Which is the most stable carbocation among the following ?

Options

A. Iso - butyl cation
B. Triphenyl methyl cation
C. Cyclopropyl methyl cation
D. t-Butyl cation

Answer

To determine the most stable carbocation among the given options (isobutyl cation, triphenylmethyl cation, cyclopropylmethyl cation, and tertiary butyl cation), we can analyze the stability of each carbocation based on their structure and the effects that stabilize them.

### Step-by-Step Solution:

1. **Identify the Structures of Each Carbocation:**
- **Isobutyl Cation:** This is a primary carbocation, represented as \( \text{C}_3\text{H}_7^+ \).
- **Triphenylmethyl Cation:** This cation has three phenyl groups attached to the carbon with the positive charge, represented as \( \text{C}^+(C_6H_5)_3 \).
- **Cyclopropylmethyl Cation:** This cation has a cyclopropyl group attached to a carbon with the positive charge, represented as \( \text{C}_3\text{H}_5^+ \).
- **Tertiary Butyl Cation:** This is a tertiary carbocation, represented as \( \text{C}^+(C_3\text{H}_7)_3 \).

2. **Evaluate the Stability Factors:**
- **Inductive Effect:** Alkyl groups can donate electron density through the inductive effect, stabilizing the positive charge. Tertiary butyl cation is stabilized by three alkyl groups.
- **Resonance Effect:** Resonance can greatly stabilize carbocations by delocalizing the positive charge. The triphenylmethyl cation benefits from resonance due to the three phenyl groups, while the cyclopropylmethyl cation also has resonance stabilization.

3. **Rank the Stability:**
- **Triphenylmethyl Cation:** Highly stable due to extensive resonance from three phenyl groups. The positive charge can be delocalized over multiple structures.
- **Cyclopropylmethyl Cation:** Stabilized by resonance, though less effectively than triphenylmethyl.
- **Tertiary Butyl Cation:** Stabilized by inductive effects from three alkyl groups but lacks resonance.
- **Isobutyl Cation:** Least stable as it is a primary carbocation with no significant resonance stabilization.

4. **Conclusion:**
Based on the analysis, the most stable carbocation among the options is the **triphenylmethyl cation** due to its extensive resonance stabilization.

### Final Answer:
**Triphenylmethyl cation is the most stable carbocation.**