• 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