Chapters

Everything NCERT Class 12 Unit 8 (Aldehydes, Ketones and Carboxylic Acids) says about carboxylic acids (its sections 8.6–8.10; aldehydes and ketones are in Carbonyl Compounds), and their derivatives as far as NCERT goes, plus what JEE Main adds beyond the book (marked Beyond NCERT), chiefly the reactions of acid chlorides, anhydrides, esters and amides. The short note stays for the last week; this is for a proper revision pass.

Formula sheet

The rules the questions turn on, on one screen. Reagents are in the maps and the named-reaction table below.

Rule	What it says	Watch
Acid strength	pKa = −log Ka; smaller pKa, stronger acid	Strong < 1; moderate 1–5; weak 5–15
Ladder	RCOOH (~4–5) > phenol (10) > ethanol (~16)	Only RCOOH (and picric acid) free CO₂ from NaHCO₃
EWG / EDG	EWG raises acidity, EDG lowers it	Nearer to –COOH, bigger effect
Group order	Ph < I < Br < Cl < F < CN < NO₂ < CF₃	NCERT's order of acid-strengthening
Small acids	HCOOH > C₆H₅COOH > CH₃COOH > CH₃CH₂COOH	pKa 3.75 (Beyond), 4.19, 4.76
Derivatives	RCOCl > (RCO)₂O > RCOOR′ > RCONH₂	Same order for hydrolysis and LiAlH₄
Carbon count	+1 C: Grignard + CO₂, R–X → RCN → acid	−1 C: soda lime, haloform, Hoffmann
Kolbe electrolysis	2 RCOONa → R–R	Twice the alkyl carbons
HVZ	X₂, red P; needs an α-H	Gives the α-halo acid
Reduction	LiAlH₄ or B₂H₆ → RCH₂OH	NaBH₄ does not reduce –COOH

0 · What JEE Main asks from this chapter

48 questions in JEE Main 2019–2026 (47 MCQ, 1 numerical), about 1 in every 6 papers. Grouped by type (counts are approximate; 8 show the structures only as pictures):

Type	Share	Typical question
Preparation and multi-step sequences	~25%	C₂H₅Br → alc. KOH → Br₂ → excess KCN → H₃O⁺: which acid?
Derivatives: reactivity, hydrolysis, reduction, anhydrides	~25%	Order of hydrolysis: acid chloride, anhydride, ester, amide
HVZ, decarboxylation, ring substitution, identify A	~18%	Propanenitrile → acid → Cl₂/red P: name the product
Picture-only products	~17%	Product of a ring compound with DIBAL-H or LiBH₄
Acidity order and the NaHCO₃ test	~15%	Order of CH₃COOH, F₃CCOOH, ClCH₂COOH, BrCH₂COOH

1 · Nomenclature and structure

A carboxyl group –COOH is a carbonyl group bonded to a hydroxyl group. Acids are aliphatic (RCOOH) or aromatic (ArCOOH). The higher aliphatic acids (C₁₂–C₁₈) are the fatty acids, found in natural fats as esters of glycerol.

Common names end in -ic acid and come from the Latin or Greek name of the source: formic (formica, ant), acetic (acetum, vinegar), butyric (butyrum, butter; from rancid butter).

IUPAC names.

• Replace the alkane's -e with -oic acid. The carboxyl carbon is C-1.
• Several –COOH: number the chain without counting the carboxyl carbons, then add dicarboxylic acid, tricarboxylic acid with arabic numerals for their positions.

Structure	Common name	IUPAC name
HCOOH	Formic acid	Methanoic acid
CH₃COOH	Acetic acid	Ethanoic acid
CH₃CH₂COOH	Propionic acid	Propanoic acid
CH₃(CH₂)₂COOH	Butyric acid	Butanoic acid
(CH₃)₂CHCOOH	Isobutyric acid	2-Methylpropanoic acid
HOOC–COOH	Oxalic acid	Ethanedioic acid
HOOC–CH₂–COOH	Malonic acid	Propanedioic acid
HOOC–(CH₂)₂–COOH	Succinic acid	Butanedioic acid
HOOC–(CH₂)₃–COOH	Glutaric acid	Pentanedioic acid
HOOC–(CH₂)₄–COOH	Adipic acid	Hexanedioic acid
HOOCCH₂CH(COOH)CH₂COOH	Tricarballylic acid	Propane-1,2,3-tricarboxylic acid
C₆H₅COOH	Benzoic acid	Benzenecarboxylic acid
C₆H₅CH₂COOH	Phenylacetic acid	2-Phenylethanoic acid
o-C₆H₄(COOH)₂	Phthalic acid	Benzene-1,2-dicarboxylic acid

Example (NCERT Intext 8.6) · PhCH₂CH₂COOH is 3-phenylpropanoic acid. (CH₃)₂C=CHCOOH is 3-methylbut-2-enoic acid. A cyclopentane ring with COOH and an adjacent CH₃ is 2-methylcyclopentanecarboxylic acid.

