Carboxylic Acids

Carboxylic Acids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Carboxylic acids are organic compounds that carry the carboxyl group (–COOH), in which a hydroxyl (–OH) and a carbonyl (C=O) are bonded to the same carbon. The general formula for a saturated aliphatic monocarboxylic acid is CₙH₂ₙO₂, written structurally as R–COOH (e.g., CH₃COOH, acetic acid; HCOOH, methanoic/formic acid).

Three high-yield facts for CUET UG:

• Acidity order: HCOOH > C₆H₅COOH > CH₃COOH > CH₃CH₂COOH. The pKa of acetic acid is ≈ 4.76, formic acid ≈ 3.75, chloroacetic acid ≈ 2.87 — the last is far stronger because of the –Cl inductive effect.
• The sodium bicarbonate test (R–COOH + NaHCO₃ → R–COONa + H₂O + CO₂↑) distinguishes carboxylic acids from phenols, which do not liberate CO₂.
• Esterification (Fischer): R–COOH + R′OH ⇌ R–COOR′ + H₂O, catalysed by concentrated H₂SO₄.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature and Structure

The IUPAC suffix is “-oic acid” on the longest chain that contains the –COOH carbon (C-1 by rule). HCOOH is methanoic acid; CH₃COOH is ethanoic acid; CH₃CH₂COOH is propanoic acid; C₆H₅COOH is benzoic acid. The carbonyl carbon is sp²-hybridised, and the O–H bond is strongly polarised because the adjacent C=O pulls electron density, making the proton ionisable.

Acidity and the Carboxylate Anion

Carboxylic acids (pKa 3–5) are far stronger than alcohols (pKa 16–18) because deprotonation gives the carboxylate ion (RCOO⁻), which is stabilised by resonance — the negative charge is delocalised equally over both oxygen atoms, producing two equivalent O–C bonds each of order 1.5. Electron-withdrawing groups (–Cl, –NO₂, –F) intensify this inductive pull and raise Ka, while electron-donating groups (–CH₃, –C₂H₅) lower it. Formic acid has no alkyl group, so it is the strongest of the simple aliphatic series.

Boiling Points and Dimerisation

Pure liquid and vapour-phase carboxylic acids exist as cyclic hydrogen-bonded dimers, roughly doubling the effective molecular mass. This raises their boiling points well above those of comparable alcohols (e.g., acetic acid b.p. 118 °C vs. 1-propanol 97 °C).

Key Reactions at a Glance

Reagent	Product	Notes
NaOH	R–COONa + H₂O	Neutralisation
NaHCO₃	R–COONa + H₂O + CO₂↑	Diagnostic test
R′OH / H₂SO₄	R–COOR′ (ester)	Reversible
PCl₅, PCl₃ or SOCl₂	R–COCl (acid chloride)	–OH → –Cl
NH₃, then Δ	R–CONH₂ (amide)	Ammonium salt first
LiAlH₄	R–CH₂OH (1° alcohol)	Reduction
Br₂ / red P	α-bromo acid	Hell–Volhard–Zelinsky
NaOH + CaO, Δ	R–H (alkane)	Decarboxylation, soda lime

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Mechanism Insight

• Formic acid (HCOOH) is unique: it has no R group, so it possesses an aldehyde-like H on the carboxyl carbon. It therefore reduces Tollens’ reagent and Fehling’s solution and gives a positive silver mirror, unlike every other carboxylic acid.
• Oxalic acid (HOOC–COOH), the simplest dicarboxylic acid, decarboxylates on heating with concentrated H₂SO₄ to give CO₂ + CO + H₂O — a frequently tested anomaly.
• In esterification, the oxygen of water comes from the carboxylic acid, not the alcohol — this is proven by isotopic labelling with ¹⁸O. The mechanism proceeds via protonation of C=O, nucleophilic addition of the alcohol, and loss of water.
• The HVZ reaction requires red phosphorus (or PCl₃) to convert the acid into the acyl halide, whose enol form is the actual species that reacts with Br₂; without P, the reaction stalls.
• Soda-lime decarboxylation works only on the sodium salt and fails with simple aromatic acids (e.g., sodium benzoate gives benzene only at very high temperature with added CaO).

Connections to Adjacent Topics

Carboxylic acids sit at the highest oxidation state of carbon in a homologous series: aldehydes ⇌ primary alcohols ⇌ carboxylic acids ⇌ CO₂. Mastering this ladder clarifies why LiAlH₄ reduces –COOH all the way to –CH₂OH, whereas NaBH₄ does not. Their esters and amides appear again in biomolecules (lipids, peptide bonds) and in the Claisen condensation at the degree level.

Common Mistakes

1. Confusing the pKa of phenol (≈ 10) with that of a carboxylic acid (≈ 4–5) — phenols do not liberate CO₂ from NaHCO₃.
2. Assuming “more carbons = more acidic” — actually the reverse for unbranched aliphatic acids.
3. Writing CH₃COONa in the bicarbonate test and forgetting to balance CO₂ as a gaseous product.

Practice Prompts

1. Arrange in increasing acid strength: CH₃CH₂COOH, ClCH₂COOH, CH₃COOH, FCH₂COOH, (CH₃)₂CHCOOH — and justify using the inductive (+I/–I) effect.
2. Identify products A, B, C in: CH₃COOH →(PCl₅) A →(NH₃, Δ) B →(LiAlH₄) C.

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