Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

Replacement of \(\mathrm{Cl}\) of chlorobenzene to give phenol requires drastic conditions, but chlorine of 2,4 dinitrochlorobenzene is readily replaced because (a) \(-\mathrm{NO}_{2}\) makes the ring electron rich at ortho and para positions (b) \(-\mathrm{NO}_{2}\) withdraws electrons at meta position (c) \(-\mathrm{NO}_{2}\) donates electrons at meta position (d) \(-\mathrm{NO}_{2}\) withdraws electrons at ortho and para positions

Step-by-Step Solution

Answer

(d) \\(-\mathrm{NO}_{2}\\) withdraws electrons at ortho and para positions.

Step 1: Understand the Nitrogen Dioxide Group Effect

Nitro groups \((-\mathrm{NO}_{2})\) are strong electron-withdrawing groups because of their \(-\text{M}\) (mesomeric or resonance) and \(-\text{I}\) (inductive) effects. They pull electron density away from the benzene ring, particularly affecting positions ortho and para to the group.

Step 2: Analyze the Effects on Chlorobenzene

In chlorobenzene, chlorine is not easily replaced because it is an electronegative group, but not as strongly electron-withdrawing. Chlorine has a partial resonance donation effect that stabilizes its bond to the benzene ring, making it less reactive under normal conditions.

Step 3: Effects of Nitro Groups on Reaction

In 2,4-dinitrochlorobenzene, there are two nitro groups at ortho and para positions relative to the chlorine. The presence of these groups withdraws electrons from the chlorine atom's ortho and para positions, making the carbon-chlorine bond more polarized and weaker. This makes chlorine more susceptible to substitution.

Step 4: Identify Correct Explanation

Option (d) mentions that \(-\mathrm{NO}_{2}\) withdraws electrons at ortho and para positions, which aligns with the increased reactivity of 2,4-dinitrochlorobenzene compared to chlorobenzene. This increases the susceptibility of the chlorine group to nucleophilic substitution.

Key Concepts

Chlorobenzene

Chlorobenzene is a chemical compound consisting of a chlorine atom bound to a benzene ring. It's represented by the chemical formula \( \text{C}_6\text{H}_5\text{Cl} \). In chlorobenzene, chlorine acts as an electronegative atom with its ability to pull electron density towards itself. This gives the chlorine atom a partial negative charge, creating a polar bond with the carbon atom it's attached to.

Despite having this polar character, chlorobenzene doesn't readily participate in nucleophilic substitution reactions. This is because chlorine can donate some electron density back into the benzene ring through resonance, which stabilizes its position.

• Chlorobenzene is less reactive due to its resonance stability.
• The carbon-chlorine bond is strong because of partial resonance donation from chlorine.

This makes removing the chlorine atom under normal conditions difficult, requiring drastic measures or specific conditions to achieve substitution.

Electron-Withdrawing Group

Electron-withdrawing groups (EWGs) are substituents that attract electron density away from other parts of a molecule.

Nitro groups \((-\text{NO}_2)\) are a classic example of EWGs because they pull electrons through both inductive and resonance effects. Inductive effects occur when electronegative atoms, like nitrogen in nitro groups, draw electron density towards themselves. Resonance effects happen when electrons can be delocalized through pi bonds, stabilizing different regions of the molecule.

• EWGs like nitro groups make rings more susceptible to substitution by weakening certain bonds.
• They especially affect positions ortho and para to the substituent on aromatic rings.

Nitro groups in particular, significantly increase the reactivity of compounds like 2,4-dinitrochlorobenzene compared to chlorobenzene, allowing for easier substitution reactions.

Nucleophilic Substitution

Nucleophilic substitution is a type of reaction where a nucleophile—a species rich in electrons—replaces a leaving group in a molecule.

In the context of chlorobenzene, substitution is challenging due to the electron distribution. However, when electron-withdrawing groups like nitro groups are present, they make the substitution easier by reducing electron density on the carbon bearing the chloride. This polarization weakens the C-Cl bond, making it susceptible to attack by nucleophiles.

• Nucleophilic substitution involves a nucleophile displacing a leaving group establishing a new bond.
• The presence of EWGs can accelerate these reactions by weakening the available bonds.

Therefore, despite chlorobenzene's resilience to nucleophilic attacks, 2,4-dinitrochlorobenzene can undergo substitution more readily, due to its altered electronic structure induced by the nitro groups.

Nitro Group Effect

Nitro groups have a profound impact on the reactivity of aromatic compounds. When attached to benzene, nitro groups act to withdraw electrons from the ring, thus stabilizing negatively charged nucleophiles that approach.

This electron-withdrawing action is effective particularly at the ortho and para positions, where it destabilizes potential transition states for nucleophilic attack.

• The nitro group's mesomeric effect distributes negative charge through the ring, making certain bonds more reactive.
• This increased reactivity is specific to structural orientations that position nitro groups away from interfering pi systems.

In reactions involving 2,4-dinitrochlorobenzene, the nitro groups make the chlorine atom's position particularly weak. This enhances nucleophilic substitution efficiency, highlighting option (d) as the correct explanation in the original exercise.