Structure. The bonds to the carboxyl carbon lie in one plane, about 120° apart. The carboxyl carbon is less electrophilic than a carbonyl carbon, because the OH lone pair is delocalised into the C=O (resonance). So acids don't give the nucleophilic additions of aldehydes and ketones.

2 · Preparation

1. From 1° alcohols and aldehydes.
   • 1° alcohols: KMnO₄ (neutral, acidic or alkaline), K₂Cr₂O₇, or CrO₃ in acid (Jones reagent).
   • Aldehydes: even mild oxidants (Tollens', Fehling's) are enough.
2. From alkylbenzenes. Chromic acid, or acidic or alkaline KMnO₄, oxidises the whole side chain to –COOH, whatever its length.
   • 1° and 2° alkyl groups are oxidised; a 3° group (no benzylic H) is not.
   • Suitably substituted alkenes are also cleaved to acids by these reagents.
3. From nitriles and amides. H⁺ or OH⁻ hydrolyses RCN → RCONH₂ → RCOOH. Mild conditions stop at the amide.
4. From Grignard reagents. RMgX + CO₂ (dry ice) → RCOOMgX → mineral acid → RCOOH.
   • Methods 3 and 4 turn an alkyl halide into the acid with one carbon more (ascending the series).
5. From acyl halides and anhydrides.
   • Acid chlorides hydrolyse with water, faster with aqueous base (to the carboxylate, then acidify).
   • Anhydrides hydrolyse with water.
6. From esters. Acid hydrolysis gives the acid directly. Base hydrolysis (saponification) gives the carboxylate; acidify.

Example (NCERT Example 8.5) · Butan-1-ol → CrO₃–H₂SO₄ → butanoic acid. Benzyl alcohol → PBr₃ → KCN → H₃O⁺ → phenylethanoic acid (one C more). 4-Methylacetophenone → KMnO₄/KOH → H₃O⁺ → benzene-1,4-dicarboxylic acid (both side chains). Cyclohexene → KMnO₄/H⁺ → hexane-1,6-dioic acid (adipic; asked again in 2024). Butanal → Tollens' → butanoic acid.

Example (NCERT Intext 8.7) · Four routes to benzoic acid: ethylbenzene, acetophenone or styrene with hot KMnO₄/KOH then H₃O⁺; bromobenzene with Mg/ether, then CO₂, then H₃O⁺.

Example (2024) · C₂H₅Br → (alc. KOH) ethene → (Br₂) BrCH₂CH₂Br → (excess KCN) NCCH₂CH₂CN → (H₃O⁺) succinic acid. Two C added, one from each CN.

Example (2021) · Which does not give propanoic acid? CH₃CH₂CH₂Br + Mg, then CO₂: that adds a carbon and gives butanoic acid. (The others work: CH₃CH₂CCl₃ + OH⁻ hydrolyses to the acid; butan-2-one + OI⁻ loses CH₃ as iodoform.)

Example (2024) · CH₃COOH → (LiAlH₄) ethanol → (PCC) ethanal → (HCN/OH⁻) cyanohydrin → (hydrolysis) CH₃CH(OH)COOH, lactic acid.

Cyanohydrins to α-hydroxy and α,β-unsaturated acids · Beyond NCERT · A cyanohydrin hydrolyses to the α-hydroxy acid. With hot 95% H₂SO₄ it also dehydrates: butan-2-one → cyanohydrin → CH₃CH=C(CH₃)COOH (2022).

3 · Physical properties

• Aliphatic acids up to C₉ are colourless liquids with unpleasant odours. Higher acids are wax-like solids, practically odourless (low volatility).
• Boiling points are higher than aldehydes, ketones and even alcohols of similar mass: more extensive intermolecular H-bonding. The H-bonds are not broken completely even in the vapour, so most acids exist as dimers in the vapour phase and in aprotic solvents.
• Solubility: acids up to four carbons mix with water (H-bonds with water). Solubility falls as the hydrocarbon part grows. Benzoic acid is nearly insoluble in cold water. All dissolve in benzene, ether, alcohol, chloroform.

