The `S_(N)2` reaction involves back - side attack and therefore results in a ''Walden Inversion.'' For which one of the substrates shown would you be able to demonstrate that such back - side attack with ''Walden Inversion'' has infact occurred?

Options

A. 1 - bromopropane
B. 2 - bromobutane
C. 3 - bromopentane
D. Methyl bromide

Answer

To determine which substrate would demonstrate a Walden inversion during an \( S_N2 \) reaction, we need to identify a substrate that has a chiral center. A chiral center is a carbon atom that is bonded to four different groups. The \( S_N2 \) reaction involves a backside attack by the nucleophile, leading to the inversion of configuration at the chiral center.

### Step-by-Step Solution:

1. **Identify the Substrates**: We need to analyze the given substrates (1-bromopropane, 2-bromobutane, and 3-bromopentane) to see if they contain a chiral center.

2. **Check for Chiral Centers**:
- **1-bromopropane**: The structure is CH3-CH2-CH2-Br. The carbon bonded to Br is not a chiral center because it is attached to two hydrogen atoms and two carbon atoms (not four different groups).
- **2-bromobutane**: The structure is CH3-CH(Br)-CH2-CH3. The carbon atom bonded to Br is attached to four different groups (Br, CH3, CH2, and H), making it a chiral center.
- **3-bromopentane**: The structure is CH3-CH2-CH(Br)-CH2-CH3. The carbon bonded to Br is not a chiral center because it is attached to two identical CH2 groups (not four different groups).

3. **Determine the Reaction Mechanism**: Since 2-bromobutane has a chiral center, it can undergo an \( S_N2 \) reaction. The backside attack by the nucleophile will lead to the inversion of configuration at this chiral center, demonstrating Walden inversion.

4. **Conclusion**: The substrate that would demonstrate a Walden inversion due to the \( S_N2 \) reaction is **2-bromobutane**.

### Final Answer:
The substrate that would demonstrate Walden inversion is **2-bromobutane**.

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