Carboxylic acids

Specification Reference Organic Chemistry, Carboxylic acids and derivatives 33.1

Quick Notes

Benzoic acid can be made by oxidising methylbenzene
(e.g. using hot alkaline KMnO₄ followed by dilute acid).
Carboxylic acids react with PCl₅, PCl₃ or SOCl₂ to form acyl chlorides.
Methanoic acid (HCOOH) can be further oxidised to CO₂ and H₂O using Fehling’s, Tollens’ reagent, or acidified KMnO₄/K₂Cr₂O₇.
Ethanedioic acid (oxalic acid) also undergoes oxidation with warm acidified KMnO₄ to give CO₂.
Acidity comparison: Carboxylic acids > Phenols > Alcohols.
Electron-withdrawing groups (like Cl) increase acid strength by weakening and stabilising the carboxylate ion.

Benzoic acid is formed by oxidising the methyl group of an alkylbenzene (e.g. methylbenzene) to a carboxylic acid group (COOH).

Reagents: Hot alkaline potassium manganate(VII) (KMnO₄) followed by dilute acid.

Acyl chlorides (R–COCl) are highly reactive derivatives of carboxylic acids and are useful intermediates in organic synthesis.

They are commonly made by replacing the –OH group of a carboxylic acid with a -Cl atom using one of the following reagents:

PCl₅ (phosphorus pentachloride)

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

PCl₃ (phosphorus trichloride) and heat

3RCOOH + PCl₃ → 3RCOCl + H₃PO₃

SOCl₂ (thionyl chloride)

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
(Cleanest method – all by-products are gases)

Certain carboxylic acids such as methanoic acid and ethanoic acid can be further oxidised.

Methanoic acid can be oxidised further to carbon dioxide and water.

This is because methanoic acid contains an H directly bonded to the –COOH group, making it behave similarly to an aldehyde.

Reaction: HCOOH + [O] → CO₂ + H₂O

Oxidising agents: Fehling’s solution, Tollens’ reagent, acidified KMnO₄ or K₂Cr₂O₇.

Reaction: (COOH)₂ + [O] → 2CO₂ + H₂O

Warm acidified potassium manganate(VII) oxidises ethanedioic acid to carbon dioxide.

Alcohols, phenol and carboxylic acids all have an OH group, however they have different acidities (ability to lose a H⁺ ion).

Explanation:
The carboxylate ion (RCOO⁻) formed from carboxylic acids is resonance-stabilised, making it easier to lose H⁺.

Phenoxide ion is less stabilised, and alkoxide ions from alcohols are not stabilised much at all.

Electron-withdrawing groups (like chlorine atoms) bonded to the COOH group increase acidity.

Example: Trichloroethanoic acid (CCl₃COOH) is much more acidic than ethanoic acid (CH₃COOH).

Chlorine atoms pull electron density away from the carboxylate ion via the inductive effect.

This weakens the O-H bond and stabilises the negative charge on the ion, making it easier to lose the H⁺ from the acid group.

Benzoic acid can be prepared by oxidation of methylbenzene.
Carboxylic acids react with PCl₅, PCl₃ or SOCl₂ to form acyl chlorides.
Methanoic acid can be further oxidised to CO₂ and H₂O.
Ethanedioic acid is oxidised by warm acidified KMnO₄ to CO₂.
Acid strength order: Alcohols < Phenols < Carboxylic acids.
Electron-withdrawing groups (like Cl) increase acidity by stabilising the carboxylate ion.