Which of the following have `+M` effect
(`overline e` - donating mesomeric effect) ?

Options

A. `-NO_2`
B. `-COOH`
C. `-NH_2`
D. `-SR`

Answer

To determine which of the given species exhibit a +M (mesomeric) effect, we need to analyze each option based on their ability to donate electrons through resonance. The +M effect is characterized by the ability of a substituent to donate electron density to an adjacent π system (like a double bond or aromatic ring) due to the presence of lone pairs of electrons.

### Step-by-Step Solution:

1. **Identify the Species**: We have four species to evaluate: NO2, COOH, NH2, and SR.

2. **Evaluate NO2**:
- The nitro group (NO2) is known to be an electron-withdrawing group due to resonance and inductive effects.
- It pulls electron density away from the double bond or aromatic system.
- **Conclusion**: NO2 exhibits a -M effect (not +M).

3. **Evaluate COOH**:
- The carboxylic acid group (COOH) is also an electron-withdrawing group.
- It has a strong -M effect due to the resonance structure that stabilizes the negative charge on the oxygen atom.
- **Conclusion**: COOH exhibits a -M effect (not +M).

4. **Evaluate NH2**:
- The amino group (NH2) has a lone pair of electrons on the nitrogen atom.
- This lone pair can be donated to the π system, enhancing electron density.
- **Conclusion**: NH2 exhibits a +M effect.

5. **Evaluate SR**:
- The thiol group (SR) has two lone pairs of electrons on sulfur.
- Similar to NH2, these lone pairs can be delocalized into the π system, providing electron density.
- **Conclusion**: SR exhibits a +M effect.

6. **Final Answer**:
- The species that exhibit the +M effect are NH2 and SR.

### Summary of Results:
- **NO2**: -M effect (wrong)
- **COOH**: -M effect (wrong)
- **NH2**: +M effect (correct)
- **SR**: +M effect (correct)

### Correct Options:
- NH2 and SR are the correct answers that exhibit the +M effect.