Step 1: Reasons for Lower Reactivity of Haloarenes [1 Mark]

Haloarenes are less reactive towards nucleophilic substitution reactions due to the following factors:

• Resonance Effect: The lone pair of electrons on the halogen atom participates in conjugation with the $\pi$-electrons of the benzene ring. This imparts a partial double bond character to the $C-X$ bond, making it shorter and stronger than the $C-X$ bond in haloalkanes, thus making it difficult to cleave.
• Difference in Hybridization: In haloarenes, the carbon atom attached to the halogen is $sp^2$ hybridized, which is more electronegative and holds the electron pair more tightly than the $sp^3$ hybridized carbon in haloalkanes.
• Instability of Phenyl Cation: The $S_N1$ mechanism is ruled out as the phenyl cation formed would not be stabilized by resonance.
• Electron Repulsion: The electron-rich nucleophile is repelled by the electron-rich arene ring.

Step 2: Effect of Nitro ($-NO_2$) Group [0.5 Mark]

The presence of an electron-withdrawing group like $-NO_2$ at ortho- and para-positions withdraws electron density from the benzene ring. This facilitates the attack of the nucleophile by stabilizing the intermediate carbanion (Meisenheimer complex) through resonance.

Step 3: Final Concluded Answer [0.5 Mark]

The negative charge on the intermediate is effectively delocalized over the $-NO_2$ group at the ortho and para positions, significantly lowering the activation energy for the reaction. The reaction proceeds as follows:

With $-NO_2$ groups, the reaction conditions become much milder (e.g., 2,4-dinitrochlorobenzene reacts with water just by warming).

💡 EL10 Concept Insight:

Think of the benzene ring as a crowded, electron-packed room that hates new guests (nucleophiles). The $-NO_2$ group acts like a vacuum cleaner, sucking out the extra electrons from the room, which clears space and makes it much easier for the new guest to enter.