pKa ≈ 4.76) is stronger than phenol (pKa ≈ 10).
• Electron-withdrawing groups (e.g., –Cl, –NO₂) increase acidity of carboxylic acids by stabilizing the conjugate base; e.g., Cl₃CCOOH (pKa = 0.65) is stronger than CH₃COOH.
• Electron-donating groups (e.g., –CH₃) decrease acidity; e.g., CH₃CH₂COOH (pKa = 4.87) is slightly weaker than CH₃COOH (pKa = 4.76).
• Formic acid (HCOOH) is stronger than acetic acid (CH₃COOH) because the +I effect of –CH₃ destabilizes the acetate ion.
• Benzoic acid (pKa = 4.20) is stronger than acetic acid due to resonance withdrawal of electrons by the phenyl ring.
• The order of acidity: HCOOH > CH₃COOH > CH₃CH₂COOH (due to increasing +I effect).
• Carboxylic acids react with NaHCO₃ to produce CO₂ gas – a test to distinguish them from phenols, which do not.
• Esterification: RCOOH + R'OH ⇌ RCOOR' + H₂O in presence of conc. H₂SO₄ (Fischer esterification).
• Acyl chlorides are prepared from carboxylic acids using SOCl₂, PCl₅, or PCl₃; e.g., CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl.
• Amides are formed when carboxylic acids react with ammonia or amines followed by heating; e.g., CH₃COOH + NH₃ → CH₃COONH₄ → CH₃CONH₂ + H₂O on heating.
• Decarboxylation of sodium salts of carboxylic acids with soda lime (NaOH + CaO) gives alkanes; e.g., CH₃COONa + NaOH → CH₄ + Na₂CO₃.
• Kolbe’s electrolytic method: Electrolysis of sodium or potassium salt of carboxylic acid gives alkane; e.g., 2CH₃COO⁻ → CH₃–CH₃ + 2CO₂ at anode.
• Reduction of carboxylic acids to primary alcohols occurs with LiAlH₄ (not NaBH₄); e.g., CH₃COOH → CH₃CH₂OH via LiAlH₄.
• Hell-Volhard-Zelinsky (HVZ) reaction: α-halogenation of carboxylic acids using X₂ and red phosphorus; e.g., CH₃CH₂COOH → CH₃CHBrCOOH with Br₂/P.
• Carboxylic acids undergo nucleophilic acyl substitution due to the presence of polar carbonyl group; reactivity order: acyl chlorides > anhydrides > esters > amides.
• Acid anhydrides are formed by dehydration of two carboxylic acid molecules with P₂O₅; e.g., (CH₃COOH)₂ → (CH₃CO)₂O + H₂O.
• Hydrolysis of esters in acidic medium gives carboxylic acid and alcohol; in basic medium (saponification), it gives carboxylate salt and alcohol.
• The pKa of carboxylic acids typically ranges from 3 to 5; verify from NCERT for specific values.
• Aromatic carboxylic acids undergo electrophilic substitution at meta position due to electron-withdrawing nature of –COOH group; e.g., nitration of benzoic acid gives m-nitrobenzoic acid.
• Diborane (B₂H₆) does not reduce carboxylic acids, but LiAlH₄ does – a key distinction in reduction reactions.

Difficulty Level

Intermediate — requires understanding of electronic effects, reaction mechanisms, and comparative acidity, but most reactions are directly from NCERT Class 12, Chapter 12.

Common CUET Traps (3 bullets)

• Trap: Assuming phenols are stronger acids than carboxylic acids because of resonance. Avoid: Carboxylic acids are stronger due to better resonance delocalization in carboxylate ion and inductive effect.
• Trap: Using NaBH₄ to reduce carboxylic acids to alcohols. Avoid: Only LiAlH₄ reduces carboxylic acids; NaBH₄ is ineffective.
• Trap: Thinking HVZ reaction occurs with all carboxylic acids regardless of α-hydrogen. Avoid: HVZ requires α-hydrogen; no reaction if α-carbon has no H (e.g., (CH₃)₃CCOOH).

