Which of the following is the most stable carbanion?

Options

A. `CH_3overset(Theta)(CH_2)`
B. `(CH_3)_2overset(Theta)(CH)`
C. `(CH_3)_3overset(-)C`
D. `C_6H_5bar(C)H_2`

Answer

To determine which of the given carbanions is the most stable, we need to analyze the stability of each carbanion based on the presence of electron-withdrawing and electron-donating groups.

### Step-by-Step Solution:

1. **Understanding Carbanion Stability**:
- Carbanion stability is influenced by the presence of electron-withdrawing groups (EWGs) and electron-donating groups (EDGs).
- Electron-withdrawing groups stabilize the negative charge through inductive effects, while electron-donating groups destabilize the carbanion by increasing electron density.

2. **Analyzing Each Carbanion**:
- **Carbanion 1: CH3CH2- (Ethyl Carbanion)**:
- Contains one methyl group (EDG).
- Stability: Moderate, as it has one EDG.

- **Carbanion 2: CH3CH-CH3 (Isopropyl Carbanion)**:
- Contains two methyl groups (EDGs).
- Stability: Less stable than the ethyl carbanion due to increased electron density from two EDGs.

- **Carbanion 3: CH3C(CH3)2- (Tertiary Carbanion)**:
- Contains three methyl groups (EDGs).
- Stability: Least stable among the three, as the three EDGs significantly increase electron density.

- **Carbanion 4: C6H5CH2- (Benzyl Carbanion)**:
- Contains a phenyl group (C6H5), which can stabilize the negative charge through resonance.
- Stability: Most stable due to resonance stabilization, as the negative charge can be delocalized over the aromatic ring.

3. **Conclusion**:
- Among the four carbanions analyzed, the benzyl carbanion (C6H5CH2-) is the most stable due to resonance stabilization.

### Final Answer:
The most stable carbanion is **C6H5CH2-** (option 4).