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Carboxylic Acids

Preparation from primary alcohols and aldehydes

  • 3 things to know
  • Unit A2 1

Overview

Why an excess of the oxidising agent is used, purpose of heating under reflux rather than distilling and colour change seen during the oxidation.

What you need to know

Answer each one before you reveal it.

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Key term

Which reagent oxidises a primary alcohol to a carboxylic acid, what are the equations for ethanol and ethanal, and what conditions are needed?

Key term

Name the oxidising agent used to prepare a carboxylic acid from a primary alcohol.

Key term

Write the equation for the oxidation of ethanol to ethanoic acid using [O] for the oxidising agent.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Name the oxidising agent used to prepare a carboxylic acid from a primary alcohol.

    Model answer

    Potassium dichromate(VI) acidified with dilute sulfuric acid is used.

  2. 2

    Write the equation for the oxidation of ethanol to ethanoic acid using [O] for the oxidising agent.

    Model answer

    The equation is CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

  3. 3

    Write the equation for the oxidation of ethanal to ethanoic acid using [O] for the oxidising agent.

    Model answer

    The equation is CH₃CHO + [O] → CH₃COOH.

  4. 4

    Describe the conditions used to convert a primary alcohol completely to a carboxylic acid.

    Model answer

    The alcohol is heated under reflux with an excess of acidified potassium dichromate(VI) for about twenty minutes.

  5. 5

    Explain why an excess of the oxidising agent is used.

    Model answer

    Two oxidation steps are needed, so enough oxidising agent must be present to take the alcohol past the aldehyde stage to the acid.

  6. 6

    Explain the purpose of heating under reflux rather than distilling.

    Model answer

    Reflux condenses the vapour and returns it to the flask, so the volatile aldehyde is not lost and remains in contact with the oxidising agent.

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