Which one is most reactive towards S N.. reaction ?

Options

A. 1 - Bromopentane
B. 1 - Bromo- 2 - methyl butane
C. 1 - Bromo- 3 - methyl butane
D. 2 - Bromo- 2 - methyl butane

Explanation

To determine which compound is most reactive towards an SN2 reaction, we need to analyze the steric hindrance around the carbon atom that is bonded to the halogen. The reactivity in SN2 reactions is inversely proportional to the steric hindrance; that is, the less hindered the carbon atom, the more reactive it is.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- Option A: 1-Bromopentane
- Option B: 1-Bromo-2-Methylbutane
- Option C: 1-Bromo-3-Methylbutane
- Option D: 2-Bromo-2-Methylbutane

2. **Draw the Structures:**
- **1-Bromopentane:** This compound has a straight-chain structure with bromine attached to the first carbon. It has minimal steric hindrance because only one carbon is adjacent to the carbon attached to bromine.
- **1-Bromo-2-Methylbutane:** This compound has a branching at the second carbon. The carbon attached to bromine is adjacent to one carbon (the methyl group), which adds some steric hindrance.
- **1-Bromo-3-Methylbutane:** Here, the bromine is attached to the first carbon, and the methyl group is on the third carbon. The steric hindrance is less than in the previous compound since the methyl group is further away.
- **2-Bromo-2-Methylbutane:** In this case, the bromine is attached to the second carbon, which is also bonded to two other carbons (the two methyl groups). This creates significant steric hindrance.

3. **Evaluate Steric Hindrance:**
- **1-Bromopentane:** Least hindrance (1 adjacent carbon).
- **1-Bromo-2-Methylbutane:** Moderate hindrance (2 adjacent carbons).
- **1-Bromo-3-Methylbutane:** Moderate hindrance (1 adjacent carbon, but one further away).
- **2-Bromo-2-Methylbutane:** Most hindrance (3 adjacent carbons).

4. **Determine Reactivity:**
- Since SN2 reactivity decreases with increasing steric hindrance, the compound with the least hindrance will be the most reactive.
- Therefore, **1-Bromopentane** (Option A) is the most reactive towards SN2 reactions due to its minimal steric hindrance.

### Conclusion:
The most reactive compound towards SN2 reaction is **1-Bromopentane (Option A)**.

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