Presentation on theme: "PHENOL Acidity of phenol Reaction with sodium Nitration and bromination of the aromatic ring."— 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 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 pK a : strength of acid decrease *lower pK a 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 1. 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 H 2 gas evolved. Reaction is more vigorous than ethanol because phenol is a stronger acid.
3) Nitration (a) Mono-nitration Reagent : Dilute HNO 3 Condition : Room temperature Product: mixture of 2-nitrophenol and 4-nitrophenol. Phenol is nitrated very easily (conc. H 2 SO 4 not required) The –OH group activates the ring towards electrophilic substitution (2-, 4-directing)
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 CCl 4 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 CH 3 CH(OH)- group gives yellow crystals of CHI 3. E.g: Used as a test for presence of CH 3 CH(OH)- group in alcohols.
Test for Phenols ReagentAlcoholPhenol Neutral solution of FeCl 3 (aq) No reactionPurple colouration PCl 5 at room temperature-vigorous reaction -Steamy fumes of HCl No reaction SOCl 2, reflux-vigorous reaction -Steamy fumes of HCl No reaction NaOH(aq)No reactionDissolves readily to form sodium phenoxide.