Which of the following carbocation is least stable ?

Options

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

Explanation

To determine which carbocation is the least stable, we need to analyze the stability of various carbocations based on the presence of electron-donating or electron-withdrawing groups. Here’s a step-by-step solution:

### Step 1: Understand Carbocation Stability
Carbocation stability is influenced by the presence of substituents on the carbon atom bearing the positive charge. Electron-donating groups (EDGs) stabilize carbocations, while electron-withdrawing groups (EWGs) destabilize them.

### Step 2: Identify Electron-Donating and Electron-Withdrawing Groups
- **Electron-Donating Groups (EDGs)**: Groups that release electron density towards the carbocation, enhancing its stability. Examples include alkyl groups and groups with lone pairs like -OCH3.
- **Electron-Withdrawing Groups (EWGs)**: Groups that withdraw electron density from the carbocation, decreasing its stability. Examples include -NO2 and -CN.

### Step 3: Analyze the Given Carbocations
Let’s consider the carbocations in the question:
1. **Carbocation with -OCH3**: The -OCH3 group has a lone pair of electrons that can donate electron density, making the carbocation more stable.
2. **Carbocation with -NO2**: The -NO2 group is a strong electron-withdrawing group due to its -I (inductive) and -R (resonance) effects, which decreases the stability of the carbocation.

### Step 4: Compare the Stability
- The carbocation with the -OCH3 group is stabilized by resonance and inductive effects.
- The carbocation with the -NO2 group is destabilized due to the withdrawal of electron density.

### Step 5: Conclusion
Among the given options, the carbocation with the -NO2 group is the least stable due to the strong electron-withdrawing effect of the nitro group.

### Final Answer
The least stable carbocation is the one with the -NO2 group.

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