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 SN2 reaction, we need to analyze the chirality of the substrates provided. The SN2 reaction involves a backside attack by the nucleophile, leading to an inversion of configuration at the chiral center. Here’s a step-by-step breakdown of how to identify the correct substrate:

### Step 1: Understand the SN2 Mechanism
- The SN2 (Substitution Nucleophilic Bimolecular) reaction is characterized by a single step where the nucleophile attacks the substrate from the opposite side of the leaving group.
- This backside attack results in a geometric inversion known as Walden inversion.

**Hint:** Remember that for a Walden inversion to occur, the carbon atom attached to the leaving group must be chiral.

### Step 2: Identify Chiral Centers
- A chiral center is a carbon atom that is bonded to four different substituents. Only chiral centers can exhibit Walden inversion.
- Examine each substrate to check if the carbon atom bonded to the halogen (in this case, bromine) is chiral.

**Hint:** Look for a carbon atom bonded to four different groups or atoms.

### Step 3: Analyze the Given Substrates
- **1-Bromopropane:** The carbon attached to bromine is bonded to two hydrogen atoms and two other groups (not chiral).
- **2-Bromobutane:** The carbon attached to bromine is bonded to an ethyl group, a methyl group, a hydrogen atom, and the bromine (chiral).
- **2-Bromopentane:** Similar to 2-bromobutane, but check if the carbon is still chiral.
- **Methyl Bromide:** The carbon attached to bromine is bonded to three hydrogen atoms (not chiral).

**Hint:** Identify which of the substrates has a chiral carbon atom.

### Step 4: Confirm the Presence of Walden Inversion
- For the substrate to demonstrate Walden inversion, it must undergo the SN2 reaction with a chiral center.
- In the case of 2-bromobutane, when the nucleophile attacks from the backside, it will lead to an inversion of configuration from R to S or vice versa.

**Hint:** Remember that only the substrate with a chiral center can show Walden inversion.

### Conclusion
Based on the analysis, **2-bromobutane** is the only substrate that can demonstrate a Walden inversion due to the presence of a chiral carbon atom.

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