CIE A Level Chemistry (9701) exams from 2022

Revision Notes

7.4.5 Nitration & Bromination of Phenol

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

Hydroxy Compounds - Reagents and conditions for nitration and bromination of phenol and benzene table, downloadable AS & A Level Chemistry revision notes

Directing Effects of Hydroxyl Group on Phenol

  • 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

Hydroxy Compounds - Directing Effects of Hydroxyl, downloadable AS & A Level Chemistry revision notes

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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