Carboxylic Acids

Carboxylic acids (R–COOH) are the most acidic of the common organic functional groups. The PMDC MDCAT 2026 syllabus expects you to name them, prepare them, convert them into acyl halides / anhydrides / esters / amides, and explain why they are stronger acids than alcohols and phenols. Expect 1–2 MCQs.

PMC Table of Specifications. Three subtopics — Nomenclature/Structure/Preparation, Reactivity, and Conversion to Derivatives (acyl halides, anhydrides, esters).

Nomenclature, Structure and Preparation

The carboxyl group –COOH is a carbonyl (>C=O) joined to a hydroxyl (–OH) on the same carbon. The two functional groups interact: the C=O withdraws electrons inductively from the O–H, weakening the bond and making the H acidic. The carboxylate ion (R–COO⁻) is stabilised by resonance over two equivalent oxygens.

IUPAC nomenclature

Replace the –e of the parent alkane with –oic acid; the carboxyl carbon is always C-1.
Examples: HCOOH = methanoic acid (formic acid), CH₃COOH = ethanoic acid (acetic acid), CH₃CH₂COOH = propanoic acid, HOOC–COOH = ethanedioic acid (oxalic acid), C₆H₅COOH = benzoic acid.

Physical properties

The first four members are pungent liquids; higher members are waxy solids.
Boiling points are higher than alcohols of similar molar mass because carboxylic acids form cyclic dimers via two hydrogen bonds.
Lower acids (C₁–C₄) are completely soluble in water; solubility falls as the alkyl tail lengthens.

Acid strength

pK_a ≈ 4–5 (e.g. acetic acid pK_a 4.76). Stronger than water and phenol, weaker than mineral acids. Electron-withdrawing substituents (–Cl, –NO₂) on the α-carbon stabilise the carboxylate and increase acidity:

HCOOH > CH₃COOH (no +I from H).
Cl₃CCOOH (pK_a 0.7) > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH.
Acidity falls down the halogen group (Cl > Br > I) at the α-position.

Preparation

From primary alcohols / aldehydes

R–CH₂OH ⟶{KMnO₄/H⁺ or K₂Cr₂O₇/H⁺} R–CHO ⟶{[O]} R–COOH.

From nitriles by hydrolysis

R–CN + H₂O ⟶{H⁺ or OH⁻, Δ} R–COOH + NH₃. Useful because R–CN can itself be made by S_N2 of R–X with KCN — net effect: extends a chain by one C.

From Grignard + CO₂

R–MgX + CO₂ (dry ice) → R–COO–MgX ⟶{H₃O⁺} R–COOH. Also extends chain by one C.

From alkylbenzenes

C₆H₅–CH₃ ⟶{KMnO₄, Δ} C₆H₅–COOH (benzoic acid). The whole side-chain is oxidised down to a single –COOH no matter how long it is, provided there's at least one benzylic H.

Reactivity

Carboxylic acids are reactive at three sites: the acidic O–H, the electrophilic C of the C=O, and the α-carbon (which can be halogenated).

Acid–base reactions

R–COOH + NaOH → R–COONa + H₂O.
R–COOH + NaHCO₃ → R–COONa + H₂O + CO₂↑. The CO₂ evolution is a diagnostic test — phenols and alcohols do not liberate CO₂.
2 R–COOH + Na₂CO₃ → 2 R–COONa + H₂O + CO₂↑.

Reduction

R–COOH + LiAlH₄ ⟶{dry ether} R–CH₂OH (1° alcohol). NaBH₄ is too mild to reduce carboxylic acids.

Decarboxylation

R–COONa + NaOH (CaO) ⟶{Δ} R–H + Na₂CO₃ (soda-lime decarboxylation). Removes the –COOH group, shortens chain by one C.

α-Halogenation — Hell–Volhard–Zelinsky reaction

R–CH₂–COOH + Cl₂/red P → R–CHCl–COOH + HCl. Introduces a halogen at the α-carbon.

Common trap. The unique acidity-confirming test is the NaHCO₃ reaction — only carboxylic acids fizz with sodium bicarbonate. Phenol does dissolve in NaOH but does NOT release CO₂ from NaHCO₃; alcohols do neither.

Conversion to Derivatives (acyl halides, anhydrides, esters)

The general reaction at the carboxyl carbon is replacement of the –OH by another nucleophilic leaving group, giving an acid derivative. Reactivity of derivatives toward nucleophiles: acyl halide > anhydride > ester > amide.

