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PHENOL

PHENOL. Acidity of phenol Reaction with sodium Nitration and bromination of the aromatic ring. Physical Properties of Phenol. Phenol organic compound containing hydroxy (-OH) group directly attached to a benzene ring. E.g : Colourless crystalline solid at room temperature.

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PHENOL

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  1. PHENOL Acidity of phenol Reaction with sodium Nitration and bromination of the aromatic ring

  2. Physical Properties of Phenol • Phenol organic compound containing hydroxy (-OH) groupdirectly attached to a benzene ring. • E.g : • Colourless crystalline solid at room temperature. • Often pale pink in colour due to impurities. • Melting point higher than hydrocarbons of comparable Mr due to intermolecular hydrogen bonding. • Phenol is partially soluble in water due to hydrogen bonding. • Most other phenols are insoluble in water.

  3. Acidity of Phenol • A weak acid (pH ≈ 5 – 6) • Dissolves in NaOH(aq) to give sodium phenoxide salt. • Weaker acid than carboxylic acids but stronger acid than water and ethanol. • Order of acid strength : phenol > water > ethanol pKa : 10.0 16.0 18.0 strength of acid decrease *lower pKa value, stronger acid.

  4. Phenoxide ion produced is stabilised by delocalisation of negative charge into the benzene ring. • Ethanol is a weaker acid than water • The negative charge in the ethoxide ion is intensified by the electron-donatingethyl group stronger base than OH-.

  5. Effects of substituent on acid strength of phenol. • Electron-withdrawing group (EWG) increases acid strength. • EWG attracts electrons away from O atom and stabilises the phenoxide ion reducing its tendency to attract protons. • Electron-donating group (EDG) decreases acid strength of phenol. • EDG supplies electrons to O atom increase negative charge density more ready to accept protons.

  6. Reaction of Phenols • Reaction with bases. • Reagent : NaOH(aq) • Product : sodium phenoxide • Phenol is recovered from sodium phenoxide by acidification with dilute acid. • Reaction with sodium. • Reagent : sodium • Product : sodium phenoxide • Sodium sinks (not melt) and bubbles of H2 gas evolved. • Reaction is more vigorous than ethanol because phenol is a stronger acid.

  7. 3) Nitration (a) Mono-nitration • Reagent : Dilute HNO3 • Condition : Room temperature • Product : mixture of 2-nitrophenol and 4-nitrophenol. • Phenol is nitrated very easily (conc. H2SO4 not required) • The –OH group activates the ring towards electrophilic substitution (2-, 4-directing)

  8. (b) Tri-nitration • Reagent : Concentrated HNO3 • Condition : Room temperature • Product : 2,4,6-trinitrophenol

  9. 4) Bromination • Undergoes bromination more readily than benzene. • Because of the electron-donating –OH group activates the ring towards electrophilic substitution. Hence : • no halogen carrier needed. • Aqueous halogen solution can be used. • More than 1 halogen atoms can be substituted. • (a) Mono-bromination • Reagent : Bromine in CCl4 • Condition : Room temperature • Product : mixture of 2-bromophenol and 4-bromophenol. • Observation : reddish-brown bromine solution decolourised and steamy white fumes evolved.

  10. 4) Bromination • (a) Tri-bromination • Reagent : Bromine water • Condition : Room temperature • Product : 2, 4, 6-tribromophenol. • Observation : reddish-brown bromine solution decolourised, white precipitate of 2, 4, 6-tribromophenol formed and steamy white fumes evolved.

  11. 5) Tri-iodomethane reaction (iodoform test) • Reagent : aqueous iodine and aqueous NaOH • Condition : warm • Observation : yellow crystals of tri-iodomethane formed. • Only alcohol with CH3CH(OH)- group gives yellow crystals of CHI3. • E.g: • Used as a test for presence of CH3CH(OH)- group in alcohols.

  12. Test for Phenols

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