PHENOL Acidity of phenol Reaction with sodium

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Presentation transcript:

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. 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.

Acidity of Phenol strength of acid decrease 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.

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-donating ethyl group stronger base than OH-.

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.

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 Sodium sinks (not melt) and bubbles of H2 gas evolved. Reaction is more vigorous than ethanol because phenol is a stronger acid.

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)

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

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.

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.

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.

Test for Phenols Neutral solution of FeCl3(aq) No reaction Reagent Alcohol Phenol Neutral solution of FeCl3(aq) No reaction Purple colouration PCl5 at room temperature -vigorous reaction -Steamy fumes of HCl SOCl2, reflux NaOH(aq) Dissolves readily to form sodium phenoxide.