Question

Increasing order of the following for the electrophilic substitution reaction as:

A) I < II < III
B) III < I < II
C) III < II < I
D) II < I < III

Answer 1

Hint: The reactions in which the functional group present in a molecule or attached to the molecule is substituted or replaced by an electrophile is known as an electrophilic substitution reaction. There are two types of electrophilic substitution reactions electrophilic aromatic substitution and electrophilic aliphatic substitution.


Complete solution:
We know that the reactions in which the functional group present in a molecule or attached to the molecule is substituted or replaced by an electrophile is known as an electrophilic substitution reaction.
In electrophilic aromatic substitution, an atom attached to an aromatic ring is substituted by an electrophile. In electrophilic aromatic substitution, benzene acts as a nucleophile. Thus, the benzene ring donates electrons from inside its ring.
When electron withdrawing groups are present on the benzene ring the ring is less reactive towards electrophilic substitution reaction. And when electron donating groups are present on the benzene ring the ring is more reactive towards electrophilic substitution reaction.
In structure (I), the methyl group is attached to the benzene ring. The methyl group is an electron donating group. Thus, the structure (I) is more reactive towards electrophilic substitution reaction.
In structure (II), neither electron donating group nor electron withdrawing group is attached. Thus, the structure (II) is less reactive towards electrophilic substitution reaction than structure (I)
In structure (III), an acidic group is attached to the benzene ring. The acidic group is an electron withdrawing group. Thus, the structure (III) is less reactive towards electrophilic substitution reaction than structure (II).
Thus, increasing order for the electrophilic substitution reaction is III < II < I.

Thus, the correct option is (C) III < II < I.


Note: The electrophilic substitution reaction proceeds through a three step mechanism. The three steps in electrophilic substitution reaction are: 1. Generation of an electrophile, 2. Formation of carbocation intermediate and 3. Removal of proton from the intermediate.