Question
Sat February 11, 2012 By: Sagar

What will happen when ARENES are reacted with halogen atom in the presence of LEWIS ACID as CATALYST and in the presence of LIGHT (instead of DARK).

Expert Reply
Wed February 15, 2012

Reaction of benzene or other aromatic hydrocarbon with halogens gives haloarenes. These reactions if carried out in the presence of Lewis acids such as ferric or aluminium halides (FeCl3, FeBr3, AlCl3) in the dark, at ordinary temperatures (310-320 K) yield products with ring substitution.  In the absence of light and in the presence of ferric chloride, substitution occurs mostly in the ring and follows electrophillic substitution mechanism. But, if halogen is bubbled into the arene that is exposed to light, then substitution occurs almost exclusively in the side chain and follows the free radical substitution mechanism.

For example:

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