Practice MCQs (5 questions)

Q1. Which of the following is the strongest acid?
A) CH₃CH₂OH
B) CH₃COOH
C) H₂O
D) C₆H₅OH

Answer: B
Explanation: CH₃COOH (pKa ~4.76) is stronger than phenol (pKa ~10), ethanol (pKa ~15.9), and water (pKa 15.7).
Why others fail: Phenol is often mistaken as stronger due to resonance, but carboxylic acids are more acidic.

Q2. Which reagent converts benzoic acid to benzoyl chloride?
A) Cl₂/hv
B) SOCl₂
C) PCl₃
D) Both B and C

Answer: D
Explanation: SOCl₂, PCl₃, and PCl₅ all convert –COOH to –COCl; NCERT lists all three.
Why others fail: Students often select only SOCl₂, missing that PCl₃ is also valid.

Q3. The HVZ reaction involves:
A) Bromination at β-carbon using Br₂ and light
B) Bromination at α-carbon using Br₂ and red P
C) Bromination at α-carbon using Br₂ alone
D) Bromination of aromatic ring with Br₂/FeBr₃

Answer: B
Explanation: HVZ reaction specifically involves α-bromination with Br₂ and red phosphorus.
Why others fail: Confusion with free radical halogenation (Br₂/hv) at β-carbon.

Q4. Which of the following does NOT reduce carboxylic acids?
A) LiAlH₄
B) B₂H₆
C) H₂/Ni
D) NaBH₄

Answer: D
Explanation: NaBH₄ cannot reduce carboxylic acids; only LiAlH₄ does among common hydrides.
Why others fail: NaBH₄ reduces aldehydes/ketones, so students assume it works for acids too.

Q5. Arrange in increasing order of acidity: (i) ClCH₂COOH, (ii) CH₃COOH, (iii) Cl₂CHCOOH, (iv) Cl₃CCOOH
A) ii < i < iii < iv
B) iv < iii < i < ii
C) i < ii < iii < iv
D) ii < iii < i < iv

Answer: A
Explanation: More Cl atoms increase –I effect, increasing acidity: CH₃COOH < ClCH₂COOH < Cl₂CHCOOH < Cl₃CCOOH.
Why others fail: Misjudging the cumulative effect of electron-withdrawing groups.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ Acidity order: HCOOH > ClCH₂COOH > CH₃COOH — verify from NCERT for exact pKa.
• ⚠️ Carboxylic acids turn blue litmus red — basic test.
• ⚠️ Only LiAlH₄ reduces –COOH to –CH₂OH; NaBH₄ does not.
• ⚠️ HVZ reaction: α-halogenation with X₂ + red P — requires α-H.
• ⚠️ Kolbe’s electrolysis: 2RCOO⁻ → R–R + 2CO₂ at anode.
• ⚠️ Soda lime decarboxylation: RCOONa + NaOH → RH + Na₂CO₃.
• ⚠️ Esterification is nucleophilic acyl substitution with alcohol + acid catalyst.
• ⚠️ Benzoic acid nitration gives m-nitrobenzoic acid — meta-directing –COOH.
• ⚠️ Acyl chlorides most reactive in nucleophilic substitution; amides least.
• ⚠️ pKa of carboxylic acids ~3–5; phenol ~10; alcohols ~15–18.
• ⚠️ CO₂ evolution with NaHCO₃ confirms carboxylic acid — not phenol.
• ⚠️ Formic acid has aldehyde-like behavior — reduces Tollens’ reagent.
• ⚠️ (CH₃)₂CHCOOH undergoes HVZ; (CH₃)₃CCOOH does not — no α-H.
• ⚠️ Anhydride formation: 2RCOOH + P₂O₅ → (RCO)₂O.
• ⚠️ Saponification: ester + NaOH → RCOO⁻Na⁺ + R'OH.
• ⚠️ Diborane reduces only carbonyls, not –COOH — verify from NCERT.
• ⚠️ Electron-withdrawing groups ↑ acidity; donating groups ↓ acidity.
• ⚠️ Fischer esterification is reversible; saponification is irreversible.
• ⚠️ Amide formation: RCOOH + NH₃ → RCOONH₄ → RCONH₂ on heating.
• ⚠️ Mnemonic: LiAlH₄ = Loves All Hydrogens — reduces acids, aldehydes, ketones, esters.