Which of the following contain `+ ` M but -I effect -

Options

A. `O = CH - `
B. `- NO_(2)`
C. `- Cl`
D. `CH_(3)`

Answer

To solve the question "Which of the following contain `+ M but -I effect", we need to analyze the effects of the given groups on a common molecule, typically benzene, which has a high electron density. Let's break down the analysis step by step.

### Step 1: Understand the Effects
- **+M Effect (Mesomeric Effect)**: This effect occurs when a substituent can donate electron density into the aromatic ring through resonance. This typically involves groups that have lone pairs of electrons.
- **-I Effect (Inductive Effect)**: This effect occurs when a substituent withdraws electron density from the aromatic ring through sigma bonds. This is usually seen with electronegative atoms or groups.

### Step 2: Analyze Each Group
We will analyze each of the four groups mentioned in the question: CHO, NO2, Cl, and CH3.

1. **CHO (Aldehyde Group)**:
- The CHO group can withdraw electron density through resonance (due to the carbonyl group) and thus shows a **-M effect**.
- It also has a **-I effect** because the carbon in the carbonyl is more electronegative than hydrogen.
- **Conclusion**: CHO shows -M and -I effects.

2. **NO2 (Nitro Group)**:
- The NO2 group is a strong electron-withdrawing group due to resonance and inductive effects. It shows a **-M effect** and a **-I effect**.
- **Conclusion**: NO2 shows -M and -I effects.

3. **Cl (Chlorine)**:
- Chlorine has lone pairs that can be donated into the ring, thus showing a **+M effect**.
- However, it is also electronegative and can withdraw electron density through sigma bonds, showing a **-I effect**.
- **Conclusion**: Cl shows +M and -I effects.

4. **CH3 (Methyl Group)**:
- The CH3 group is an electron-donating group through hyperconjugation, showing a **+I effect**. It does not have a lone pair to donate through resonance.
- **Conclusion**: CH3 shows +I effect only.

### Step 3: Identify the Correct Group
From the analysis:
- CHO: -M, -I
- NO2: -M, -I
- Cl: +M, -I
- CH3: +I only

The only group that shows **+M but -I effect** is **Chlorine (Cl)**.

### Final Answer:
**Chlorine (Cl) is the group that shows +M but -I effect.**

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