7.4.4 Acidity of Phenols

• Although phenol compounds contain an alcohol (-OH) group, they are weakly acidic
• This is due to the delocalisation of one of the lone pairs from the oxygen atom into the aromatic ring
• This increases the electron density of the ring and increases the acidic behaviour

Delocalisation of charge density

• The conjugate base of phenol is the phenoxide ion
• In the phenoxide ion, the negative charge on the oxygen is spread out over the entire ion
• This is possible as one of the lone pairs on the oxygen atom overlaps with the delocalised π system of the ring
• Because of this delocalisation, there is less charge density on the oxygen atom
• The H⁺ ions are therefore not strongly attracted to the phenoxide ion and are less likely to reform the phenol molecule
• This means that phenol is more likely to lose a proton (and act as an acid) rather than to gain a proton (and act as a base )

The negative charge is spread over the ion, causing the electrons to become less available for bonding with an incoming proton

Stability of the conjugate base

• Phenol ionises to form a more stable negative phenoxide ion with its negative charge spread out
• This means that phenol is more likely to undergo ionisation
• The equilibrium position, therefore, lies further to the right and a higher proportion of phenol molecules donate a proton compared to for example water and ethanol
• The phenol compound is, therefore, more likely to act as an acid rather than a base

Since the phenoxide ion formed from the ionisation of phenol is more stable than phenol itself, the equilibrium position lies further to the right-hand-side and phenol is more likely to act as an acid rather than a base

• 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