4 · Acidity

Reactions as an acid (O–H cleavage):

• Like alcohols, acids give H₂ with electropositive metals.
• Like phenols, they form salts with alkalis.
• Unlike phenols, they react with the weaker bases carbonate and hydrogencarbonate and give CO₂. This effervescence is the test for –COOH.

Example (2025) · CO₂ with NaHCO₃: benzoic acid, picric acid (2,4,6-trinitrophenol) and cyclohexanecarboxylic acid. Not phenol, not 4-methoxyphenol. Picric acid is the phenol that passes: three NO₂ groups take its pKa below 1.

Dissociation and pKa.

Acid	pKa
HCl	−7.0
CF₃COOH (the strongest carboxylic acid)	0.23
4-Nitrobenzoic acid	3.41
Benzoic acid	4.19
4-Methoxybenzoic acid	4.46
Acetic acid	4.76
Phenol	10
Ethanol	~16

Strong acids pKa < 1; moderately strong 1–5; weak 5–15; extremely weak > 15. Carboxylic acids are weaker than mineral acids but stronger than alcohols and most simple phenols.

Why stronger than phenol (NCERT exercise 8.20). Phenoxide has more resonance structures, yet carboxylate is more stable:

• Carboxylate has two equivalent structures, each with the charge on an electronegative O.
• Phenoxide's structures are not equivalent, and in most of them the charge sits on a less electronegative C.
• So the charge is better spread in carboxylate, over two O atoms.

Substituents.

• An electron-withdrawing group (−I or −R) stabilises the carboxylate and strengthens the acid.
• An electron-donating group destabilises it and weakens the acid.
• The effect falls off with distance from –COOH; more EWGs add up.

NCERT's order of increasing effect: Ph < I < Br < Cl < F < CN < NO₂ < CF₃. Its list of acids, strongest first:

CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NCCH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH

• Phenyl or vinyl directly on –COOH raises acidity (benzoic > acetic), against what resonance would suggest. The sp² carbon is more electronegative.
• On the ring of benzoic acid, an EWG raises acidity and an EDG lowers it: 4-nitro (3.41) > benzoic (4.19) > 4-methoxy (4.46).

Example (NCERT Intext 8.8) · CH₂FCO₂H > CH₃CO₂H. CH₂FCO₂H > CH₂ClCO₂H (F more electronegative). CH₃CHFCH₂CO₂H > CH₂FCH₂CH₂CO₂H (F nearer the COOH). A CF₃ on the ring beats a CH₃ on the ring.

Example (NCERT exercise 8.12) · (CH₃)₂CHCOOH < CH₃CH₂CH₂COOH < CH₃CH(Br)CH₂COOH < CH₃CH₂CH(Br)COOH. And 4-methoxybenzoic < benzoic < 4-nitrobenzoic < 3,4-dinitrobenzoic.

Example (2023, 2024) · F₃CCOOH > ClCH₂COOH > BrCH₂COOH > CH₃COOH. HCOOH > CH₃COOH > CH₃CH₂COOH > CH₃CH₂CH₂COOH (each extra alkyl donates a little more).

Ortho effect · Beyond NCERT · Almost any ortho substituent, even an EDG like –CH₃, makes benzoic acid stronger. It twists the –COOH out of the ring's plane, so the ring can no longer donate into it. 2-Hydroxybenzoic (salicylic) acid is stronger still: intramolecular H-bonding holds the carboxylate.

5 · Reactions at the C–OH bond

1. Anhydride. Heat with a mineral acid such as H₂SO₄, or with P₂O₅: 2 RCOOH → (RCO)₂O + H₂O.
2. Esterification. With an alcohol or a phenol and a mineral acid catalyst (conc. H₂SO₄ or HCl gas). Reversible; remove the water or the ester as it forms to push it forward (NCERT exercise 8.18).

Mechanism (nucleophilic acyl substitution):

1. H⁺ protonates the carbonyl O and activates the C=O.
2. The alcohol adds; a tetrahedral intermediate forms.
3. A proton moves to the OH, making –⁺OH₂, a good leaving group.
4. Water leaves; the protonated ester loses H⁺ to give the ester.

The water's O comes from the acid; the ester's alkyl O comes from the alcohol. Beyond NCERT: an ¹⁸O-labelled alcohol puts the label in the ester. 2022 stated both "esterification is nucleophilic acyl substitution" and "an EWG on the acid speeds it up" as true.

