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Alcohols and Phenols

Acidity of phenol

  • 3 things to know
  • Component 2

Overview

Anion produced when phenol loses a proton, why phenol is a far stronger acid than ethanol and acid dissociation constant and pKa of phenol.

What you need to know

Answer each one before you reveal it.

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Key term

Explain why phenol is a far stronger acid than ethanol.

Key term

Write the equation for the reaction of phenol with sodium metal, and explain why this reaction does not identify phenol.

Key term

Write the equation for the ionisation of phenol in aqueous solution.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Give the formula of phenol and describe its appearance and melting temperature.

    Model answer

    Phenol is C₆H₅OH, a white crystalline solid melting at 43 °C, only slightly soluble in cold water and with a sharp antiseptic smell.

  2. 2

    Write the equation for the ionisation of phenol in aqueous solution.

    Model answer

    C₆H₅OH(aq) ⇌ C₆H₅O⁻(aq) + H⁺(aq).

  3. 3

    Name the anion produced when phenol loses a proton.

    Model answer

    It is the phenoxide ion.

  4. 4

    Explain why phenol is a far stronger acid than ethanol.

    Model answer

    In the phenoxide ion the lone pair left on the oxygen is drawn into the delocalised π system of the ring, so the negative charge is spread over the whole ion and the anion is stabilised, whereas in the ethoxide ion the charge stays concentrated on one oxygen atom. The more stable anion means the dissociation equilibrium lies much further to the right.

  5. 5

    Explain, in terms of the O—H bond, how attaching the hydroxyl group to a benzene ring increases acidity.

    Model answer

    The ring draws electron density away from the oxygen towards itself, which weakens the O—H bond and so lets the proton be released more readily.

  6. 6

    State the acid dissociation constant and pKa of phenol.

    Model answer

    Ka is about 1.0 × 10⁻¹⁰ mol dm⁻³, which corresponds to a pKa of about 10.0.

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