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Carboxylic Acids

Part of the JEE Main study roadmap. Chemistry topic chem-024 of Chemistry.

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Carboxylic Acids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Carboxylic acids are organic compounds containing the carboxyl group (–COOH): a hydroxyl (–OH) attached to a carbonyl (C=O) carbon. Saturated aliphatic acids follow CₙH₂ₙO₂, named R–COOH. The carbonyl carbon is sp² and the –OH oxygen is sp³, allowing two molecules to form a cyclic H-bonded dimer in non-polar solvents — even in the gas phase.

Acidity arises from resonance stabilisation of the carboxylate anion (RCOO⁻), where the negative charge is delocalised equally over both oxygens. Acetic acid pKa = 4.76; alcohols ≈ 16–18. Electron-withdrawing groups (–Cl, –F, –NO₂) sharpen acidity; alkyl groups weaken it. Formic acid (HCOOH) is the strongest simple carboxylic acid because it lacks an electron-donating alkyl group.

Must-know reactions: Fischer esterification (RCOOH + R’OH ⇌ RCOOR’ + H₂O, acid-catalysed), Hell–Volhard–Zelinsky (α-bromination), Kolbe electrolysis (2 RCOO⁻ → R–R), soda-lime decarboxylation, and Rosenmund reduction. Derivative reactivity: acid chloride > anhydride > ester ≈ acid > amide.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature & Structure

The –COOH carbon is sp² hybridised (planar, 120° bond angles) with the C=O π bond, while the hydroxyl O is sp³. The acidic hydrogen is the –OH hydrogen. IUPAC names end in -oic acid (e.g. methanoic, ethanoic, butanoic acid); the carboxyl carbon is C1.

Acidic Character & Substituent Effects

The conjugate base RCOO⁻ is stabilised by two equivalent resonance structures, lowering pKa dramatically compared with alcohols. Acidity order: HCOOH > CH₃COOH > C₂H₅COOH > … (inductive donation by alkyls destabilises the anion). Substituted benzoic acids: o-, m-, p-NO₂ all increase acidity; –I groups (Cl, F) raise acidity when ortho; the ortho effect makes ortho-substituted benzoics more acidic than para even for –I groups, attributed to steric inhibition of resonance plus a field effect.

Henderson–Hasselbalch for Buffer Calculations

For an acid HA and its salt A⁻: pH = pKa + log₁₀([A⁻]/[HA]). JEE routinely tests this with benzoic acid / benzoate buffer calculations.

Preparation Methods

  • Oxidation of primary alcohols or aldehydes (K₂Cr₂O₇/H⁺, or Tollens for HCOOH from HCHO).
  • Hydrolysis of nitriles (R–CN + H₂O/H⁺ or OH⁻ → RCOOH), amides, esters, acid chlorides, anhydrides.
  • Grignard + CO₂: RMgX + CO₂ → RCOOMgX → RCOOH (adds one carbon).
  • Kolbe electrolysis: 2 RCOO⁻ —electrolysis→ R–R + 2 CO₂ + 2 e⁻.

Key Reactions

Fischer esterification: RCOOH + R’OH ⇌ RCOOR’ + H₂O, driven forward by removing water or using excess alcohol; reversible, acid-catalysed (H₂SO₄). Saponification: ester + NaOH (aq) → RCOONa + R’OH. Hell–Volhard–Zelinsky (HVZ): R–CH₂–COOH + Br₂/red P → R–CHBr–COOH (only α-H acids react; formic acid fails). Decarboxylation: soda-lime (RCOONa + NaOH/CaO → RH + Na₂CO₃) removes –COOH as CO₂. Rosenmund: RCOCl + H₂/Pd-BaSO₄ → RCHO. Stephen: RCN + SnCl₂/HCl → RCHO. Hunsdiecker: RCOOAg + Br₂ → RBr + CO₂ + AgBr.

