Question

The correct order of reactivity in electrophilic substitution reaction of the following compounds is :

• A. \( B > C > A > D \)
• B. \( D > C > B > A \)
• C. \( A > B > C > D \)
• D. \( B > A > C > D \)

Answer 1

The Correct Option is D

In electrophilic substitution reactions, the reactivity of aromatic compounds is influenced by the substituents attached to the benzene ring. These substituents can be either activating or deactivating, and they can direct the incoming electrophile to specific positions on the ring.

Let's examine the given compounds:

1. Compound A: Benzene
2. Compound B: Toluene (methylbenzene)
3. Compound C: Chlorobenzene
4. Compound D: Nitrobenzene

Analysis:

• Toluene (B): The methyl group is an electron-donating group, which activates the benzene ring towards electrophilic substitution. This makes toluene more reactive than benzene.
• Chlorobenzene (C): The chlorine atom is an electron-withdrawing group due to its electronegativity, but it also has lone pairs that can participate in resonance. This makes chlorobenzene less reactive than benzene but not as deactivating as nitrobenzene.
• Nitrobenzene (D): The nitro group is a strong electron-withdrawing group, severely deactivating the benzene ring towards electrophilic substitution.

Therefore, the reactivity order in electrophilic substitution is:

B > A > C > D

This means Toluene is the most reactive, followed by Benzene, then Chlorobenzene, and Nitrobenzene being the least reactive.

Answer 2

The reactivity of aromatic compounds in electrophilic substitution reactions depends on the presence and nature of substituents attached to the benzene ring. Substituents can be classified as either activating or deactivating groups based on their electronic influence on the ring.

Let's analyze the given compounds:

1. Compound A: Benzene (without any substituent) - Neutral in terms of activation. It serves as a reference point.
2. Compound B: Toluene (\( \text{CH}_3 \) group) - The methyl group is an electron-donating group, which activates the benzene ring towards electrophilic substitution by increasing the electron density on the ring.
3. Compound C: Chlorobenzene (\( \text{Cl} \) group) - The chlorine atom is slightly electron-withdrawing due to its inductive effect, but it can donate electrons through resonance. Overall, it is less activating than a methyl group.
4. Compound D: Nitrobenzene (\( \text{NO}_2 \) group) - The nitro group is a strong electron-withdrawing group. It greatly decreases the electron density on the benzene ring, making it the least reactive towards electrophilic substitution.

Considering these effects, the order of reactivity in electrophilic substitution reactions is determined by the ability of the substituents to donate or withdraw electrons.

The correct order of reactivity is:

\( B > A > C > D \)

Explanation:

• Toluene (B): The \( \text{CH}_3 \) group donates electrons and increases the ring's reactivity.
• Benzene (A): Being unsubstituted, it is less reactive than toluene but more reactive than chlorobenzene and nitrobenzene.
• Chlorobenzene (C): The \( \text{Cl} \) group slightly withdraws electrons, reducing reactivity compared to benzene and toluene.
• Nitrobenzene (D): The \( \text{NO}_2 \) group strongly withdraws electrons, making it the least reactive.

Thus, option \( B > A > C > D \) is the correct order.