In which of the following pairs A is more stable than B?

Options

A.
B.
C.
D. `{:(A,B),(Ph_(3)C^(.),(CH_3)_(3)C^(.)):}`

Answer

To determine which of the given pairs has A being more stable than B, we need to analyze the stability of the free radicals in each pair based on their structure and the principles of radical stability.

### Step-by-Step Solution:

1. **Identify the Free Radicals:**
- We have four pairs of radicals to analyze. For each pair, we need to identify whether they are primary, secondary, tertiary, or other types of radicals.

2. **Evaluate Each Pair:**
- **Pair 1:**
- A: Secondary radical (attached to 2 carbon atoms)
- B: Tertiary radical (attached to 3 carbon atoms)
- **Stability:** B is more stable than A (Tertiary > Secondary).

- **Pair 2:**
- A: Cyclopropyl radical (small ring, high angle strain)
- B: Cyclohexyl radical (larger ring, lower angle strain)
- **Stability:** B is more stable than A (Cyclohexyl > Cyclopropyl).

- **Pair 3:**
- A: Vinyl radical (less stable due to sp2 hybridization)
- B: Allyl radical (more stable due to resonance)
- **Stability:** B is more stable than A (Allyl > Vinyl).

- **Pair 4:**
- A: Tertiary alkyl radical (attached to three carbon atoms)
- B: Phenyl radical (delocalized over phenyl rings)
- **Stability:** A is more stable than B (due to resonance stabilization in the phenyl radical).

3. **Conclusion:**
- The only pair where A is more stable than B is the last pair (D: CH3-CH(CH3)-CH3 radical vs. Ph-CH2 radical).

### Final Answer:
- The pair where A is more stable than B is **D**.