Reactions of Phenol (Cambridge (CIE) A Level Chemistry)

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

Phenol in acid-base reactions

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

Phenol with metals reaction

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

Phenol in diazonium ion reactions

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

Phenol in nitration reactions

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

Phenol in bromination reactions

Phenols undergo bromination when reacted with bromine water at room temperature