PHENOL

Acidity of phenolReaction 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 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 Phenols1. Reaction with bases. Reagent : NaOH(aq) Product : sodium phenoxide

Phenol is recovered from sodium phenoxide by acidification with dilute acid.

2. 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.

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

Reagent Alcohol Phenol

Neutral solution of FeCl3(aq)

No reaction Purple colouration

PCl5 at room temperature -vigorous reaction-Steamy fumes of HCl

No reaction

SOCl2, reflux -vigorous reaction-Steamy fumes of HCl

No reaction

NaOH(aq) No reaction Dissolves readily to form sodium phenoxide.