Hyperconjugation is most useful for stabilizing which of the following carbocations?

Options

A. Neo-pentyl
B. Tert. Butyl
C. Iso propyl
D. Ethyl

Answer

To determine which carbocation is most stabilized by hyperconjugation, we will analyze each of the given options: neopentyl, tertiary butyl, isopropyl, and ethyl carbocations.

### Step-by-Step Solution:

1. **Understanding Hyperconjugation**:
- Hyperconjugation, also known as no bond resonance, involves the delocalization of sigma electrons from C-H bonds that are adjacent to a positively charged carbon atom (the carbocation). The more alpha hydrogens present, the greater the potential for hyperconjugation and stabilization.

2. **Analyzing Tertiary Butyl Carbocation**:
- Structure: (CH3)3C^+
- This carbocation has three alpha carbons (CH3 groups) attached to the positively charged carbon.
- Each of these CH3 groups has 3 hydrogen atoms, leading to a total of 9 alpha hydrogens.
- Therefore, there are 9 possible hyperconjugating structures.

3. **Analyzing Neopentyl Carbocation**:
- Structure: (CH3)4C^+CH2^+
- The positively charged carbon is adjacent to only one carbon (the CH2 group).
- This alpha carbon has no hydrogen atoms attached.
- Therefore, there is no hyperconjugation possible in this case.

4. **Analyzing Isopropyl Carbocation**:
- Structure: (CH3)2C^+CH3
- This carbocation has two alpha carbons (CH3 groups) attached to the positively charged carbon.
- Each of these CH3 groups has 3 hydrogen atoms, leading to a total of 6 alpha hydrogens.
- Therefore, there are 6 possible hyperconjugating structures.

5. **Analyzing Ethyl Carbocation**:
- Structure: CH3CH2^+
- The positively charged carbon is adjacent to one carbon (the CH2 group).
- This alpha carbon has 3 hydrogen atoms attached.
- Therefore, there are 3 possible hyperconjugating structures.

6. **Comparing Stabilization**:
- Tertiary Butyl: 9 hyperconjugating structures
- Neopentyl: 0 hyperconjugating structures
- Isopropyl: 6 hyperconjugating structures
- Ethyl: 3 hyperconjugating structures

7. **Conclusion**:
- The carbocation that is most stabilized by hyperconjugation is the **tertiary butyl carbocation** (Option B), as it has the highest number of hyperconjugating structures.

### Final Answer:
**Option B: Tertiary Butyl**