CIE A Level Chemistry复习笔记7.4.3 Reactions of Phenol

• Phenols can undergo many types of reactions as both the electron-rich benzene ring and the polar -OH group can participate in chemical reactions
• Some of the reactions of phenols include:
  • With bases
  • With reactive metals
  • With diazonium salts
  • Nitration
  • Bromination

Reactions of the -OH group in phenols

• The -OH group in phenols has a slightly acidic character
• It can therefore act as an acid and take part in acid-base reactions

Reaction with bases

• Phenols are only slightly soluble in water due to the large non-polar benzene ring
• However, they do dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water

Phenols are weak acids and undergo acid-base reactions in alkaline solutions

Reaction with reactive metals

• Molten phenols react vigorously with reactive metals such as sodium (Na)
• This is also an acid-base reaction
• Now, a soluble salt is formed and hydrogen gas is given off

Molten phenols react vigorously with reactive metals to form a soluble salt and hydrogen gas

Reaction with diazonium ions

• Diazonium ions are very reactive compounds containing an -N₂⁺ group
• When phenols are dissolved in sodium hydroxide (NaOH), a solution of sodium phenoxide is obtained
• This solution is cooled in ice and cold diazonium ion is added to the sodium phenoxide
• After the reaction has occurred, a yellow-orange solution or precipitate of an azo compound is formed
• These are compounds in which two benzene rings are linked by a nitrogen bridge

Azo compounds are formed from the reaction of phenols with diazonium ions

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

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

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