Subtopics - Carboxylic Acid and Their Derivatives (NEET)
Acidity, preparation, named reactions, and chemistry of acyl chlorides, amides, esters, and anhydrides
1) Physical Properties and Acidity of Carboxylic Acids
Carboxylic acids form dimers via intermolecular H-bonding, giving higher boiling points than alcohols. Alternation effect: even-carbon acids have higher melting points than odd-carbon acids due to better crystal packing. Acidity depends on resonance stabilisation of carboxylate anion (two equivalent structures). Electron-withdrawing substituents (Cl, NO2) stabilise the anion and increase acidity. Effect increases with number of substituents (CH2ClCOOH < CHCl2COOH < CCl3COOH) and decreases with distance from COOH. Ortho effect: all ortho-substituted benzoic acids are more acidic than para/meta isomers.
2) Chemical Reactions of Carboxylic Acids
Reactions with metals/alkali/bicarbonates (CO2 effervescence test). Esterification with alcohols (H+ catalyst, reversible, steric effects on rate). Formation of acid chlorides (SOCl2, PCl5, PCl3) and anhydrides (P2O5). Decarboxylation: sodalime gives alkane, beta-keto acids decarboxylate on heating. Salt reactions: Kolbe electrolysis (2RCOO- gives R-R at anode), Hunsdiecker (AgRCOO + Br2/CCl4 gives RBr), HVZ reaction (alpha-halogenation with Cl2 or Br2/P). Schmidt reaction gives primary amine from RCOOH + HN3.
3) Acyl Chlorides and Acid Amides
Acyl chlorides (RCOCl): most reactive acid derivative. Prepared from acids using SOCl2/PCl5/PCl3. Reactions: acylation of alcohols/amines/phenols, Friedel-Crafts acylation, Rosenmund reduction (H2/Pd-BaSO4 gives RCHO), dialkylcadmium gives ketones. Acid amides (RCONH2): prepared from acyl chlorides + NH3, anhydrides + NH3, or heating ammonium carboxylate. Amides are amphoteric (feeble base due to resonance delocalisation of N lone pair). Hoffmann bromamide degradation: RCONH2 + Br2/KOH gives RNH2 (one carbon less, isocyanate intermediate).
4) Esters and Acid Anhydrides
Esters (RCOOR): prepared by esterification or acylation. Hydrolysis: acid-catalysed (reversible) or base-catalysed (saponification, irreversible). Transesterification with different alcohol/H+. Claisen condensation: ester with alpha-H + NaOEt gives beta-keto ester (e.g., ethyl acetoacetate from ethyl acetate). Grignard addition gives tertiary alcohol. LiAlH4 reduction gives two alcohols. Acid anhydrides: prepared from RCOOH + RCOCl/pyridine or RCOONa + RCOCl. Cyclic anhydrides from dicarboxylic acids (5/6-membered rings only). Reactivity: RCOCl > (RCO)2O > RCOOR > RCONH2.
Carboxylic Acid and Their Derivatives Download Notes & Weightage Plan
For each topic in the Carboxylic Acid and Their Derivatives chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Physical Properties and Acidity of Carboxylic Acids
Dimerisation, alternation effect, acidity order, substituent effects, ortho effect.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Acidity comparison is the most tested concept. Know the ortho effect exception and halogen electronegativity order.
- High-risk Area: Students forget that ortho-hydroxybenzoic acid (salicylic acid) is more acidic than para-hydroxybenzoic acid despite OH being electron-releasing.
- Best Practice Style: Solve 10 acidity comparison MCQs. The pattern will become automatic.
Chemical Reactions of Carboxylic Acids
Esterification, decarboxylation, Kolbe, Hunsdiecker, HVZ, Schmidt.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Hunsdiecker and HVZ are direct name-the-reaction questions. Kolbe electrolysis product prediction is common.
- High-risk Area: In Kolbe electrolysis, students forget that the alkane product has twice the carbon count minus two (2R from 2RCOO-). In Hunsdiecker, the product has one fewer carbon.
