6.1.3 Phenol

The structure of phenol 

• In phenol the OH group is directly bonded to the benzene ring
• Phenol can behave as a very weak acid
• Depending on the concentration, the typical pH of a solution of phenol will be 5-6
• A hydrogen ion can break away from the -OH group and transfer to a base

C₆H₅OH + H₂O ⇌ C₆H₅O^- + H₃O⁺

• Phenol is able to lose a hydrogen ion because the phenoxide ion (C₆H₅O^-) formed is fairly stable
• The more stable the ion is, the more likely it is to form
• One of the lone pairs of electrons on the oxygen is delocalised with the electron cloud of the whole phenoxide ion
• Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen

Delocalisation in the phenoxide ion

• Oxygen is the most electronegative element in the ion and the delocalised electrons will be drawn towards it
• That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again
• This is why phenol behaves as just a weak acid
• This behaviour is demonstrated by the reactions of phenol with sodium hydroxide and sodium metal 

• Phenols dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water

Reaction of phenol with sodium NaOH 

• Phenols react vigorously with reactive metals such as sodium (Na)
• A soluble salt is formed and hydrogen gas is given off

Reaction of phenol with Na 

Reactions of the aromatic ring in phenols

• Phenols react more readily with electrophiles compared to benzene
• This is because one of the lone pairs of electrons on the oxygen atom in -OH overlaps with the π bonding system
• This increases the electron density of the benzene ring making it more susceptible to electrophilic attack
• The -OH group in phenols is activating and directs incoming electrophiles to the 2, 4, and 6 positions

Bromination

• Phenols also undergo electrophilic substitution reactions when reacted with bromine water at room temperature
• Phenol decolourises the orange bromine solution to form a white precipitate of 2,4,6-tribromophenol
• This is also known as the bromination of phenol

Phenols undergo bromination when reacted with bromine water at room temperature

Nitration

• Phenols can undergo electrophilic substitution reactions when reacted with dilute nitric acid (HNO₃) at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol
  • When concentrated HNO₃ is used, the product will be 2,4,6-trinitrophenol instead

• A hydrogen atom in the benzene ring is substituted by a nitro (-NO₂) group
• This is also known as the nitration of phenol

Phenols undergo nitration when reacted with dilute HNO₃ at room temperature