3. Acid chloride. PCl₅, PCl₃ or SOCl₂ replace the OH by Cl, as with alcohols. SOCl₂ is preferred: both by-products are gases and escape, so the acid chloride is easy to purify.

4. Amide. With ammonia an acid gives the ammonium salt; strong heating turns it into the amide. CH₃COONH₄ → CH₃CONH₂ + H₂O. Benzoic acid gives benzamide. Phthalic acid with NH₃ and strong heating gives phthalimide (used in Gabriel synthesis, Amines).

6 · Reactions of the –COOH group

Reduction. LiAlH₄, or better diborane (B₂H₆), reduces the acid to the 1° alcohol. Diborane does not easily reduce an ester, nitro or halo group, so it reduces –COOH selectively (2020). NaBH₄ does not reduce –COOH.

Decarboxylation. The sodium salt heated with soda lime (NaOH and CaO, 3 : 1) loses CO₂ and gives the hydrocarbon with one carbon less.

Kolbe electrolysis. Electrolysis of an aqueous alkali-metal salt also decarboxylates. The hydrocarbon has twice the carbons of the alkyl group: 2 CH₃COONa → CH₃–CH₃ (+ 2CO₂ at the anode, H₂ and NaOH at the cathode).

Example (2024) · Ethanol → (Jones, then KMnO₄) acetic acid → (NaOH, CaO, heat) methane.

Example (2020) · P gives a single ring monobromide C₈H₇O₂Br with Br₂/FeBr₃, and toluene with soda lime. So P is 4-methylbenzoic acid: soda lime removes the COOH and leaves toluene, and CH₃ (o/p) and COOH (meta) send Br to the same two equivalent positions. Phenylacetic acid also gives toluene but two bromo isomers.

7 · Substitution in the hydrocarbon part

Hell–Volhard–Zelinsky (HVZ) reaction. An acid with an α-H, with Cl₂ or Br₂ and a small amount of red phosphorus, is halogenated at the α-carbon.

Example (2026) · CH₃CH₂CN → (OH⁻, heat; H₃O⁺) propanoic acid → (Cl₂, red P; H₂O) 2-chloropropanoic acid.

Example (2023) · Glycine + Cl₂/red P gives ClCH(NH₂)COOH, which has a chiral carbon: assertion true. "A molecule with 2 chiral carbons is always optically active" is false (meso compounds).

Ring substitution. In aromatic acids the –COOH deactivates the ring and directs meta. Aromatic acids do not give Friedel–Crafts reactions: the ring is deactivated and AlCl₃ (a Lewis acid) binds to the carboxyl group.

Example (2021) · 6.1 g benzoic acid (0.05 mol) → m-bromobenzoic acid. Theoretical: 0.05 × 201 = 10.05 g. Actual 7.8 g → 7.8/10.05 × 100 = 78%.

Example (2021) · Benzene → 3-nitrobenzoic acid: HNO₃/H₂SO₄ first (–NO₂ directs meta), then Br₂/AlBr₃ (m-bromonitrobenzene), then Mg/ether, CO₂, H₃O⁺. NCERT Example 8.5 does the last step the same way. The routes through NaCN fail: an aryl bromide does not substitute with CN⁻.

8 · Acid derivatives

NCERT names them (acid chlorides, anhydrides, esters, amides), makes them from acids and hydrolyses them back. JEE goes further.

Reactivity order · Beyond NCERT

• The order follows how good the leaving group is: Cl⁻ > RCOO⁻ > R′O⁻ > NH₂⁻. The weaker the base, the better it leaves.
• It also follows how much the group donates into the C=O: N donates most, so amides are least reactive.
• The same order holds for hydrolysis (2021, 2025) and for reduction by LiAlH₄ (2019).
• A more reactive derivative can be made from a less reactive one only by going downhill: acid chloride → anhydride → ester → amide.

Reactions JEE uses · Beyond NCERT

Derivative	Reagent	Product
RCOCl	H₂O / R′OH / NH₃	acid / ester / amide
RCOCl	H₂, Pd–BaSO₄	RCHO (Rosenmund)
RCOOR′	NaOH, then H₃O⁺	RCOOH + R′OH (saponification)
RCOOR′	LiAlH₄	RCH₂OH + R′OH
RCOOR′	DIBAL-H (1 equiv, cold)	RCHO
RCOOR′	2 R″MgX; H₃O⁺	3° alcohol
RCONH₂	LiAlH₄	RCH₂NH₂ (same carbons)
RCONH₂	Br₂, NaOH	RNH₂ (one C less; Hoffmann, in Amines)
RCONH₂	P₂O₅, heat	RC≡N

• Base hydrolysis of esters is faster with an EWG on the acid part: ethyl 4-nitrobenzoate > 4-chloro > benzoate > 4-methoxy (2019).
• An ester is the one alcohol derivative that falls apart in aqueous base; ethers, THP and t-butyl ethers survive (2020).

