Most stable carbonium ion is :

Options

A. (a)`p-NO_(2)-C_(6)H_(4)-^(+)CH_(2)`
B. (b)`C_(6)H_(5).^(+)CH_(2)`
C. (c)`p-Cl-C_(6)H_(4)-.^(+)CH_(2)`
D. (d)`p-CH_(3)O-C_(6)H_(4)-.^(+)CH_(2)`

Answer

To determine the most stable carbonium ion (carbocation) among the given options A, B, C, and D, we need to analyze the effects of substituents on the stability of the carbocation. Here’s a step-by-step solution:

### Step 1: Identify the Structures
- **A**: Para-nitro methylenium ion (CH2^+ with a NO2 group at the para position).
- **B**: Para-methylenium ion (CH2^+ with no substituents).
- **C**: Para-chloro methylenium ion (CH2^+ with a Cl group at the para position).
- **D**: Para-methoxy methylenium ion (CH2^+ with an OCH3 group at the para position).

### Step 2: Analyze the Substituents
- **A (NO2)**: The nitro group is a strong electron-withdrawing group (−R, −I). It destabilizes the carbocation by pulling electron density away from the positively charged carbon, making the carbocation less stable.

- **B (No substituent)**: There are no groups present, so the stability of the carbocation is neutral. It does not have any destabilizing or stabilizing effects.

- **C (Cl)**: The chlorine atom is a weak electron-withdrawing group (−I) but has a +R effect as well. However, its overall effect is still destabilizing, though less so than the nitro group.

- **D (OCH3)**: The methoxy group is an electron-donating group (+R). It donates electron density to the positively charged carbon, which stabilizes the carbocation.

### Step 3: Compare Stability
- **Stability Order**:
- **D (OCH3)**: Most stable due to electron donation.
- **B (No substituent)**: Neutral stability.
- **C (Cl)**: Less stable than B but more stable than A.
- **A (NO2)**: Least stable due to strong electron-withdrawing effect.

### Conclusion
The most stable carbonium ion is **D (para-methoxy methylenium ion)**.

### Stability Order Summary
1. D (OCH3) - Most stable
2. B (No substituent) - Neutral stability
3. C (Cl) - Less stable than B
4. A (NO2) - Least stable

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