Replacement of \(Cl\) of \(C_6H_5Cl\) to give \(C_6H_5OH\) occur only in drastic conditions but \(Cl\) od 2, 4-dinitrochlorobenzene is readily replaced. This is because:

\(-NO_2\) withdraws electrons from ortho and para position.

The correct answer is option 1. \(-NO_2\) withdraws electrons from ortho and para position.

Let us explore why 2,4-dinitrochlorobenzene undergoes nucleophilic substitution much more readily than chlorobenzene, with a focus on the electronic effects of substituents like the nitro group \((-NO_2)\) and how they influence the reactivity.

Understanding the Structure and Reactivity of Chlorobenzene (\(C_6H_5Cl\)):

Chlorobenzene has a chlorine atom attached directly to a benzene ring. Chlorine is an electronegative atom, and it has two significant effects on the benzene ring:

Inductive Effect: Chlorine withdraws electron density through the sigma bond due to its electronegativity. This makes the carbon atom bonded to chlorine slightly positive, but it also withdraws electron density from the ring, making the ring less reactive toward nucleophiles.

Resonance Effect: Chlorine has lone pairs of electrons that can participate in resonance with the benzene ring. This resonance donation somewhat counterbalances the inductive electron withdrawal, making the ring more stable.

The overall effect of chlorine is a balance between electron withdrawal (inductive effect) and electron donation (resonance effect). This balance results in a benzene ring that is not highly activated for nucleophilic substitution. Therefore, nucleophilic substitution of chlorine in chlorobenzene is difficult and requires drastic conditions (high temperature, pressure, or strong nucleophiles).

Introducing Nitro Groups (\(-NO_2\)) in 2,4-Dinitrochlorobenzene:

In this compound, two nitro groups are attached to the benzene ring at the 2nd and 4th positions (ortho and para to the chlorine atom). The nitro group is a strongly electron-withdrawing group due to its resonance and inductive effects:

Inductive Effect: The nitro group pulls electron density away from the benzene ring through the sigma bond, making the entire ring more electron-deficient.

Resonance Effect: The nitro group also withdraws electron density from the ring through resonance. It pulls electron density from the ring by stabilizing negative charge on the oxygen atoms while leaving a positive charge on the ring. This resonance effect is particularly strong at the ortho and para positions relative to the nitro group.

Effect on Reactivity: With two nitro groups at the ortho and para positions, the electron density in the ring is significantly reduced. This makes the carbon atom bonded to chlorine highly electron-deficient, increasing its susceptibility to attack by nucleophiles.

Mechanism of Nucleophilic Substitution in 2,4-Dinitrochlorobenzene:

Activation of the Benzene Ring: The strong electron-withdrawing effects of the two nitro groups make the benzene ring very electron-poor, especially at the positions where the nitro groups are located. The carbon atom attached to the chlorine becomes highly positive due to the withdrawal of electron density, making the carbon-chlorine bond more polarized. This polarization weakens the carbon-chlorine bond, making it more prone to nucleophilic attack.

Nucleophilic Attack: In the presence of a nucleophile like \(OH^-\) (from a strong base like NaOH), the nucleophile is attracted to the electron-deficient carbon atom bonded to chlorine. The nucleophile attacks the carbon, leading to the displacement of the chlorine atom and the formation of a new bond, resulting in the substitution of chlorine with the nucleophile (in this case, forming phenol if the nucleophile is \(OH^-\)).

Why Chlorobenzene Requires Drastic Conditions:

In chlorobenzene, the electron-withdrawing inductive effect of chlorine is partially offset by its resonance donation, leading to a less reactive ring. The nucleophile has a harder time attacking the carbon atom bonded to chlorine because the carbon is not as electron-deficient as in 2,4-dinitrochlorobenzene. Therefore, nucleophilic substitution in chlorobenzene requires more energy (higher temperatures, stronger nucleophiles) to occur.

Why 2,4-Dinitrochlorobenzene is More Reactive: The presence of two electron-withdrawing nitro groups at the ortho and para positions makes the ring much more electron-deficient, especially at the carbon bonded to chlorine. This electron deficiency makes the carbon more susceptible to nucleophilic attack, facilitating the substitution of chlorine under milder conditions.

Conclusion:

The reason why the \(Cl\) in 2,4-dinitrochlorobenzene is more easily replaced compared to chlorobenzene is because the nitro groups \((-NO_2)\) at the ortho and para positions strongly withdraw electron density from the ring. This withdrawal makes the carbon-chlorine bond more polarized and the carbon atom more positive, enhancing the reactivity toward nucleophiles.

Thus, the correct explanation is option 1: \(-NO_2\) withdraws electrons from ortho and para positions.

This electron withdrawal leads to increased reactivity of 2,4-dinitrochlorobenzene in nucleophilic substitution reactions.