The Question is from neet-chemistry haloalkanes-haloarenes arylhalides, and it was created by the author Mr. S M HUSSAIN

Question

Which of the following aryl halides undergoes nucleophilic aromatic substitution with aqueous NaOH at room temperature most readily?

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: D
## CONCEPT FRAMEWORK
 **Primary Concept**: Nucleophilic Aromatic Substitution (SNAr)
 **Related Topics**:  Aromatic reactivity, electron withdrawing groups, leaving groups
 **Prerequisites**: Understanding of SNAr mechanism and the effect of electron withdrawing groups on aromatic rings.
## SOLUTION PATHWAY
**Given Information**:
Aqueous NaOH as the nucleophile, room temperature reaction conditions.
Four aryl halides: Chlorobenzene, 4-Nitrochlorobenzene, 2,4-Dinitrochlorobenzene, 2,4,6-Trinitrochlorobenzene. 
**Method & Steps**:
1. **Understanding SNAr:** SNAr reactions involve a nucleophile attacking an electron-deficient aromatic ring, followed by the departure of a leaving group. The rate of SNAr is significantly enhanced by the presence of strong electron-withdrawing groups (EWGs) on the aromatic ring, which stabilize the negatively charged intermediate formed during the reaction.
2. **Analyzing the effect of Nitro groups:** Nitro groups ($-NO_2$) are strong electron-withdrawing groups. The more nitro groups present on the ring, the more electron-deficient the ring becomes, thus facilitating the nucleophilic attack.
3. **Comparing the given compounds:**
    * Chlorobenzene ($C_6H_5Cl$): No electron-withdrawing groups, least reactive.
    * 4-Nitrochlorobenzene ($C_6H_4(NO_2)Cl$): One nitro group, moderately reactive.
    * 2,4-Dinitrochlorobenzene ($C_6H_3(NO_2)_2Cl$): Two nitro groups, more reactive than 4-Nitrochlorobenzene.
    * 2,4,6-Trinitrochlorobenzene ($C_6H_2(NO_2)_3Cl$): Three nitro groups, most reactive among the given options.  The three nitro groups strongly deactivate the ring, making it highly susceptible to nucleophilic attack.
4. **Conclusion:** 2,4,6-Trinitrochlorobenzene reacts most readily with aqueous NaOH at room temperature due to the presence of three strong electron-withdrawing nitro groups.
**Result:** 2,4,6-Trinitrochlorobenzene
**Verification:** The reactivity trend aligns with the understanding of SNAr reactions and the effect of electron-withdrawing groups.
## OPTION ANALYSIS
A :x: Chlorobenzene lacks electron-withdrawing groups, making it unreactive under these conditions.
B :x: 4-Nitrochlorobenzene has only one nitro group, making it less reactive than the others with multiple nitro groups.
C :x: 2,4-Dinitrochlorobenzene is more reactive than 4-nitrochlorobenzene, but less than 2,4,6-trinitrochlorobenzene.
D ✓: 2,4,6-Trinitrochlorobenzene has three nitro groups, making it the most electron-deficient and therefore the most reactive towards nucleophilic aromatic substitution.
## LEARNING AIDS
 **Common Mistakes**:
  1.Ignoring the effect of electron-withdrawing groups on SNAr reactivity:  Students might focus solely on the leaving group without considering the importance of activating the ring for nucleophilic attack.
  2.Overestimating the role of the leaving group: While the leaving group's ability to depart is important, the electronic effects of substituents on the ring are more dominant in determining the rate of SNAr.
 **Quick Tips**:
  1.Look for the presence and number of strong electron-withdrawing groups (like nitro groups) on the aromatic ring. More EWGs mean faster reaction.
  2.Remember that SNAr reactions are favored by electron-deficient aromatic rings.

Mr. S M HUSSAIN · Answer

Chlorobenzene

Mr. S M HUSSAIN · Answer

4-Nitrochlorobenzene

Mr. S M HUSSAIN · Answer

2,4-Dinitrochlorobenzene

Mr. S M HUSSAIN · Answer

The correct Option is "2,4,6-Trinitrochlorobenzene"

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