The Question is from neet-chemistry organic-chemistry-basics , and it was created by the author THIRUPATHI REDDY

Question

In which of the following compounds does the -NO2 group exert the STRONGEST -M effect?

THIRUPATHI REDDY · Accepted Answer

## CORE INFORMATION
✓ **Answer**: 4
## CONCEPT FRAMEWORK
 **Primary Concept**: Resonance and Mesomeric Effect (-M effect)
 **Related Topics**: Electron withdrawing groups, Hammett substituent constants, Electrophilic Aromatic Substitution
 **Prerequisites**: Understanding of resonance structures, electron density distribution in molecules.
## SOLUTION PATHWAY
**Given Information**:
  Four compounds: Nitrobenzene, p-nitrotoluene, p-nitrophenol, p-nitrobenzaldehyde.
  We need to identify the compound where the -NO2 group exerts the strongest -M effect.
**Method & Steps**:
 1. Understand the -M effect: The -M effect (mesomeric effect) is the electron-withdrawing effect through resonance.  The -NO2 group withdraws electrons through resonance, creating a partial positive charge on the ring.
 2. Analyze each compound's resonance structures:
  a. Nitrobenzene ($	ext{C}_6	ext{H}_5	ext{NO}_2$): The -NO2 group directly attached to the benzene ring withdraws electron density through resonance.
  b. p-nitrotoluene ($	ext{CH}_3	ext{C}_6	ext{H}_4	ext{NO}_2$): The methyl group ($	ext{CH}_3$) is an electron-donating group (+I effect), which somewhat counteracts the electron-withdrawing effect of the -NO2 group.
  c. p-nitrophenol ($	ext{HOC}_6	ext{H}_4	ext{NO}_2$): The hydroxyl group (-OH) is an electron-donating group (+M and +I effects), which also counteracts the electron-withdrawing effect of the -NO2 group. However, the -OH group is also capable of hydrogen bonding, which can influence the electron distribution but not as directly as resonance.
  d. p-nitrobenzaldehyde ($	ext{O=CHC}_6	ext{H}_4	ext{NO}_2$): The aldehyde group (-CHO) is an electron-withdrawing group (-I and -M effects).  This enhances the electron-withdrawing effect of the -NO2 group through resonance. The combined -M effect is stronger.
 3. Compare the -M effects: The -M effect of -NO2 is strongest when it is not opposed or even enhanced by another group's electron-withdrawing nature. In p-nitrobenzaldehyde, both -NO2 and -CHO groups withdraw electrons through resonance, resulting in the strongest overall -M effect.
  Result: p-nitrobenzaldehyde exhibits the strongest -M effect due to the synergistic effect of both -NO2 and -CHO groups.
  Verification: The stronger the electron withdrawing group attached to the benzene ring, the greater the -M effect.  The aldehyde group (-CHO) is a stronger electron-withdrawing group than the methyl group (-CH3) or the hydroxyl group (-OH).
## OPTION ANALYSIS
A :x: Nitrobenzene has a strong -M effect from the -NO2 group, but it's less than p-nitrobenzaldehyde.
B :x: The methyl group in p-nitrotoluene donates electrons, reducing the overall -M effect.
C :x: The hydroxyl group in p-nitrophenol donates electrons, reducing the overall -M effect.
D ✓: p-nitrobenzaldehyde experiences a synergistic electron-withdrawing effect from both the -NO2 and -CHO groups, leading to the strongest -M effect.
## LEARNING AIDS
 **Common Mistakes**:
  1.Ignoring the combined effects of multiple substituents:  Consider all substituents and their electronic effects.
  2.Overestimating the inductive effect compared to resonance: Resonance effects generally dominate over inductive effects in determining the overall electron distribution.
 **Quick Tips**:
  1.Prioritize the resonance effect when comparing -M effects.
  2.Remember that electron-withdrawing groups enhance the -M effect of other electron-withdrawing groups.

THIRUPATHI REDDY · Answer

Nitrobenzene

THIRUPATHI REDDY · Answer

p-nitrotoluene

THIRUPATHI REDDY · Answer

p-nitrophenol

THIRUPATHI REDDY · Answer

The correct Option is "p-nitrobenzaldehyde"

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