Question

The most reactive compound towards nucleophilic substitution with an aqueous NaOH is:

• A.
• B.
• C.
• D.

Hint:

Electron-withdrawing groups (like $ NO}_2 $) increase the reactivity of aromatic rings in nucleophilic substitution reactions. Para-substituted groups have the strongest effect.

Answer 1

The Correct Option is B

Step 1: Understand Nucleophilic Substitution.
This is a nucleophilic aromatic substitution reaction (\( S_NAr \)), where a nucleophile (e.g., \( \text{OH}^- \) from aqueous NaOH) replaces a leaving group (e.g., \( \text{Cl}^- \)) on an aromatic ring.
In such reactions, the presence of electron-withdrawing groups (EWGs) like \( \text{NO}_2 \) increases the reactivity by stabilizing the intermediate formed during the reaction.
Step 2: Analyze Each Option.
(1) Chlorobenzene:
No activating groups present.
The benzene ring is relatively electron-rich.
Poorly reactive towards nucleophilic substitution.
(2) p-Nitrochlorobenzene:
A strong electron-withdrawing nitro group (\( \text{NO}_2 \)) is at the para position relative to chlorine.
This strongly activates the ring for nucleophilic attack by withdrawing electrons and stabilizing the intermediate.
Most reactive among the options.
(3) p-Chloronitrobenzene:
Equivalent to option (2); same molecule.
However, the correct IUPAC name is \( \text{p-Nitrochlorobenzene} \), so this naming may be misleading or incorrect.
(4) m-Dinitrochlorobenzene:
Two nitro groups are present at meta positions.
Although EWGs are present, their positioning is less effective than a single nitro group at the para position.
Less reactive than p-nitrochlorobenzene.
Step 3: Choose the Most Reactive Compound.
Among all the given compounds, p-nitrochlorobenzene has a nitro group in the para position, which provides maximum stabilization of the negative charge developed during the transition state, making it the most reactive towards nucleophilic substitution.
Final Answer: \( \boxed{(2) \, \text{p-Nitrochlorobenzene}} \)