Cyclic anhydrides · Beyond NCERT · A dicarboxylic acid closes to a 5- or 6-membered ring on heating: succinic, glutaric, phthalic and maleic (cis-butenedioic) acids. Fumaric (trans) cannot reach (2021). Adipic acid would need a 7-membered ring and is the least able (2019).

Example (2019) · A (C₇H₆O₂, aromatic) + aq. NH₃, heat → B; B + Br₂/KOH → C (C₆H₇N). C is aniline, so B is benzamide and A is benzoic acid.

Example (2025) · A has N, molar mass 121; NaOH then H₃O⁺ gives B, which fizzes with NaHCO₃; B + EtOH/H₂SO₄ gives a fruity C. A = benzamide (121), B = benzoic acid, C = ethyl benzoate.

9 · Reaction map and named reactions

Name	Reagents	Change
Esterification (Fischer)	R′OH, conc. H₂SO₄, Δ	RCOOH → RCOOR′
Hell–Volhard–Zelinsky	X₂, red P; H₂O	RCH₂COOH → RCHXCOOH
Decarboxylation	soda lime (NaOH : CaO = 3 : 1), Δ	RCOONa → RH
Kolbe electrolysis	electrolysis of aq. RCOONa	→ R–R + CO₂
Jones oxidation	CrO₃, H₂SO₄	RCH₂OH → RCOOH
Grignard carboxylation	CO₂ (dry ice); H₃O⁺	RMgX → RCOOH
Rosenmund (Carbonyl)	H₂, Pd–BaSO₄	RCOCl → RCHO
Hoffmann bromamide (Amines)	Br₂, NaOH	RCONH₂ → RNH₂

10 · Traps

Trap	What goes wrong	Do this
Grignard + CO₂	Forgetting the extra carbon (2021)	RMgX + CO₂ gives an acid with one C more than R–X
NaBH₄	Reducing –COOH or an ester	NaBH₄ stops at aldehydes and ketones; use LiAlH₄ or B₂H₆
HVZ	Halogenating (CH₃)₃CCOOH or the ring	Needs an α-H; it goes to the α-carbon only
Phenol and NaHCO₃	Counting phenol as fizzing (2025)	Only acids with pKa below ~6.4 (RCOOH, picric acid)
Benzoic vs acetic	Thinking the phenyl ring weakens it	sp² carbon pulls: benzoic 4.19 < acetic 4.76
Side-chain oxidation	Oxidising a tert-butyl group	No benzylic H, no oxidation; any longer chain → one COOH
Friedel–Crafts on benzoic acid	Expecting acylation	No F–C on a ring carrying –COOH; AlCl₃ binds the COOH
Derivative order	Ranking ester above anhydride (2021, 2025)	Chloride > anhydride > ester > amide
Maleic vs fumaric	Heating fumaric acid for the anhydride (2021)	Only the cis acid closes the ring
Decarboxylation	Heating the free acid with soda lime and losing count	Salt + soda lime: one C less; Kolbe: double the alkyl

11 · Numbers worth knowing

• pKa: CF₃COOH 0.23; 4-nitrobenzoic 3.41; benzoic 4.19; 4-methoxybenzoic 4.46; acetic 4.76; phenol 10; ethanol ~16; HCl −7.
• Soda lime: NaOH : CaO = 3 : 1. Fatty acids: C₁₂–C₁₈.

Compound	g mol⁻¹
HCOOH	46
CH₃COOH	60
benzoic acid	122
benzamide	121
salicylic acid	138
adipic acid	146
phthalic acid	166
m-bromobenzoic acid	201
oxalic acid dihydrate	126

12 · For the boards only

JEE Main doesn't ask this, but the CBSE paper can.

• Methanoic acid: rubber, textile, dyeing, leather and electroplating industries.
• Ethanoic acid: solvent; vinegar in the food industry.
• Hexanedioic (adipic) acid: manufacture of nylon-6,6.
• Esters of benzoic acid: perfumery. Sodium benzoate: food preservative.
• Higher fatty acids: soaps and detergents.
• Many acids were among the first organic compounds isolated from nature, which is why so many keep common names.