Exam Patterns for JEE Main

Carboxylic acids appear in Single Correct MCQs (1–2 questions per shift), Numerical Value Questions (NVQ), and Assertion–Reason items. High-yield numericals: pH of a buffer using Henderson–Hasselbalch, comparison of acid strength with reasoning, and identifying products of HVZ / Kolbe / Rosenmund.

Common Mistakes to Avoid

  • Treating formic acid as merely an acid — it also gives Tollens’ and Fehling’s tests because of its aldehydic H.
  • Assuming esterification needs an acid catalyst — base-catalysed transesterification differs mechanistically.
  • Confusing Clemmensen (Zn-Hg/HCl) with Wolff–Kishner (NH₂NH₂/KOH, heat) — both reduce C=O to CH₂ but for different substrates.
  • Forgetting that amides need strong heating with H⁺ or OH⁻ to hydrolyse, unlike acid chlorides which hydrolyse in cold water.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Resonance, Inductive Effects, and Quantitative Acidity

The carboxylate anion is a 3-atom, 4-electron π system: −O–C=O ↔ O=C–O⁻. This delocalisation equalises the two C–O bond lengths (~127 pm) in solution, between single (~136 pm) and double (~123 pm) bond lengths. Quantitative substituent constants (Hammett σ) correlate well with pKa shifts in benzoic acids: σₘ(NO₂) = +0.71, σₚ(NO₂) = +0.78, σₘ(OCH₃) = +0.12, σₚ(OCH₃) = −0.27. σₚ values are amplified by resonance for –M groups, while σₘ reflects mainly inductive effects.

Mechanism: Fischer Esterification

Protonation of carbonyl O → nucleophilic attack by R’OH → tetrahedral intermediate → proton transfer → loss of water → deprotonation. The rate-determining step is the nucleophilic addition; the reaction is AAC2 (acid-catalysed, acyl–oxygen cleavage, bimolecular). Retention of configuration at chiral α-carbons confirms acyl–oxygen cleavage.

Decarboxylation Mechanisms

  • Soda-lime: RCOONa + NaOH/CaO, Δ → RH + Na₂CO₃. Mechanism: hydroxide abstracts the α-H of the sodium salt; the resulting carbanion expels CO₂, forming a hydride that picks up a proton. Works for all but formic acid; acetic acid gives methane.
  • Hunsdiecker: RCOOAg + Br₂, CCl₄, Δ → RBr + CO₂ + AgBr. Goes through an acyl hypobromite (RCOOBr) and a free-radical chain; stereochemistry is mostly retention at the α-carbon for simple aliphatic substrates.
  • Kolbe electrolysis: anodic oxidation of RCOO⁻ gives RCOO• → R• + CO₂; two radicals couple to R–R. Useful for symmetric alkane synthesis.

Adjacent Topics & Cross-Links

  • Carbonic acid (H₂CO₃, pKa₁ ≈ 6.35) is weaker than carboxylic acids — that’s why NaHCO₃ liberates CO₂ from –COOH: a classic JEE test of relative acidity.
  • Phenols (pKa ≈ 10) are weaker than carboxylic acids but stronger than alcohols.
  • Acid derivatives interconvert via nucleophilic acyl substitution; reactivity tracks leaving-group ability: Cl⁻ > RCOO⁻ > RO⁻ > NH₂⁻.

Worked Micro-Example

Q: Benzoic acid (pKa = 4.20) 0.10 M is mixed with sodium benzoate 0.30 M. Find pH. Solution: pH = 4.20 + log(0.30/0.10) = 4.20 + log 3 = 4.20 + 0.477 = 4.68.

Practice Prompts

  1. Assertion–Reason: “Formic acid reduces Tollens’ reagent because it is the strongest carboxylic acid.” Is the assertion true? Is the reason the correct explanation? (Answer: Assertion true, reason false — reduction is due to the aldehydic H, not acid strength.)
  2. Numerical: An unknown saturated monoacid has M = 102 g mol⁻¹ and decolourises bromine in the presence of red P, giving a monobromo acid. The original acid is? (102 = C₅H₁₀O₂ = valeric acid; HVZ gives 2-bromovaleric acid.)

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