Phenol

Phenols and phenol

  • Type of organic chemical containing a hydroxyl group, -OH, bonded directly to an aromatic ring. 
  • The simplest phenol, C6H5OH, has the same name as the group, phenol. 
  • Although alcohols and phenols have some common reactions, many reactions are different as the proximity of the delocalised ring influences the -OH group. 
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Phenol as a weak acid

  • Phenol is less soluble in water due to the presence of the non-polar benzene ring. 
  • When dissolved in water, phenol partially dissociates froming the phenoxide ion and a proton. 
  • Becasue of this ability to partially dissociate to produce protons, phenol is described as a weak acid. 
  • Phenol is more acidic than alcohols but less acidic than carboxylic acids. 
  • Ethanol does not react with sodium hydroxide (strong base) or sodium carbonate (weak base).
  • Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide. 
  • Only carboxylic acids are strong enough to react with the weak base, sodium carbonate. 
  • A reaction with sodium carbonate can be used to distinguish between a phenol and carboxylic acid - the carboxylic acid reacts with sodium carbonate to produce carbon dioxide, which is evolved as a gas. 
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Reaction of phenol with sodium hydroxide

  • Reacts with sodium hydroxide to form the salt, sodium phenoxide, and water in a neutralisation reaction. 

C6H5OH + NaOH ---> C5H5O-Na+ + H2O 

Phenol + sodium hydroxide ---> sodium phenoxide + water 

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Electrophilic substitution reactions of phenol

  • Phenols are aromatic compounds and they undergo electrophilic substitution reactions.
  • The reactions take place under milder conditions and more readily than the reactions of benzene. 
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Bromination of phenol

  • Reacts with an aqueous solution of bromine to form a white precipitate of 2,4,6-tribromophenol. 
  • The reaction decolourises the bromine water. 
  • A halogen carrier is not required and the reaction is carried out at room temperature. 

C6H5OH + 3Br2 ---> C6H2Br3OH + 3HBr 

Phenol + bromine ---> 2,4,6-tribromophenol + hydrogenbromide 

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Nitration of phenol

  • Phenol reacts with dilute nitric acid at room temperature. A mixture of 2-nitrophenol and 4-nitrophenol is formed. 

C6H5OH + HNO3 ---> C6H4NO2OH + H2O 

Phenol + nitric acid ---> nitrophenol + water 

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Comparing reactivity of phenol and benzene

  • Increased reactivity of phenol is caused by a lona pair of electrons from the oxygen p orbital of the -OH group being donated to the pi system of phenol. 
  • The electron density is increased. The increased electron density attracts electrophiles more readily than with benzene. 
  • The aromatic ring in phenol is more susceptible to attack from electrophiles than in benzene. 
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