Core reactions to know

· Carboxylic acids contain the –COOH / –CO₂H functional group.
· The key A-Level focus is benzoic acid formation, conversion to acyl chlorides, further oxidation of selected acids, and relative acidity.
· General formula: RCOOH; aromatic example: C₆H₅COOH = benzoic acid.
· Key exam skill: link acid strength to stability of the conjugate base.

This image shows the carbonyl oxygen, hydroxyl oxygen, and acidic hydrogen in the –COOH group. It is useful for visualising why carboxylic acids contain both C=O and O–H features in one functional group. Source

Benzoic acid from alkylbenzenes

· Alkylbenzene → benzoic acid using hot alkaline KMnO₄, then dilute acid.
· Example: methylbenzene → benzoic acid.
· The alkyl side-chain is oxidised to –COOH; the benzene ring remains intact.
· Reagents/conditions: hot alkaline potassium manganate(VII), KMnO₄, followed by dilute acidification.
· Typical equation summary: C₆H₅CH₃ + [O] → C₆H₅COOH.
· Exam phrase: complete oxidation of the side-chain gives benzoic acid.

This reaction scheme directly matches the syllabus example of making benzoic acid from methylbenzene. It shows that the –CH₃ side-chain is converted into –COOH under strong oxidation conditions. Source

Formation of acyl chlorides

· Carboxylic acid → acyl chloride by replacing –OH in –COOH with Cl.
· General reaction: RCOOH → RCOCl.
· Reagents: PCl₃ and heat, PCl₅, or SOCl₂.
· With PCl₅: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl.
· With SOCl₂: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
· With PCl₃: 3RCOOH + PCl₃ → 3RCOCl + H₃PO₃.
· Exam phrase: product is an acyl chloride, not a halogenoalkane.

This diagram shows the product formed when a carboxylic acid reacts with PCl₃, PCl₅, or SOCl₂. It highlights the key structural change: –OH in –COOH is replaced by Cl. Source

Further oxidation of selected carboxylic acids

· Most carboxylic acids are resistant to further oxidation, but methanoic acid and ethanedioic acid are exceptions.
· Methanoic acid, HCOOH, is oxidised to carbon dioxide and water.
· Oxidising agents for HCOOH: Fehling’s reagent, Tollens’ reagent, acidified KMnO₄, or acidified K₂Cr₂O₇.
· Simplified oxidation: HCOOH + [O] → CO₂ + H₂O.
· Expected observations: Tollens’ → silver mirror, Fehling’s → brick-red precipitate, acidified KMnO₄ → purple decolourises, acidified K₂Cr₂O₇ → orange to green.
· Ethanedioic acid, HOOCCOOH, is oxidised by warm acidified KMnO₄ to carbon dioxide.
· Simplified oxidation: HOOCCOOH + [O] → 2CO₂ + H₂O.
· Exam phrase: warm acidified KMnO₄ oxidises ethanedioic acid to CO₂.

Relative acidity: carboxylic acids, phenols and alcohols

· Acidity order: carboxylic acids > phenols > alcohols.
· Carboxylic acids are strongest because their conjugate base, the carboxylate ion RCOO⁻, is strongly stabilised by delocalisation over two oxygen atoms.
· Phenols are weaker than carboxylic acids because the phenoxide ion is stabilised by delocalisation into the benzene ring, but less effectively than a carboxylate ion.
· Alcohols are weakest because the alkoxide ion RO⁻ has its negative charge largely localised on one oxygen atom.
· Key explanation: greater conjugate base stability = stronger acid.
· Avoid saying “more H atoms means more acidic”; acid strength depends mainly on ease of H⁺ loss and anion stability.

Chlorine-substituted carboxylic acids

· Chlorine-substituted carboxylic acids are more acidic than the corresponding non-chlorinated carboxylic acid.
· Cl is an electron-withdrawing group and has a negative inductive effect, –I.
· The –I effect pulls electron density away from COO⁻, stabilising the carboxylate ion.
· More Cl atoms = stronger acid, because the electron-withdrawing effect is greater.
· Cl closer to –COOH = stronger acid, because the inductive effect decreases with distance.
· Example acidity trend: CH₃COOH < ClCH₂COOH < Cl₂CHCOOH < CCl₃COOH.
· Exam phrase: chlorine atoms stabilise the conjugate base by the –I effect.

These diagrams show why adding chlorine atoms increases carboxylic acid strength. Chlorine atoms pull electron density through σ bonds, stabilising the carboxylate ion and lowering pKa. Source

High-yield exam equations

· Methylbenzene oxidation: C₆H₅CH₃ → C₆H₅COOH using hot alkaline KMnO₄, then dilute acid.
· Acyl chloride formation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
· Acyl chloride formation: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl.
· Acyl chloride formation: 3RCOOH + PCl₃ → 3RCOCl + H₃PO₃.
· Methanoic acid oxidation: HCOOH + [O] → CO₂ + H₂O.
· Ethanedioic acid oxidation: HOOCCOOH + [O] → 2CO₂ + H₂O.

Common exam traps

· Do not confuse benzoic acid formation with electrophilic substitution: this is side-chain oxidation, not ring substitution.
· Do not write RCH₂Cl as the product of carboxylic acid + PCl₅ / SOCl₂; the product is RCOCl.
· Do not claim all carboxylic acids are easily oxidised; only syllabus examples are methanoic acid and ethanedioic acid.
· Do not explain acidity using “number of oxygens” alone; explain using conjugate base stability.
· Do not forget conditions: hot alkaline KMnO₄ then dilute acid for benzoic acid from methylbenzene.