Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol
The acidic nature of phenol can be represented by the following two reactions:-
(a) Phenols react with sodium to give sodium phenoxide, liberating H2.
(b) Phenols react with sodium hydroxide to give sodium phenoxide and water as a by-product.
The acidity of phenol is more than that of ethanol. This is because phenol after losing a proton becomes phenoxide ion which undergoes resonance and is stabilized whereas ethoxide ion does not.
The resonating structures of phenoxide ion are shown as below:
The lone pair of electrons on oxygen delocalizes into the benzene (mesomeric effect) which reduces the electron density in the O-H bond. The O-H bonds are weaker and therefore breaks easily whereas in ethanol the electron releasing inductive effect of the alkyl group increases the electron density on the O-H bond. This strengthens the bond so the bond does not break easily. Therefore making ethanol less acidic than phenol.