In which of the following pairs of carbarion the first one is more stable than correct.

Options

A. `overset(o.)CF_(3),overset(o.)"CC"l_(3)`
B. `HC-=overset(" "o.)C,H_(2)C=overset(o.)CH`
C.
D. `(CH_(3)),overset(o.)C,H_(3)C-overset(o.)CH_(2)`

Answer

To solve the question of which of the following pairs of carbanions has the first one being more stable, we will analyze each pair step by step.

### Step 1: Analyze CF3- and CCl3-
- **CF3- (Trifluoromethyl carbanion)**: The fluorine atoms are highly electronegative, which means they will attract electron density towards themselves. This results in a destabilization of the carbanion because the negative charge is less stable when surrounded by highly electronegative atoms.
- **CCl3- (Trichloromethyl carbanion)**: Chlorine is less electronegative than fluorine, so it will not pull electron density as strongly. Additionally, chlorine has a larger atomic size, allowing for better accommodation of the negative charge in its larger orbitals.

**Conclusion**: CCl3- is more stable than CF3-. Therefore, the first one (CF3-) is not more stable than the second one (CCl3-).

### Step 2: Analyze CH≡C- and CH2=CH-
- **CH≡C- (Acetylide carbanion)**: This carbanion has sp hybridization, which means it has 50% s character. Higher s character means the electrons are held closer to the nucleus, making the negative charge more stable.
- **CH2=CH- (Vinyl carbanion)**: This carbanion has sp2 hybridization, which means it has 33.33% s character. The electrons are not held as tightly as in the sp hybridized carbanion.

**Conclusion**: CH≡C- is more stable than CH2=CH-. Therefore, the first one (CH≡C-) is more stable than the second one (CH2=CH-).

### Step 3: Analyze Cyclohexyl- and Benzyl-
- **Cyclohexyl-**: This carbanion is sp3 hybridized and has a certain stability due to its saturated structure.
- **Benzyl-**: This carbanion is sp2 hybridized and is stabilized by resonance due to the adjacent aromatic ring.

**Conclusion**: Benzyl- is more stable than Cyclohexyl-. Therefore, the first one (Cyclohexyl-) is not more stable than the second one (Benzyl-).

### Step 4: Analyze Tertiary alkyl- and Primary alkyl-
- **Tertiary alkyl- (e.g., (CH3)3C-)**: This carbanion is stabilized by hyperconjugation and inductive effects from the three alkyl groups.
- **Primary alkyl- (e.g., CH3CH2-)**: This carbanion has only one alkyl group providing stabilization.

**Conclusion**: Tertiary alkyl- is more stable than Primary alkyl-. Therefore, the first one (Tertiary alkyl-) is more stable than the second one (Primary alkyl-).

### Final Conclusion:
From the analysis above, the only pair where the first carbanion is more stable than the second is the pair involving tertiary and primary alkyl carbanions.