In which of the following options chlorine will acts as the best leaving group.

Options

A. `CH_(3) - Cl`
B. `CH_(3) - CH_(2)-Cl`
C. `H-underset(CH_3)underset(|)overset(CH_3)overset(|)(C )-Cl`
D. `CH_(3)-CH_(2)-underset(CH_3)underset(|)overset(CH_3)overset(|)(C )-Cl`

Answer

To determine which option allows chlorine to act as the best leaving group, we need to analyze the stability of the carbocations formed when chlorine leaves in each compound. The more stable the carbocation, the better the leaving group will be.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We have several compounds where chlorine is attached. We need to analyze each one to see what happens when chlorine leaves.

2. **Analyze the First Compound**:
When chlorine leaves, it forms a methyl carbocation (CH3+). Methyl carbocations are not very stable because they have no alkyl groups to stabilize the positive charge. Thus, chlorine is not a good leaving group here.

**Hint**: Look for the type of carbocation formed (methyl, primary, secondary, tertiary) to assess stability.

3. **Analyze the Second Compound**:
In this case, when chlorine leaves, an ethyl carbocation (CH3CH2+) is formed. Ethyl carbocations are more stable than methyl carbocations due to the presence of one alkyl group that can provide some stabilization. However, they are still not as stable as secondary or tertiary carbocations.

**Hint**: Remember that the stability of carbocations increases with the number of alkyl groups attached.

4. **Analyze the Third Compound**:
Here, when chlorine leaves, a secondary carbocation is formed. Secondary carbocations (R2CH+) are more stable than primary and methyl carbocations because they have two alkyl groups that can help stabilize the positive charge through hyperconjugation and inductive effects.

**Hint**: Secondary carbocations are more stable than primary and methyl carbocations.

5. **Analyze the Fourth Compound**:
In this case, when chlorine leaves, a tertiary carbocation (R3C+) is formed. Tertiary carbocations are the most stable type of carbocation because they have three alkyl groups that provide significant stabilization through hyperconjugation and inductive effects.

**Hint**: Tertiary carbocations are the most stable and thus favor the leaving of the leaving group.

6. **Conclusion**:
Based on the stability of the carbocations formed:
- Methyl carbocation (least stable)
- Ethyl carbocation (more stable than methyl)
- Secondary carbocation (more stable than ethyl)
- Tertiary carbocation (most stable)

Therefore, chlorine will act as the best leaving group in the compound that forms a tertiary carbocation.

### Final Answer:
Chlorine will act as the best leaving group in the last option (the one forming a tertiary carbocation).