11.7 Carboxylic Acids

2026 Syllabus Objectives

Core:

  1. Describe the reaction of ethanoic acid with:
    • (a) metals
    • (b) bases
    • (c) carbonates including names and formulae of the salts produced

Supplement: 2. Describe the formation of ethanoic acid by the oxidation of ethanol:

  • (a) with acidified aqueous potassium manganate(VII)
  • (b) by bacterial oxidation during vinegar production
  1. Describe the reaction of a carboxylic acid with an alcohol using an acid catalyst to form an ester

What are Carboxylic Acids? 🔑

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Carboxylic acids are a homologous series of organic compounds that all contain the functional group −COOH-\text{COOH} (or −CO2H-\text{CO}_2\text{H}) attached to a hydrocarbon chain.

General Formula

The general formula for carboxylic acids is:

CnH2n+1COOH\text{C}_n\text{H}_{2n+1}\text{COOH}

where nn is the number of carbon atoms in the alkyl group.

Important: When naming a carboxylic acid, remember that the carbon atom of the acid group is counted as the first carbon in the chain.

First Two Members of the Series

Carboxylic AcidMolecular FormulaDescription
Methanoic acidHCOOH\text{HCOOH}Present in nettle stings and ant stings
Ethanoic acidCH3COOH\text{CH}_3\text{COOH}The acid in vinegar (previously called acetic acid)

Physical Properties

  • The first two acids in the series are liquids at room temperature
  • Ethanoic acid will solidify if the temperature falls only slightly
  • These acids dissolve in water to produce solutions that are weakly acidic

Formation of Ethanoic Acid ⚡

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Ethanoic acid can be formed by the oxidation of ethanol through two main methods:

Method 1: Bacterial Oxidation (Vinegar Production) 🍷

Vinegar is a weak solution of ethanoic acid produced commercially from wine by biochemical oxidation using bacteria called Acetobacter.

Key Features:

  • This is an example of traditional biotechnology
  • The bacteria are naturally present in the air
  • Wine can simply become "vinegary" if left open to the air
  • This process occurs slowly over time

Method 2: Chemical Oxidation with Acidified Potassium Manganate(VII)

The same oxidation can be achieved quickly using powerful oxidising agents such as warm acidified potassium manganate(VII).

Apparatus Setup:

  • The condenser is arranged vertically to prevent the volatile alcohol escaping
  • This would be dangerous as ethanol is highly flammable
  • This setup is known as refluxing

Refluxing: A practical technique using a (reflux) condenser fitted vertically to condense vapours from an experiment back into a flask.

Observable Change:

  • The colour of the potassium manganate(VII) solution turns from purple to colourless

Word Equation:

ethanol+oxygen from oxidising agent→ethanoic acid+water\text{ethanol} + \text{oxygen from oxidising agent} \rightarrow \text{ethanoic acid} + \text{water}

Chemical Equation:

C2H5OH+2[O]→CH3COOH+H2O\text{C}_2\text{H}_5\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}

where [O][\text{O}] represents oxygen from the oxidising agent.

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