7.4.5 Nitration & Bromination of Phenol

• Compared to benzene, phenol reacts more readily with electrophiles
• This is because one of the lone pairs of electrons on the oxygen atom in phenol overlaps with the π bonding system of the benzene ring
• As a result, there is now an increased electron density in the ring
• The electron-donating -OH group in phenol, therefore, activates the benzene ring and directs incoming electrophiles to the 2, 4, and 6 positions
• The increased reactivity of phenol means that different reagents and conditions are used for electrophilic substitution reactions of phenols compared to benzene

Nitration

• Nitration is an example of an electrophilic substitution reaction
• The nitration of benzene requires a mixture of concentrated nitric acid (HNO₂) and sulfuric acid (H₂SO₄) refluxed with benzene between 25 ^oC and 60 ^oC
• Since phenol is more reactive, nitration can occur under milder conditions by reacting it with dilute nitric acid at room temperature
  • If concentrated nitric acid is used, 2,4,6-trinitrophenol is formed

Bromination

• Bromination is another example of an electrophilic substitution reaction
• Benzene will undergo bromination only when reacted with pure bromine (not a solution) and in the presence of an anhydrous aluminium bromide (AlBr₃) catalyst at room temperature
• Phenol on the other hand readily reacts with bromine water in the absence of a catalyst

Reagents & conditions for nitration and bromination of phenol & benzene table

• Phenols consist of a hydroxyl (-OH) group attached to a benzene ring
• The oxygen atom in this hydroxyl group donates electron density into the ring
• One of the lone pairs of the oxygen atom overlaps with the π system of the benzene ring and become delocalised causing an increased electron density in the aromatic ring
• Due to the increased electron density, the benzene ring is now more likely to undergo electrophilic attack and becomes activated
• The incoming electrophiles are directed by the hydroxyl group of the phenol to the 2, 4, and 6 positions
• An example is the bromination of phenol
  • The bromine acts as an electrophile and substitutes a hydrogen atom in the benzene ring
  • The substitution of the hydrogen atom can occur on the 2, 4, or 6 positions

The hydroxyl group in phenol directs bromination in the 2, 4, and 6 positions