Which of the following statement is not correct about `S_(N^2)` reactions of alkyl halides?

Options

A. Nucleophile attacks the carbon from the side opposite to where the leaving group is attached.
B. The bond formation and bond breaking take place in one step.
C. The rate of reaction depends upon the concentration of nucleophile.
D. `S_(N^2)` mechanism is predominant in tertiary alkyl halides.

Answer

To determine which statement is not correct about \( S_N2 \) reactions of alkyl halides, let's analyze each option step by step.

### Step 1: Understand the \( S_N2 \) Mechanism
The \( S_N2 \) (Substitution Nucleophilic Bimolecular) reaction involves a nucleophile attacking an electrophilic carbon atom, leading to the simultaneous breaking of the bond with the leaving group. This reaction occurs in a single concerted step.

### Step 2: Analyze Each Statement
1. **Nucleophile attacks a carbon from one side and opposite to where the leaving group is**: This statement is correct. In \( S_N2 \) reactions, the nucleophile approaches the carbon atom from the side opposite to the leaving group, leading to a backside attack.

2. **Bond formation and bond breaking takes place in one step**: This statement is also correct. The \( S_N2 \) mechanism is characterized by a single transition state where the bond to the nucleophile is forming while the bond to the leaving group is breaking.

3. **Rate of the reaction depends upon the concentration of nucleophile**: This statement is correct as well. The rate of an \( S_N2 \) reaction is dependent on the concentration of both the nucleophile and the substrate (alkyl halide), as it involves two species in the rate-determining step.

4. **\( S_N2 \) mechanism is predominant in tertiary alkyl halides**: This statement is incorrect. \( S_N2 \) reactions are not favored for tertiary alkyl halides due to steric hindrance. Tertiary carbons are too crowded for the nucleophile to effectively attack, making \( S_N2 \) reactions more favorable for primary and secondary alkyl halides.

### Conclusion
The statement that is not correct about \( S_N2 \) reactions of alkyl halides is:
**"The \( S_N2 \) mechanism is predominant in tertiary alkyl halides."**