Nucelophilic aliphatic substitution reaction is given by those compounds which have nucleophiic groups as leaving. Less is the basicity of the leaving group, more is its leaving power. Leaving group can be in the form of nucleophile (charged of neutral)
Leaving power of which group is maximum ?

Answer

To determine which leaving group has the maximum leaving power, we need to analyze the basicity and stability of the leaving groups provided. The less basic a leaving group is, the better its leaving power.

### Step-by-Step Solution:

1. **Understand the Concept of Leaving Groups**:
- Leaving groups are atoms or groups that can depart from the parent molecule during a nucleophilic substitution reaction. Their ability to leave is influenced by their basicity; less basic groups are better leaving groups.

2. **Identify the Given Leaving Groups**:
- The problem mentions three leaving groups:
- Group 1: O⁻ (with resonance stabilization)
- Group 2: O S(O)₂ (with a - charge and a C₄F₉ group)
- Group 3: Br⁻

3. **Analyze Group 1: O⁻**:
- The oxygen anion (O⁻) can participate in resonance stabilization, which helps to delocalize the negative charge. This stabilization increases its leaving power compared to a non-resonance-stabilized group.

4. **Analyze Group 2: O S(O)₂ with C₄F₉**:
- This group has a - charge and is bonded to a carbon chain (C₄F₉) that has strong electron-withdrawing properties due to the presence of fluorine atoms. The -I (inductive effect) from the fluorinated carbon chain further stabilizes the negative charge through electron withdrawal, making this leaving group very stable.

5. **Analyze Group 3: Br⁻**:
- Bromide (Br⁻) is a good leaving group, but it lacks resonance stabilization and the strong -I effect that the second group has. Therefore, it is less stable compared to the other two groups.

6. **Compare the Stability of Leaving Groups**:
- Group 2 (O S(O)₂ with C₄F₉) is the most stable due to both resonance stabilization and the strong -I effect from the fluorinated carbon chain.
- Group 1 (O⁻) is also a good leaving group but is less stable than Group 2.
- Group 3 (Br⁻) is the least stable leaving group.

7. **Conclusion**:
- The leaving group with the maximum leaving power is **Group 2: O S(O)₂ with C₄F₉** due to its resonance stabilization and strong inductive effect.