- Best Practice Style: Write complete equations with reagents and conditions from memory.
Acyl Chlorides and Acid Amides
SOCl2 preparation, acylation, Rosenmund, Hoffmann bromamide degradation.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Hoffmann bromamide degradation product (amine with one fewer carbon) is a direct NEET question.
- High-risk Area: Students confuse Hoffmann bromamide (RCONH2 gives RNH2, loses CO) with Hoffmann elimination (quaternary ammonium gives less substituted alkene). Completely different reactions.
- Best Practice Style: Practice interconversion: acid to acid chloride to amide to amine in a single reaction sequence.
Saponification, Claisen condensation, cyclic anhydrides, reactivity order.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Reactivity order of acid derivatives is a direct question. Claisen condensation product identification is common.
- High-risk Area: Students confuse Claisen condensation (ester + NaOEt gives beta-keto ester) with aldol condensation (aldehyde + NaOH gives beta-hydroxy aldehyde).
- Best Practice Style: Compare Claisen vs aldol side by side: both need alpha-H, but substrate and base differ.
Carboxylic Acid and Their Derivatives Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Carboxylic Acid and Their Derivatives chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Thinking EDG at ortho decreases acidity: Even electron-donating groups like OH, NH2, or CH3 at the ortho position make benzoic acid MORE acidic than the para isomer. The ortho effect overrides normal inductive/resonance predictions.
- Ignoring intramolecular H-bonding: In ortho-hydroxybenzoic acid (salicylic acid), the phenoxide oxygen forms an intramolecular hydrogen bond with the carboxylate, stabilising the anion extra. This makes salicylic acid much more acidic than para-hydroxybenzoic acid.
Ortho-methylbenzoic acid is more acidic than para-methylbenzoic acid despite CH3 being electron releasing. Ortho-hydroxybenzoic acid (salicylic acid, pKa 2.97) is far more acidic than para-hydroxybenzoic acid (pKa 4.58).
How NEET Frames The Trap
NEET gives ortho and para isomers of substituted benzoic acids and asks which is more acidic. Students apply only inductive/resonance effects and miss the ortho effect.
Q. Among o-hydroxybenzoic acid, m-hydroxybenzoic acid, and p-hydroxybenzoic acid, the strongest acid is:
A. o-hydroxybenzoic acid B. m-hydroxybenzoic acid C. p-hydroxybenzoic acid D. All are equally acidic
Trick: o-hydroxybenzoic acid (salicylic acid) is strongest due to the ortho effect. The carboxylate anion is extra-stabilised by intramolecular hydrogen bonding with the nearby OH group.
Mistake Snapshot (What Students Do Wrong)
- Confusing carbon counts: Hunsdiecker: RCOOAg + Br2 gives RBr (one carbon LESS than the acid). Kolbe: 2RCOONa electrolysis gives R-R (each acid loses CO2, then two R radicals couple). Students mix up which reaction gains or loses carbons.
- Wrong Kolbe product: In Kolbe electrolysis, the product alkane has carbon count = 2(n-1) where n is carbons in the acid. From CH3COONa (C2), product is C2H6 (ethane, C2). From C2H5COONa (C3), product is C4H10 (butane, C4).
CH3CH2COOAg + Br2/CCl4 gives CH3CH2Br (propanoic acid C3 gives bromoethane C2). 2CH3COONa electrolysis gives CH3-CH3 (acetic acid C2 gives ethane C2).
How NEET Frames The Trap
NEET asks for the product of Kolbe electrolysis of sodium propanoate. Students who do not track carbon count from each acid molecule write the wrong alkane.
Q. Kolbe electrolysis of sodium propanoate gives:
A. Ethane B. Propane C. Butane D. Hexane
Trick: Butane because each propanoate ion (C3) loses CO2 to give C2H5 radical, and two ethyl radicals couple to form C4H10 (butane).
