The chemistry of phenol

• phenols are a type of organic chemical containing a hydroxyl -OH functional group directly bonded to an aromatic ring
• simplest member of the phenols, C6H5OH has the same name as the group - phenol
• some compounds such as C6H5CH2OH contain an -OH group bonded to a carbon side chain rather than the aromatic ring - alcohols rather than phenols
• although alcohols and phenols have some common reactions, many reactions are different as the proximity of the delocalised ring influences the -OH group
• phenols are used in many everyday antiseptics
• phenol is an important chemical used in the production of disinfectants, detergents, plastics, paints and even aspirin
• original manufacture:

  • phenol used to be made from benzene, C6H6, using sulfuric acid and sodium hydroxide in a multi-stage process
  • C6H6 + H2SO4 + 2NaOH = C6H5OH + Na2SO3 + 2H2O
• current manufacture of phenol:

  • majority of phenol is manufactured from benzene and propene, C3H6 in a multistep reaction
  • C6H6 + C3H6 + 02 = C6H5OH +CH3COCH3
  • reaction has an 86% yield of phenol from benzene
  • other product is propanone (acetone) - useful product but less in demand than phenol, and companies can find it difficult to make use of all the propanone produced
• future manufacture of phenol:

  • research chemists are investigating another way of producing phenol using benzene and nitrogen (I) oxide, N2O (nitrous oxide)
  • C6H6 + N2O = C6H5OH + N2
  • reaction has a 95% yield of phenol from benzene
  • nitrous oxide is a gaseous waste product from the production of nylon and cannot be allowed to escape into the atmosphere - greenhouse gas
• Phenol as a weak acid:

  • less soluble in water than alcohols due to the presence of the non-polar benzene ring
  • when dissolved in water, phenol partially dissociated forming the phenoxide ion and a proton
  • bc of this ability to partially dissociate to produce protons - classed as a weak acid
  • other phenols act as weak acids
• • phenol is more acidic than alcohols but less acidic than carboxylic acids
  • can be seen by comparing the acid dissociation constant, Ka of an alcohol with a phenol and carboxylic acid
• • 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 acids to react with the weak base sodium carbonate
• a reaction with sodium carbonate can be used to distinguish between a phenol and a COOH - the COOH reacts to sodium carbonate to produce CO2 - evolved as a gas
• Reaction of phenol with sodium hydroxide:

  • phenol reacts with sodium hydroxide to produce the salt, sodium phenoxide and water in a neutralisation reaction
• Electrophilic substitution reactions of phenol:

  • phenols are aromatic compounds and they undergo electrophilic substitution reactions
  • the reacts of phenols take place under milder conditions and more readily than the reactions of benzene
• bromination of phenol:

  • phenol reacts with aqueous solution of bromine (bromine water) to form a white ppt of 2,4,6-tribromophenol
  • the reaction decolourises that bromine water (orange to colourless)
  • with phenol, a halogen carrier catalyst is not required and the reaction is carried out at RT
• Nitration of phenol:

  • phenol reacts readily with dilute nitric acid at RT
  • a mixture of 2-nitrophenol and 4-nitrophenol is formed
• comparing the reactivity of phenol and benzene:

  • bromine and nitric acid react more readily with phenol than they do benzene
  • phenol is nitrated with dilute nitric acid rather than needing conc. nitric and sulfuric acids as with benzene
• • The increased reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi-system of phenol
  • the electron density of the benzene ring in phenol is increased
  • the increased electron density attracts electrophiles more strongly than with benzene
  • aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene
  • for bromine, the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required