Phenol (OCR A Level Chemistry)

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Acidity of Phenol

Organic Chemistry - 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

Hydroxy Compounds - Delocalisation in Phenol

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

Hydroxy Compounds - Acid-base Reaction of Phenols

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

phenol

Reaction of phenol with Na 

Reactions of Phenol

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

Hydroxy Compounds - Bromination, downloadable AS & A Level Chemistry revision notes

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

Hydroxy Compounds - Nitration, downloadable AS & A Level Chemistry revision notes

Phenols undergo nitration when reacted with dilute HNO3 at room temperature

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.