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Chemistry 3% exam weight

Carboxylic Acids

Part of the MDCAT study roadmap. Chemistry topic chem-16 of Chemistry.

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Carboxylic Acids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Carboxylic acids contain the carboxyl group (–COOH): a hydroxyl (–OH) bonded directly to a carbonyl carbon (C=O). General formula R–COOH, where R = H (formic acid, HCOOH), alkyl (CH₃COOH acetic acid), or aryl (C₆H₅COOH benzoic acid).

  • Acidity arises because the carboxylate anion (RCOO⁻) is resonance-stabilised across two equivalent oxygens, delocalising the negative charge. Acetic acid pKₐ ≈ 4.76 — a weak acid, far weaker than HCl but stronger than phenol (≈10) and water (14).
  • Boiling points are exceptionally high because two molecules hydrogen-bond into a cyclic dimer, effectively doubling the molecular mass for van der Waals calculations.
  • MDCAT high-yield: brisk CO₂ effervescence with NaHCO₃ distinguishes –COOH from phenol; Fischer esterification with alcohol + concentrated H₂SO₄ gives sweet-smelling esters; decarboxylation of sodium salts with soda lime (NaOH/CaO) yields the alkane R–H, not an alcohol.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure and Hybridisation

The carboxyl carbon is sp² hybridised with trigonal-planar geometry, leaving one p-orbital for π-bonding with oxygen. The O–H oxygen is sp³. In the conjugate base RCOO⁻, both C–O bonds become equivalent (bond length ≈127 pm), and the negative charge is delocalised equally over both oxygens — this resonance is the structural reason carboxylic acids donate protons readily.

Acidity and the Inductive Effect

Using Kₐ = [H⁺][A⁻]/[HA], acetic acid has Kₐ ≈ 1.8 × 10⁻⁵. Electron-withdrawing substituents (Cl, NO₂) on the α-carbon increase acidity by stabilising the anion; alkyl groups exert a +I (positive inductive) effect, pushing electron density into the carboxylate and decreasing acidity down the homologous series (formic > acetic > propanoic > butanoic).

Physical Properties

PropertyReason
High boiling pointsCyclic dimer formation via two H-bonds
Solubility in water (≤C4)–COOH hydrogen-bonds with H₂O
Pungent smell (C1–C3), rancid (C4–C10), waxy (>C10)Increasing hydrophobic chain

Key Reactions (MDCAT-favoured)

  • With NaHCO₃: RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂↑ (brisk effervescence — diagnostic test).
  • Fischer esterification: RCOOH + R’OH ⇌ RCOOR’ + H₂O (conc. H₂SO₄ catalyst, reversible).
  • Decarboxylation: RCOONa + NaOH/CaO (soda lime) → R–H + Na₂CO₃ (product is an alkane, not an alcohol).
  • Reduction: RCOOH + LiAlH₄ → RCH₂OH (primary alcohol); NaBH₄ does not reduce carboxylic acids.
  • Conversion to acid chloride: RCOOH + SOCl₂ → RCOCl + SO₂↑ + HCl↑.

Naming

IUPAC suffix is -oic acid (methanoic, ethanoic, propanoic); common names — formic, acetic, benzoic — appear frequently in MCQs.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Resonance and Why –COOH Outranks Phenol

The carboxylate anion has two equivalent resonance structures, each placing the negative charge on one oxygen — this is more effective stabilisation than phenol’s phenoxide (charge delocalised onto carbon). Hence pKₐ(acetic) ≈ 4.76 ≪ pKₐ(phenol) ≈ 10. MDCAT assertion-reason questions often hinge on this comparison.

Mechanism Spotlight: Fischer Esterification

Step 1: Protonation of carbonyl O by H₂SO₄ activates the carbon toward nucleophilic attack. Step 2: Alcohol O attacks the carbonyl C, forming a tetrahedral intermediate. Step 3: Proton transfer and loss of water yields the oxocarbenium ion. Step 4: Deprotonation gives the ester. The reaction is reversible and acid-catalysed; Le Chatelier’s principle is applied by removing water or using excess alcohol.

Edge Cases and Common Traps

  • HCOOH is unique: it contains both –COOH and an aldehyde-like H–C=O, so it gives a positive Tollens’ (silver mirror) and Fehling’s test — unlike any other carboxylic acid.
  • α-hydrogen chemistry: halogenation at the α-carbon (Hell–Volhard–Zelinsky reaction uses P/Br₂) introduces Cl/Br adjacent to –COOH, increasing acidity further.
  • Dicarboxylic acids: oxalic (HOOC–COOH, pKₐ₁ = 1.25) is far stronger than acetic due to two electron-withdrawing COOH groups; adipic acid (C6) is industrial feedstock for nylon-6,6.
  • Decarboxylation trap: students often write RCOONa + NaOH/CaO → ROH. Correct product is R–H (alkane); soda lime supplies CaO to keep NaOH dry and non-fusing.
  • Salt formation with Na₂CO₃ (not just NaHCO₃) also releases CO₂ — examiners sometimes substitute reagents to test recognition.

Exam Strategy

MDCAT Chemistry allocates ~3% weight to this chapter, typically 1–2 MCQs. Expect identification-by-reagent questions (NaHCO₃ vs NaOH vs Na), naming conversions, and one assertion-reason on acidity ordering: carboxylic acid > carbonic acid > phenol > water > alcohol > alkyne.

Practice Prompts

  1. Arrange formic, acetic, propanoic, and benzoic acids in increasing pKₐ. Justify using the +I effect and resonance with the ring.
  2. Write balanced equations for: (a) benzoic acid + NaHCO₃, (b) sodium acetate + soda lime, (c) acetic acid + ethanol with conc. H₂SO₄. Name each organic product.

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