Q46 / 180 · COMEDK 2026 (9 May Shift 2) · Chemistry
Among the given resonating structures of molecules negative mesomeric effect is represented by:
- A
- B
- Correct answerC
- D
Solution
The negative mesomeric effect ($-M$ effect) is observed when a substituent withdraws electron density from a conjugated system (such as a benzene ring) through resonance. This results in a decreased electron density in the ring, typically placing positive charges at the ortho and para positions.
In option (A), the $-\text{OCH}_3$ group donates a lone pair of electrons to the ring, showing a $+M$ effect and creating negative charges at the ortho and para positions.
In option (B), the $-\text{OH}$ group donates a lone pair of electrons to the ring, showing a $+M$ effect.
In option (C), the $-\text{NO}_2$ group withdraws $\pi$ electrons from the ring towards itself, creating positive charges at the ortho and para positions. This represents the $-M$ effect.
In option (D), the $-\text{NH}_2$ group donates a lone pair of electrons to the ring, showing a $+M$ effect.
Answer:
In option (A), the $-\text{OCH}_3$ group donates a lone pair of electrons to the ring, showing a $+M$ effect and creating negative charges at the ortho and para positions.
In option (B), the $-\text{OH}$ group donates a lone pair of electrons to the ring, showing a $+M$ effect.
In option (C), the $-\text{NO}_2$ group withdraws $\pi$ electrons from the ring towards itself, creating positive charges at the ortho and para positions. This represents the $-M$ effect.
In option (D), the $-\text{NH}_2$ group donates a lone pair of electrons to the ring, showing a $+M$ effect.
Answer:
Hence, the correct answer is option: 'C'.