Question

Use resonance structures to show how and why the hydroxyl group directs incoming electrophiles in phenol.

Answer 1

The -OH group attached to the benzene ring in phenol has the effect of making the ring much more reactive than it would otherwise be. For example, as you will find below, phenol will react with a solution of bromine in water (bromine water) in the cold and in the absence of any catalyst. It also reacts with dilute nitric acid, whereas benzene itself needs a nitrating mixture of concentrated nitric acid and concentrated sulfuric acid.

            

Figure: One of the lone pairs on the oxygen atom in the -OH group overlaps with the delocalised ring electron system giving a structure rather like the right.

The donation of the oxygen's lone pair into the ring system increases the electron density around the ring. A benzene ring undergoes substitution reactions in which the ring electrons are attacked by positive ions or the slightly positive parts of molecules. In other words, it undergoes electrophilic substitution. If you increase the electron density around the ring, it becomes even more attractive to incoming electrophiles. That's what happens in phenol.