Which is not correct about `S_(N)2` ?

Options

A. Role of `S_(N)2` is faster in polar aprotic solvents.
B. Rate of `S_(N)2` is directly proportional to the nature of leaving group.
C. Rate of `S_(N)2` inversely proportional to the steric hindrance present in substrate.
D. Rate of `S_(N)2` increases as stability of carbocation increases.

Answer

To determine which statement is not correct about the \( S_N2 \) reaction, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding the \( S_N2 \) Mechanism:**
- The \( S_N2 \) (Substitution Nucleophilic Bimolecular) reaction involves a nucleophile attacking an electrophile (usually a carbon atom bonded to a leaving group) in a single concerted step. This means that both the nucleophile and the substrate (alkyl halide) are involved in the rate-determining step.

**Hint:** Recall that in \( S_N2 \) reactions, the mechanism is a single step where the nucleophile attacks and the leaving group departs simultaneously.

2. **Analyzing the Role of Solvents:**
- The \( S_N2 \) reaction is faster in polar aprotic solvents. These solvents stabilize the nucleophile without solvation, allowing it to be more reactive.

**Hint:** Consider how solvent polarity affects nucleophilicity in \( S_N2 \) reactions.

3. **Effect of Leaving Group:**
- The rate of the \( S_N2 \) reaction is directly proportional to the nature of the leaving group. A better leaving group will facilitate a faster reaction.

**Hint:** Think about how the stability of the leaving group affects the reaction rate.

4. **Steric Hindrance:**
- The rate of the \( S_N2 \) reaction is inversely proportional to steric hindrance. More steric hindrance around the carbon atom makes it harder for the nucleophile to attack.

**Hint:** Recall that steric hindrance impacts the accessibility of the electrophilic center.

5. **Carbocation Stability:**
- The statement that the rate of \( S_N2 \) increases as the stability of carbocation increases is incorrect. In \( S_N2 \) reactions, there is no carbocation intermediate formed. Instead, the reaction proceeds through a direct attack by the nucleophile.

**Hint:** Remember that \( S_N2 \) does not involve carbocation formation, unlike \( S_N1 \) reactions.

### Conclusion:
The statement that is not correct about \( S_N2 \) is: "Rate of \( S_N2 \) increases as the stability of carbocation increases." Therefore, the correct answer is option 4.