Why is phenol more reactive than benzene?
Phenol (which is a benzene ring with one substituted carbon with a hydroxyl or alcohol group) does not require a halogen carrier to be halogenated whereas benzene itself cannot react with a halogen alone.
What's the reason for this difference in reactvity between benzene and phenol? Let's take the bromination of phenol and benzene as examples and have a look at the electronic structure of both reactants.
The electonic structure of benzene consists of delocalised pi-electrons above and below the ring of 6 carbon atoms with the electron density is shared throughout the ring.
Phenol has an OH group bonded to one of the carbons and this oxygen has two lone pairs in p-orbitals. Now these electrons can overlap with the electrons in the benzene ring and if we look at the molecule as a whole, the oxygen shares these electrons with the rest of the system and so, increases the electron density. This means that there is enough electron density in phenol to induce a dipole in a Br2 molecule, making it an electrophile that can then be attacked by the nucleophilic phenol.
Benzene on the otherhand does not have sufficient electron density to induce this dipole so a strong enough electrophile must be produced using a halogen carrier like FeBr3 if benzene is to react at all.
