Which of the following is the most stable carbanion?

Options

A. `(CH_(3))_(3)C:^(-)`
B. `CH_(3):^(-)`
C. `CH_(3)CH_(2):^(-)`
D. `(CH_(3))_(2)CH:^(-)`

Explanation

To determine which of the given carbanions is the most stable, we need to analyze the effects of substituents on the stability of carbanions. Carbanions are negatively charged species, and their stability is influenced by the presence of electron-donating or electron-withdrawing groups.

### Step-by-Step Solution:

1. **Identify the Carbanions**:
Let's denote the carbanions as follows:
- A: C(CH3)3^-
- B: CH3^-
- C: CH2(CH3)2^-
- D: CH(CH3)2^-

2. **Understand the Effects of Substituents**:
- **+I Effect (Inductive Effect)**: Alkyl groups are electron-donating due to the +I effect. They increase electron density on the carbon atom, which can destabilize a carbanion by concentrating the negative charge.
- **-I Effect**: Electron-withdrawing groups stabilize carbanions by dispersing the negative charge.

3. **Analyze Each Carbanion**:
- **Carbanion A (C(CH3)3^-)**: Has three methyl groups. The +I effect from these groups increases electron density, making it less stable.
- **Carbanion B (CH3^-)**: Has no substituents, hence no +I effect. This carbanion is relatively stable as there are no destabilizing effects.
- **Carbanion C (CH2(CH3)2^-)**: Has one +I effect from the two methyl groups. This will destabilize the carbanion but not as much as A.
- **Carbanion D (CH(CH3)2^-)**: Has two methyl groups, which also contribute to the +I effect, making it less stable than B but more stable than A.

4. **Rank the Stability**:
- The stability order from most stable to least stable is:
- B (CH3^-) > C (CH2(CH3)2^-) > D (CH(CH3)2^-) > A (C(CH3)3^-)

5. **Conclusion**:
The most stable carbanion among the options is **B (CH3^-)** because it has no +I effect destabilizing it.