Question

Increasing order of the nucleophilic substitution of following compounds is

• A. I $<$ III $<$ II $<$ IV
• B. II $<$ I $<$ III $<$ IV
• C. II $<$ III $<$ I $<$ IV
• D. IV $<$ III $<$ II $<$ I

Hint:

Using a simple frame or just bolding for the box Key Points: Nucleophilic Aromatic Substitution (S$_N$Ar) rate depends on the stability of the Meisenheimer intermediate (negatively charged). Electron-Withdrawing Groups (EWGs like -NO$_2$, -CN, -CF$_3$) at ortho/para positions increase the rate by stabilizing the negative charge. Electron-Donating Groups (EDGs like -OCH$_3$, -NH$_2$, -alkyl) decrease the rate by destabilizing the negative charge. The more EWGs at ortho/para positions, the higher the reactivity.

Answer 1

The Correct Option is B

This question asks for the increasing order of reactivity towards nucleophilic aromatic substitution (S_NAr). The S_NAr mechanism involves the attack of a nucleophile on the carbon atom bearing the leaving group (Cl in this case), forming a negatively charged intermediate called the Meisenheimer complex. The reaction rate is enhanced by electron-withdrawing groups (EWGs) at the ortho and/or para positions relative to the leaving group, as these groups stabilize the negative charge in the intermediate through resonance and/or inductive effects. Conversely, electron-donating groups (EDGs) destabilize the intermediate and decrease the reaction rate. Let's analyze the substituents:

• I (Chlorobenzene): No activating or deactivating groups other than Cl. This serves as a baseline.
• II (p-Methoxychlorobenzene): The methoxy group (-OCH₃) is an electron-donating group (+R effect>-I effect). It increases electron density on the ring, destabilizing the negative charge of the Meisenheimer complex. Thus, it deactivates the ring towards S_NAr compared to chlorobenzene.
• III (p-Nitrochlorobenzene): The nitro group (-NO₂) is a strong electron-withdrawing group (-R and -I effects). Located para to the Cl, it effectively stabilizes the negative charge in the Meisenheimer complex, activating the ring towards S_NAr compared to chlorobenzene.
• IV (2,4-Dinitrochlorobenzene): This molecule has two strong electron-withdrawing nitro groups, one ortho and one para to the Cl. Both groups effectively stabilize the negative charge of the intermediate. This compound is highly activated towards S_NAr, much more so than compound III.

Comparing the effects:

• Deactivating group: -OCH₃ (II)
• No activating/strong deactivating group: (I)
• One activating group: p-NO₂ (III)
• Two activating groups: o-NO₂ and p-NO₂ (IV)

Therefore, the order of increasing reactivity is: Deactivated<Baseline<Activated (1 EWG)<Highly Activated (2 EWGs).
II<I<III<IV
This corresponds to option (B).