Acyl halides (R–COX)

R–COOH + SOCl₂ → R–COCl + SO₂↑ + HCl↑ (cleanest method).
R–COOH + PCl₅ → R–COCl + POCl₃ + HCl.
R–COOH + PCl₃ (3 equiv) → 3 R–COCl + H₃PO₃.
Acyl chlorides are the most reactive derivative — they react vigorously with water, alcohols, and amines.

Acid anhydrides (R–CO–O–CO–R′)

2 R–COOH ⟶{P₂O₅, Δ} R–CO–O–CO–R + H₂O.
R–COCl + R′–COONa → R–CO–O–CO–R′ + NaCl (mixed anhydride).
Examples: ethanoic anhydride (CH₃CO)₂O, used to acetylate alcohols/amines.

Esters (R–COO–R′) — Fischer esterification

R–COOH + R′–OH ⟶{conc. H₂SO₄} R–COO–R′ + H₂O.
The reaction is reversible; driven forward by removing water or using excess alcohol. The mechanism involves protonation of the C=O, attack by R′OH, and loss of water (acid-catalysed nucleophilic acyl substitution).
Esters have fruity smells — ethyl ethanoate (pear), pentyl ethanoate (banana), octyl ethanoate (orange).

Amides (R–CONH₂)

R–COOH + NH₃ → R–COO⁻NH₄⁺ ⟶{Δ} R–CONH₂ + H₂O.
Or more conveniently: R–COCl + 2 NH₃ → R–CONH₂ + NH₄Cl.

Mnemonic for derivative reactivity. "HAEA" — Halide > Anhydride > Ester > Amide. The better the leaving group (Cl⁻ > carboxylate > alkoxide > amide), the more reactive the derivative.

Worked MCQs

Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — that's where the deeper concept lives.

Q1. Which of the following gives a brisk effervescence with NaHCO₃?

Ethanol
Phenol
Ethanoic acid
Diethyl ether

Only carboxylic acids are strong enough to liberate CO₂ from sodium bicarbonate. Phenol (pK_a ≈ 10) is too weak; ethanol and ether are essentially non-acidic toward NaHCO₃.

Q2. The strongest acid among the following is:

CH₃COOH
ClCH₂COOH
Cl₂CHCOOH
Cl₃CCOOH

Three chlorines withdraw electron density inductively, stabilising the carboxylate ion most strongly. Trichloroacetic acid has pK_a 0.7 — almost as strong as a mineral acid.

Q3. Treatment of ethanoic acid with thionyl chloride (SOCl₂) gives:

Ethyl chloride
Acetyl chloride (ethanoyl chloride)
Acetic anhydride
Ethyl acetate

SOCl₂ converts a carboxylic acid into the corresponding acyl chloride. The by-products SO₂ and HCl escape as gases, leaving a clean product — that's why it's the preferred reagent over PCl₅.

Q4. Fischer esterification of ethanoic acid with ethanol uses which catalyst?

NaOH
Concentrated H₂SO₄
NaBH₄
KMnO₄

The reaction is acid-catalysed: H₂SO₄ protonates the carbonyl oxygen, making the carboxyl carbon more electrophilic and accelerating attack by ethanol. The acid is also a dehydrating agent, helping drive the equilibrium toward the ester.

Q5. Reduction of butanoic acid with LiAlH₄ followed by aqueous workup gives:

Butanal
Butan-1-ol
Butan-2-ol
Butane

LiAlH₄ reduces a carboxylic acid all the way to the primary alcohol — the –COOH becomes –CH₂OH. NaBH₄ would not reduce the acid at all.

Quick Recap

–COOH = carbonyl + hydroxyl on the same C; conjugate base stabilised by resonance over two oxygens.
Acid strength: HCOOH > acetic acid; EWG on α-C boost acidity (Cl₃CCOOH > CH₃COOH).
Diagnostic test: carboxylic acid liberates CO₂ from NaHCO₃; phenol does not.
Preparation: oxidation of 1° alcohol/aldehyde, hydrolysis of nitrile, RMgX + CO₂, alkylbenzene + KMnO₄.
Derivatives reactivity: acyl halide > anhydride > ester > amide.
SOCl₂ = best for –COCl; conc. H₂SO₄ = catalyst for Fischer esterification.
LiAlH₄ reduces –COOH to 1° alcohol; NaBH₄ cannot.
HVZ reaction installs α-halogen using Cl₂/red P.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.