Question

Reactivity order of the following towards nucleophilic substitution reactions:
(A) 4-Nitro-1-chlorobenzene
(B) 2,4-Dinitro-1-chlorobenzene
(C) Chlorobenzene
(D) 2,4,6-Trinitro-1-chlorobenzene
Choose the correct answer from the options given below:

• A. A $<$ B $<$ C $<$ D
• B. A $<$ C $<$ B $<$ D
• C. B $<$ A $<$ D $<$ C
• D. C $<$ A $<$ B $<$ D

Answer 1

The Correct Option is D

In nucleophilic substitution reactions, the reactivity order of halogenated aromatic com-pounds is influenced by the electron-withdrawing or electron-donating properties of sub-stituents on the aromatic ring. The nitro group \((−NO_2)\) is a strong electron-withdrawing group, while the chlorine atom itself is a mild electron-withdrawing group. 

(C)  Chlorobenzene:  This compound is the least reactive due to the stability of the aromatic system and the weak withdrawing effect of chlorine. 2. 4-Nitro-1-chlorobenzene
(A) : The nitro group at the para position increases reactivity compared to chlorobenzene. 3.2,4-Dinitro-1-chlorobenzene 
(B) : The presence of two nitro groups enhances the electron- withdrawing effect, significantly increasing the reactivity. 4. 2,4,6-Trinitro-1-chlorobenzene
(D) : With three nitro groups, this compound exhibits the highest reactivity due to maximal stabilization of the leaving group.

Thus, the reactivity order is: \((C) < (A) < (B) < (D)\)

Answer 2

In nucleophilic substitution reactions, the reactivity order of halogenated aromatic compounds is influenced by the electron-withdrawing or electron-donating properties of substituents on the aromatic ring. The nitro group \((−NO_2)\) is a strong electron-withdrawing group, while the chlorine atom itself is a mild electron-withdrawing group.

1. Chlorobenzene (C): This compound is the least reactive due to the stability of the aromatic system and the weak withdrawing effect of chlorine. Chlorine, being a mild electron-withdrawing group, does not significantly increase the leaving group's stability.

2. 4-Nitro-1-chlorobenzene (A): The nitro group at the para position increases the reactivity compared to chlorobenzene. The electron-withdrawing effect of the nitro group at the para position stabilizes the transition state, making the reaction faster.

3. 2,4-Dinitro-1-chlorobenzene (B): The presence of two nitro groups enhances the electron-withdrawing effect, significantly increasing the reactivity. The additional nitro group at the second position further stabilizes the transition state and facilitates the nucleophilic substitution process.

4. 2,4,6-Trinitro-1-chlorobenzene (D): With three nitro groups, this compound exhibits the highest reactivity due to maximal stabilization of the leaving group. The electron-withdrawing nature of the three nitro groups creates a highly stabilized transition state, thus promoting the reaction the most.

Thus, the reactivity order is:

\((C) < (A) < (B) < (D)\)