CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.4.3 Acidity of Phenols

Relative Acidities of Water, Phenol & Ethanol

  • The pKa is a measure of the acidity of a substance
  • The values of water, phenol, and ethanol show that phenol is a stronger acid than ethanol and water

Relative acidity of ethanol, water & phenol table

Hydroxy Compounds - Relative acidity of ethanol, water and phenol table, downloadable AS & A Level Chemistry revision notes

  • The order of acidity can be explained by looking at their conjugate bases which are formed from the dissociation of the compounds

Delocalisation of charge density

  • In the phenoxide ion (which is the conjugate base of phenol) the charge density on the oxygen atom is spread out over the entire ion
  • As a result, the electrons on the oxygen atom are less available for bond formation with a proton (H+ ion)
  • The conjugate base of ethanol is the ethoxide ion
  • The ethyl group in the ion is an electron-donating group that donates electron density to the oxygen atom
  • As a result, the electron density on the oxygen atom is more readily available for bond formation with an H+ ion

Hydroxy Compounds - Ethoxide Ion, downloadable AS & A Level Chemistry revision notes

The electron-donating alkyl group in the ethoxide ion concentrates charge density on the oxygen atom which can more easily bond an H+ ion

  • The conjugate base of water is the hydroxide ion
  • Since the charge density of the oxygen atom cannot become delocalised over a ring, the hydroxide ion more readily accepts an H+ ion compared to the phenoxide ion
    • Water is, therefore, a stronger base compared to phenol
  • However, as there are no electron-donating alkyl groups, less negative charge is concentrated on the oxygen atom which therefore less readily accepts an H+ ion compared to the ethoxide ion
    • Water is, therefore, a weaker base compared to ethanol

Hydroxy Compounds - Hydroxide Ion, downloadable AS & A Level Chemistry revision notes

The hydroxide ion lacks an aromatic ring and electron-donating alkyl groups so water is a stronger base than phenol but a weaker base than ethanol

  • Therefore, the position of equilibrium lies:
    • Further to the right-hand side favouring the dissociated phenoxide ions
    • Further to the left-hand side favouring the undissociated ethoxide and hydroxide ions

 

Hydroxy Compounds - Equilibrium Positions, downloadable AS & A Level Chemistry revision notes

Relative equilibrium positions for the dissociation of ethanol, water, and phenol

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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