Phenol

Phenol and its uses

• -OH group bonded to an aromatic ring

• Alcohols and phenols have similar reactions, but the proximity of the delocalised ring influences some reactions

• Used in manufacture of disinfectants, detergents, aspirin, etc. 

• Carbolic acid - Joseph Lister in 19th century 

Phenol as a weak acid

• Less soluble in water than alcohol

• Partially dissociates, forming the phenoxide ion and a proton (H⁺)

• The highly electronegative oxygen atom from the ion attracts negative charge strongly

• More acidic than alcohols, but less than carboxylic acid

• To distinguish between phenols and carboxylic acid: Na₂CO₃

  • phenol has no reaction

Electrophilic substitution - bromination

• Phenol reacts with an aqueous solution of bromine

• Reaction decolourises the bromine water

• Forms a white precipitate of 2,4,6-tribromophenol

• With phenol, a halogen carrier is not required and reaction is carried out at room temperature

Electrophilic substitution - nitration

• Phenol reacts readily with dilute nitric acid at room temperature

• A mixture of 2-nitrophenol and 4-nitrophenol is formed

• Gives a mixture of ortho (1,2) and para (1,4) nitrophenols

Reactivity compared to benzene

• Reactivity of phenol > reactivity of benzene 

  • A lone pair of electrons from the p-orbital of the -OH group gets donated to the π-system of phenol 

  • The electron density of the benzene ring in phenol is increased

  • Electrophiles are more attracted to phenol than benzene 

• Br₂ and HNO₃ react more readily with phenol than with benzene 

  • Phenol can be nitrated with dilute nitric acid; concentrated acid is not necessary 

  • Halogen carrier catalyst is not required for bromination - increased electron density can polarise Br2 molecules