Which one of the following species is most stable

Options

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

Explanation

To determine which of the given species is the most stable, we need to analyze the stability of each carbocation based on the substituents attached to the aromatic ring and their effects on the carbocation.

### Step-by-Step Solution:

1. **Identify the Species**: We have four different carbocations to analyze. Let's denote them as A, B, C, and D based on the substituents attached to the aromatic ring.

2. **Analyze Species A**:
- Structure: Para-NO2 and CH2+ (carbocation).
- Effect of NO2: The nitro group (NO2) is a strong electron-withdrawing group. It pulls electron density away from the ring.
- Stability: The positive charge on the carbocation (CH2+) will increase due to the electron-withdrawing effect of NO2, making this carbocation unstable.

3. **Analyze Species B**:
- Structure: Para-OCH3 and CH2+ (carbocation).
- Effect of OCH3: The methoxy group (OCH3) has lone pairs that can participate in resonance with the ring, donating electron density.
- Stability: The electron density in the ring increases, which helps to stabilize the positive charge on the carbocation. This makes species B the most stable among the options.

4. **Analyze Species C**:
- Structure: Para-Cl and CH2+ (carbocation).
- Effect of Cl: The chlorine atom has both a -I (inductive) effect and a resonance effect. However, the -I effect is stronger, pulling electron density away from the ring.
- Stability: This results in increased positive charge density on the carbocation, making it unstable.

5. **Analyze Species D**:
- Structure: A different substituent (not specified in the transcript) and CH2+ (carbocation).
- Effect: The substituent's effect on the carbocation needs to be analyzed, but it is stated that it is less stabilizing than species B.
- Stability: While it may provide some stabilization, it does not stabilize the carbocation as effectively as species B.

6. **Conclusion**: Based on the analysis, species B (with the OCH3 group) is the most stable due to the resonance effect that increases electron density and stabilizes the positive charge on the carbocation.

### Final Answer:
**The most stable species is B (CH2+ with para-OCH3).**