sp hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of –OH group to be positive. The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water. Let us examine how a compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group.
The ionisation of an alcohol and a phenol takes place as follows: Due to the higher electronegativity of sp hybridised carbon of phenol to which –OH is attached, electron density decreases on oxygen. This increases the polarity of O–H bond and results in an increase in ionisation of phenols than that of alcohols. Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.
The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion. o -Nitrophenol o –O N–C H –OH . m -Nitrophenol m –O N–C H –OH .
p -Nitrophenol p -O N–C H –OH . Phenol C H –OH . o -Cresol o -CH –C H –OH . m -Cresol m -CH C H –OH .
p -Cresol p -CH –C H –OH . Ethanol C H OH . Table . : p K a Values of some Phenols and Ethanol Compound Formula p K a From the above data, you will note that phenol is million times more acidic than ethanol.
Arrange the following compounds in increasing order of their acid strength: Propan- -ol, , , -trinitrophenol, -nitrophenol, , -dinitrophenol, phenol, -methylphenol. Propan- -ol, -methylphenol, phenol, -nitrophenol, , -dinitrophenol, , ,