33.1 Carboxylic Acids

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall how benzoic acid is produced from an alkylbenzene (specifically methylbenzene) using hot alkaline KMnO₄ followed by dilute acid.
  2. Describe how carboxylic acids react with PCl₃ (and heat), PCl₅, or SOCl₂ to form acyl chlorides.
  3. Recognise that some carboxylic acids can be further oxidised:
    • Methanoic acid (HCOOH) is oxidised by Fehling's reagent, Tollens' reagent, acidified KMnO₄, or acidified K₂Cr₂O₇ to give CO₂ and H₂O.
    • Ethanedioic acid (HOOCCOOH) is oxidised by warm acidified KMnO₄ to give CO₂.
  4. Describe and explain the relative acidities of carboxylic acids, phenols, and alcohols.
  5. Describe and explain the relative acidities of chlorine-substituted carboxylic acids.

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Background: What Is a Carboxylic Acid?

A carboxylic acid is an organic compound that contains the carboxyl group, written as –COOH. This group is made up of a C=O (carbonyl) and an O–H (hydroxyl) attached to the same carbon atom.

Examples:

  • Methanoic acid: HCOOH (the simplest carboxylic acid — just one carbon)
  • Ethanoic acid: CH₃COOH (vinegar is a dilute solution of this)
  • Benzoic acid: C₆H₅COOH (a carboxylic acid attached to a benzene ring)

Objective 1 — Production of Benzoic Acid from Methylbenzene

What is methylbenzene?

Methylbenzene (also called toluene) is a benzene ring with a –CH₃ (methyl) group attached to it. It belongs to a group of compounds called alkylbenzenes — these are benzene rings that have an alkyl (carbon-containing) side chain attached.

What is benzoic acid?

Benzoic acid (C₆H₅COOH) is a carboxylic acid where the –COOH group is directly attached to a benzene ring.

How is benzoic acid made from methylbenzene?

The reaction happens in two stages:

Stage 1 — Oxidation with hot alkaline KMnO₄

Methylbenzene is heated with hot alkaline potassium manganate(VII) (KMnO₄ dissolved in alkali). This is a powerful oxidising agent — it has the ability to take electrons away from other substances (i.e., it breaks things down by adding oxygen).

The –CH₃ side chain on the benzene ring is oxidised (broken down and oxygen is added). The ring itself is NOT broken — benzene rings are very stable and resist being destroyed.

The product at this stage is a carboxylate salt (benzoate salt), because the reaction is done under alkaline conditions.

Stage 2 — Acidification with dilute acid

The carboxylate salt produced in Stage 1 is then treated with dilute acid (e.g., dilute hydrochloric acid or dilute sulfuric acid). This converts the carboxylate salt into benzoic acid.

Summary equation (overall):

Methylbenzene → (hot alkaline KMnO₄, then dilute acid) → Benzoic acid

C₆H₅CH₃ → C₆H₅COOH

Key point to remember: Any alkylbenzene (a benzene ring with any alkyl side chain) will produce benzoic acid when treated this way, regardless of how long the side chain is. The entire side chain is oxidised down to just –COOH.

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