Which of the following group show `-m` effect?

Options

A. `-CMe_(3)`
B. `-O-overset(O)overset(||)underset(O)underset(||)(S)-O-R`
C. `-NH-overset(O)overset(||)(C)-CH_(3)`
D. `-overset(O)overset(||)underset(O)underset(||)(S)-O-R`

Answer

To determine which of the following groups show a minus M effect (−M effect), we need to understand the concept of mesomeric effects. The −M effect indicates that a group withdraws electron density from the rest of the molecule through resonance.

Let's analyze the groups one by one:

1. **Group A: CME3 (Alkyl group)**
- Alkyl groups are known to donate electron density through hyperconjugation and inductive effects. Therefore, they exhibit a +M effect (electron donating) rather than a −M effect.
- **Conclusion**: Does not show −M effect.

2. **Group B: O=S=O (Sulfonyl group)**
- In this group, the sulfur atom is bonded to oxygen atoms with double bonds. The oxygen atoms have lone pairs that can participate in resonance, allowing them to donate electron density. Thus, this group shows a +M effect.
- **Conclusion**: Does not show −M effect.

3. **Group C: NH-C(=O)-CH3 (Amide group)**
- The nitrogen atom has a lone pair that can participate in resonance with the carbonyl group (C=O). This allows the nitrogen to donate electron density into the system, indicating a +M effect.
- **Conclusion**: Does not show −M effect.

4. **Group D: O=C=O (Carbonyl group)**
- In this case, the carbonyl group (C=O) has a high electronegativity. The oxygen atoms pull electron density away from the carbon atom, leading to a decrease in electron density. This group exhibits a −M effect because it withdraws electron density through resonance.
- **Conclusion**: Shows −M effect.

**Final Answer**: The group that shows −M effect is **Group D** (O=C=O).