Mistake Snapshot (What Students Do Wrong)
- Confusing two different Hoffmann reactions: Hoffmann bromamide degradation converts RCONH2 to RNH2 (one carbon less). Hoffmann elimination converts quaternary ammonium salt to the less substituted alkene. These are completely unrelated reactions that share the name Hoffmann.
- Wrong carbon count in bromamide product: The amine product has one fewer carbon than the starting amide because the carbonyl carbon is lost as CO2. Acetamide (C2) gives methylamine (C1).
Hoffmann bromamide: CH3CONH2 + Br2/KOH gives CH3NH2 + K2CO3. Hoffmann elimination: (CH3)3C-CH2-N+(CH3)3 OH- heated gives (CH3)2C=CH2.
How NEET Frames The Trap
A question asks for the Hoffmann reaction product of an amide. Students who recall the elimination version write an alkene instead of an amine.
Q. Hoffmann bromamide reaction of propanamide (CH3CH2CONH2) with Br2/KOH gives:
A. Propylamine B. Ethylamine C. Propene D. Ethanol
Trick: Ethylamine (CH3CH2NH2) because the carbonyl carbon of propanamide is lost as CO2 during the rearrangement to isocyanate and subsequent hydrolysis. The product amine has one fewer carbon.
Mistake Snapshot (What Students Do Wrong)
- Thinking P directly halogenates the acid: Red phosphorus does not halogenate the acid. It converts the acid to an acid halide (via PBr3 formation), and the acid halide then undergoes alpha-halogenation via enolisation. The halogenated acid halide exchanges with fresh acid to propagate the cycle.
- Forgetting alpha-H requirement: HVZ reaction requires at least one alpha-hydrogen. Acids like (CH3)3CCOOH (pivalic acid) or benzoic acid cannot undergo this reaction.
CH3CH2COOH + Br2/P gives CH3CHBrCOOH (alpha-bromopropionic acid). The mechanism: P + Br2 gives PBr3, then RCOOH + PBr3 gives RCOBr, then RCOBr enolises and brominates at alpha position.
How NEET Frames The Trap
NEET asks what product forms when a carboxylic acid without alpha-H is treated with Br2/P. Students who do not check for alpha-H predict a brominated product.
Q. Which acid does NOT undergo the Hell-Volhard-Zelinsky reaction?
A. Acetic acid B. Propionic acid C. Benzoic acid D. Butyric acid
Trick: Benzoic acid does not undergo HVZ because it has no alpha-hydrogen atom. The reaction requires at least one H on the carbon adjacent to COOH.
Mistake Snapshot (What Students Do Wrong)
- Reversing the reactivity order: Correct order: RCOCl > (RCO)2O > RCOOR > RCONH2. Students sometimes think amides are most reactive because nitrogen is a good nucleophile, but reactivity refers to susceptibility to nucleophilic attack, not nucleophilicity of the leaving group.
- Confusing basicity of leaving group with reactivity: Weaker base = better leaving group = higher reactivity. Cl- is the weakest base (best leaving group), NH2- is the strongest base (worst leaving group). Higher resonance stabilisation also decreases reactivity.
Acetyl chloride reacts with water instantly (hydrolysis). Acetic anhydride reacts with water more slowly. Ethyl acetate requires acid/base catalyst. Acetamide barely hydrolyses without heating.
How NEET Frames The Trap
A question asks which acid derivative undergoes nucleophilic acyl substitution fastest. Students who do not connect leaving group basicity to reactivity choose incorrectly.
Q. The correct order of reactivity towards nucleophilic acyl substitution is:
A. RCONH2 > RCOOR > (RCO)2O > RCOCl B. RCOCl > RCOOR > (RCO)2O > RCONH2 C. RCOCl > (RCO)2O > RCOOR > RCONH2 D. RCOOR > RCOCl > RCONH2 > (RCO)2O
Trick: RCOCl > (RCO)2O > RCOOR > RCONH2 because Cl- is the weakest base and best leaving group, while NH2- is the strongest base and worst leaving group.