Chloroarenes (aryl chlorides) and bromoarenes (aryl bromides) can be easily prepared by direct chlorination or bromination of benzene or other aromatic hydrocarbons. These reactions are carried out in dark, at ordinary temperatures (310–320 K) in the presence of Lewis acid catalyst such as anhydrous ferric or aluminium halides (FeCl₃, FeBr₃, AlCl₃). These reactions are called electrophilic substitution of arenes. The Lewis acid acts as a catalyst and its function is to carry the halogen to the aromatic hydrocarbon and is also called halogen carrier. In actual practice, iron filings in the presence of chlorine or bromine are commonly used. The halogens react with iron filings to form corresponding Lewis acids.

What happens when chlorobenzene reacts with excess chlorine in the presence of anhydrous FeCl₃ (Major product)?

p-Dichlorobenzene

The correct answer is option 3. p-Dichlorobenzene.

The major product of the reaction is p-dichlorobenzene, also known as 1,4-dichlorobenzene. This is because the chlorine atoms are more likely to attack the para position of the benzene ring. The para position is the most stable position for the chlorine atoms because it is the most symmetrical.

Here is the balanced chemical equation for the reaction of chlorobenzene with excess chlorine in the presence of anhydrous FeCl₃:

The reaction is also known as the Sandmeyer reaction. This reaction is a type of electrophilic substitution reaction. In this reaction, the chlorine atoms are electrophiles that attack the benzene ring. The anhydrous FeCl₃ catalyst activates the benzene ring by making it more electron-rich.

The reaction proceeds in two steps:

1. The first step is the formation of an intermediate called chlorobenzene dichloride. This is formed when one chlorine atom attacks the benzene ring and another chlorine atom is displaced.
2. The second step is the decomposition of the chlorobenzene dichloride. This is catalyzed by the anhydrous FeCl₃, and it produces p-dichlorobenzene.