Note : In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.

Assertion : Like bromination of benzene, bromination of phenol is also carried out in the presence of Lewis acid.

Reason : Lewis acid polarises the bromine molecule.

So what is a Lewis acid?

Lewis acids are electron deficient species. They contain an empty orbital which are capable of accepting an electron pair, from Lewis base(electron enriched species) thereby forming Lewis adduct. Eg. FeCl₃, FeBr_3, BF₃, anhydrous AlCl₃, etc.

Now, we know that benzene reacts by electrophilic substitution reaction. In the bromination reaction of benzene, we need to produce an electrophile which can attach itself on the benzene ring due to which reaction would proceed.

So, how can we generate this electrophile?

For the generation of electrophile, we need Lewis acid(FeBr₃).Since the Lewis acid need electrons, it gets them when the Br-Br bond breaks and FeBr₃ accepts the Br^- and we get FeBr₄^- along with the generated electrophile Br⁺.

Now, this electrophile attacks the benzene ring and through resonance and removal of a H⁺ from the ring, we get bromobenzene.

Now, we are left with a H⁺ and FeBr₄^- . They combine among themselves thereby forming AlCl₃ and HBr.

Since FeBr₃ is formed again, it is said to be a catalyst.

This was the mechanism of bromination of benzene.

The step by step explanation of the formation of bromobenzene.

We got to see that the Lewis acid actually polarizes bromine molecule into Br⁺ and Br^-.

Now, in case of bromination of phenol, we do not need any Lewis acid. This is because of the strong activating effect(+R effect) of the –OH group (due to the presence of lone pairs on Oxygen atom) on the benzene ring towards electrophilic reaction. Thus Lewis acids are not required here to produce electrophiles.

Such is the effect of –OH group on benzene that instead of producing mono bromo derivative on the ring, the reaction readily proceeds to form tri bromo derivative that is 2,4,6-tribromophenol which is observed as a white precipitatewhen the reaction happens with bromine water.

As we discussed earlier the three positions where negative charges are largely located, we can relate here that the electrophiles would love to attack at these three positions thereby giving the tribromo derivative.

Here is the figure once again.