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, –NO?) increase acidity of carboxylic acids by stabilizing the conjugate base; e.g., Cl?CCOOH (pKa = 0.65) is stronger than CH?COOH.
• Electron-donating groups (e.g., –CH?) decrease acidity; e.g., CH?CH?COOH (pKa = 4.87) is slightly weaker than CH?COOH (pKa = 4.76).
• Formic acid (HCOOH) is stronger than acetic acid (CH?COOH) because the +I effect of –CH? 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 > CH?COOH > CH?CH?COOH (due to increasing +I effect).
• Carboxylic acids react with NaHCO? to produce CO? gas – a test to distinguish them from phenols, which do not.
• Esterification: RCOOH + R'OH-RCOOR' + H?O in presence of conc. H?SO? (Fischer esterification).
• Acyl chlorides are prepared from carboxylic acids using SOCl?, PCl?, or PCl?; e.g., CH?COOH + SOCl?-CH?COCl + SO? + HCl.
• Amides are formed when carboxylic acids react with ammonia or amines followed by heating; e.g., CH?COOH + NH?-CH?COONH?-CH?CONH? + H?O on heating.
• Decarboxylation of sodium salts of carboxylic acids with soda lime (NaOH + CaO) gives alkanes; e.g., CH?COONa + NaOH-CH? + Na?CO?.
• Kolbe’s electrolytic method: Electrolysis of sodium or potassium salt of carboxylic acid gives alkane; e.g., 2CH?COO?-CH?–CH? + 2CO? at anode.
• Reduction of carboxylic acids to primary alcohols occurs with LiAlH? (not NaBH?); e.g., CH?COOH-CH?CH?OH via LiAlH?.
• Hell-Volhard-Zelinsky (HVZ) reaction: ?-halogenation of carboxylic acids using X? and red phosphorus; e.g., CH?CH?COOH-CH?CHBrCOOH with Br?/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 P?O?; e.g., (CH?COOH) (CH?CO)?O + H?O.
• 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 (B?H?) does not reduce carboxylic acids, but LiAlH? 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 NaBH? to reduce carboxylic acids to alcohols. Avoid: Only LiAlH? reduces carboxylic acids; NaBH? is ineffective.
• Trap: Thinking HVZ reaction occurs with all carboxylic acids regardless of ?-hydrogen. Avoid: HVZ requires ?-hydrogen; no reaction if ?-carbon has no H (e.g., (CH?)?CCOOH).

Practice MCQs (5 questions)

Q1. Which of the following is the strongest acid?
A) CH?CH?OH
B) CH?COOH
C) H?O
D) C?H?OH

Answer: B
Explanation: CH?COOH (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) Cl?/hv
B) SOCl?
C) PCl?
D) Both B and C

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

Q3. The HVZ reaction involves:
A) Bromination at ?-carbon using Br? and light
B) Bromination at ?-carbon using Br? and red P
C) Bromination at ?-carbon using Br? alone
D) Bromination of aromatic ring with Br?/FeBr?

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

Q4. Which of the following does NOT reduce carboxylic acids?
A) LiAlH?
B) B?H?
C) H?/Ni
D) NaBH?

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

Q5. Arrange in increasing order of acidity: (i) ClCH?COOH, (ii) CH?COOH, (iii) Cl?CHCOOH, (iv) Cl?CCOOH
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: CH?COOH < ClCH?COOH < Cl?CHCOOH < Cl?CCOOH.
Why others fail: Misjudging the cumulative effect of electron-withdrawing groups.

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

• Acidity order: HCOOH > ClCH?COOH > CH?COOH — verify from NCERT for exact pKa.
• Carboxylic acids turn blue litmus red — basic test.
• Only LiAlH? reduces –COOH to –CH?OH; NaBH? does not.
• HVZ reaction: ?-halogenation with X? + red P — requires ?-H.
• Kolbe’s electrolysis: 2RCOO?-R–R + 2CO? at anode.
• Soda lime decarboxylation: RCOONa + NaOH-RH + Na?CO?.
• 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.
• CO? evolution with NaHCO? confirms carboxylic acid — not phenol.
• Formic acid has aldehyde-like behavior — reduces Tollens’ reagent.
• (CH?)?CHCOOH undergoes HVZ; (CH?)?CCOOH does not — no ?-H.
• Anhydride formation: 2RCOOH + P?O (RCO)?O.
• 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 + NH?-RCOONH?-RCONH? on heating.
• Mnemonic: LiAlH? = Loves All Hydrogens — reduces acids, aldehydes, ketones, esters.