Question

Toluene reacts with bromine in the presence of light to give benzoyl bromide, while in the presence of \[FeB{r_3}\], it gives p-bromotoluene. Give an explanation for the above observations.

Answer 1

Hint: The reaction of toluene with bromine in presence of light is an example of a free radical mechanism. Free radicals are electronic species which contain unpaired electrons in their atomic orbital.

Complete Step by Step Solution:
The reaction between toluene and bromine in the presence of light is shown below.

Image: Reaction of toluene with bromine

In the above reaction, toluene reacts with bromine in the presence of light to form benzyl bromide. In the presence of light, the side chain bromination reaction takes place due to the formation of free radicals.

The reaction between toluene and iron (III) bromide is shown below.

Image: Reaction of toluene with iron(III) bromide

When toluene reacts with iron (III) bromide, the benzene undergoes electrophilic substitution which leads to formation of the main product p-bromo toluene as the methyl group is o- and p- directing.

An electrophilic substitution reaction is a type of chemical reaction where a functional group attached to a compound is displaced by an electrophile group. The functional group which is displaced is mainly hydrogen atoms.
The electrophilic substitution reactions involve three steps:
1. In the first step, an electrophile is generated.
2. In the second step, carbocation is formed which acts as an intermediate.
3. In the third step removal of a proton from the intermediate takes place.

Note: In the electrophilic substitution reaction of toluene and iron (III) oxide, iron (III) bromide acts as a lewis acid. Lewis acids are the species that accept lone pairs of electrons.