Nucleophilic substitution reactions generally expressed as
`Nu^(-) +R-L rarr R-Nu +L^(-)`
Where `Nu^(-) rarr` Nucleophile , `R-L rarr` substrate, `L rarr` leaving group
The best leaving groups are those that become the most stable ions after they depart. since most leaving group leave as a negatibe ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be
(a) electron-withdrawing to polarize the carbon
(b) stable once it has left (not a strong base)
(c) polaristable to maintain partial bonding with the carbon in the transition state (both `S_(N)1` and `S_(N)2)`. This bonding helps to stabilise the transition state and reduces the activation energy.
Among the following which is false statement?

Options

A. The weaker the base after the group departs, the better the leaving group
B. A reactive leaving group would raise the energy of the product, driving the equilibrium towards the reactants
C. Relative leaving group ability may very with change of solvent.
D. Better leaving group only increases `S_(N)2` rate, not `S_(N)1`.

Answer

To determine which statement among the provided options is false regarding nucleophilic substitution reactions, we will analyze each statement based on the principles of leaving groups in these reactions.

### Step-by-Step Solution:

1. **Understanding the Role of Leaving Groups**:
- In nucleophilic substitution reactions, the leaving group (L) departs as a stable ion. The stability of this ion is crucial for the leaving group's effectiveness.
- A good leaving group is typically a weak base because weaker bases are more stable when they leave.

2. **Analyzing Statement 1**:
- **Statement**: "The weaker the base after the group departs, the better the leaving group."
- **Analysis**: This statement is true. Weaker bases are more stable as ions, making them better leaving groups. Thus, this statement is correct.

3. **Analyzing Statement 2**:
- **Statement**: "The reactive leaving group would raise the energy of the product and drive the equilibrium towards the reactant."
- **Analysis**: This statement is also true. If the leaving group is reactive, it can increase the energy of the product, causing the equilibrium to shift towards the reactants. Thus, this statement is correct.

4. **Analyzing Statement 3**:
- **Statement**: "Relative leaving group ability may vary with the change of solvent."
- **Analysis**: This statement is true. The ability of a leaving group can indeed vary depending on the solvent used, as solvents can stabilize or destabilize ions differently. Thus, this statement is correct.

5. **Analyzing Statement 4**:
- **Statement**: "Better leaving group only increases SN2 rate not SN1."
- **Analysis**: This statement is false. A better leaving group increases the rate of both SN1 and SN2 reactions. In SN1 reactions, the leaving group departs first, forming a carbocation, and in SN2 reactions, the leaving group leaves as the nucleophile attacks. Thus, this statement is incorrect.

### Conclusion:
The false statement among the options provided is:
- **Option D**: "Better leaving group only increases SN2 rate not SN1."