• The pK_a 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

• 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

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

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

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