Question

Arrange the following compounds in increasing order towards nucleophilic substitution reaction.

Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (A), (D), (C), (B)
• C. (D), (B), (C), (A)
• D. (B), (A), (C), (D)

Hint:

Each additional \(-\text{NO}_2\) group at an ortho or para position significantly stabilizes the Meisenheimer complex, increasing \(\text{S}_\text{N}\text{Ar}\) reactivity:
Chlorobenzene (\(623\text{ K}\)) \(\rightarrow\) 4-nitro (\(443\text{ K}\)) \(\rightarrow\) 2,4-dinitro (\(368\text{ K}\)) \(\rightarrow\) 2,4,6-trinitro (warm \(\text{H}_2\text{O}\)).

Answer 1

The Correct Option is B

Concept:
Aryl halides are typically unreactive towards nucleophilic aromatic substitution (\(\text{S}_\text{N}\text{Ar}\)) due to resonance stabilization of the \(\text{C}-\text{Cl}\) bond, \(sp^2\) hybridization of the aromatic carbon, and electronic repulsion from the \(\pi\)-cloud.
However, the presence of strong electron-withdrawing groups (\(-\text{NO}_2\)) at the ortho and para positions relative to the halogen greatly accelerates substitution by stabilizing the anionic Meisenheimer intermediate through \(-M\) and \(-I\) effects.

Step 1: Assessing Reactivity of Chlorobenzene:
In chlorobenzene (A), there are no electron-withdrawing substituents on the ring.
The lone pairs on the chlorine atom are delocalized into the aromatic ring, giving the \(\text{C}-\text{Cl}\) bond partial double bond character.
Consequently, chlorobenzene requires severe conditions (\(623\text{ K}\), \(300\text{ atm}\)) to undergo nucleophilic attack and is the least reactive among the group.

Step 2: Assessing the Effect of Successive Nitro Groups:
In 4-nitrochlorobenzene (D), one \(-\text{NO}_2\) group is positioned at the para position.
During nucleophilic attack, the negative charge delocalizes onto the electronegative oxygen atoms of the para-nitro group, facilitating the reaction at a milder temperature (\(443\text{ K}\)).
In 2,4-dinitrochlorobenzene (C), two \(-\text{NO}_2\) groups are present at the ortho and para positions, providing additional resonance stabilization to the carbanion intermediate and allowing the reaction to proceed smoothly at \(368\text{ K}\).
In 2,4,6-trinitrochlorobenzene (B), three \(-\text{NO}_2\) groups occupy both ortho positions and the para position.
This configuration heavily delocalizes the negative charge, making the compound extremely reactive such that substitution occurs simply upon warming with warm water.

Step 3: Ordering by Increasing Reactivity:
Comparing the four compounds, the reactivity towards nucleophiles increases directly with the number of ortho- and para-nitro substituents:
\[ \text{(A)} < \text{(D)} < \text{(C)} < \text{(B)} \] Final Answer:
The increasing order of reactivity is (A), (D), (C), (B), corresponding to option (B).