Explain why benzene requires use of a halogen carrier catalyst to react with bromine while phenol does not.

The oxygen atom in the phenol molecule has a lone pair of electrons. These electrons become delocalised into the aromatic ring, increasing its electron density. This activates the ring. This causes Br₂ molecules to be polarised to a greater extent, which allows the reaction to proceed.In benzene there are no lone pairs, so this activating effect does not occur. As such, there is insufficient electron density to polarise Br₂. Instead, a halogen carrier catalyst (such as FeBr₃) is required to generate a Br⁺ ion which is then able to react with benzene.