Consider the following alkyl halides
1. `(CH_3)_3 "CCH"_2Br`
2. `ClCH_2CH=CH_2`
3. `ClCH_2 CH_2 CH_3`
4. `BrCH_2CH_2CH_3`
Arrange these alkyl halides in decreasing order of reactivity in Williamson reaction.

Options

A. `2 gt 1 gt 3 gt 4`
B. `2 gt 1 gt 4 gt 3`
C. `1 gt 2 gt 3 gt 4`
D. `2 gt 4 gt 3 gt 1`

Answer

To determine the reactivity of the given alkyl halides in the Williamson reaction, we need to analyze the structure of each compound and understand how the reaction mechanism works. The Williamson ether synthesis involves the nucleophilic substitution of a halide by an alkoxide ion, which is a strong nucleophile. The reactivity of alkyl halides in this reaction can be influenced by several factors, including the type of halide, the degree of substitution of the carbon atom to which the halide is attached, and steric hindrance.

### Step-by-Step Solution:

1. **Identify the Alkyl Halides:**
- 1. `(CH₃)₃CCH₂Br` (tert-butyl bromide)
- 2. `ClCH₂CH=CH₂` (vinyl chloride)
- 3. `ClCH₂CH₂CH₃` (1-chloropropane)
- 4. `BrCH₂CH₂CH₃` (1-bromopropane)

2. **Understand the Williamson Reaction:**
- The Williamson reaction is a nucleophilic substitution reaction where an alkoxide ion attacks the carbon atom bonded to the halide, resulting in the formation of an ether and the release of the halide ion.

3. **Assess the Reactivity of Each Alkyl Halide:**
- **Alkyl Halide 1: `(CH₃)₃CCH₂Br` (tert-butyl bromide)**
- This is a tertiary halide. Tertiary halides are less reactive in SN2 reactions due to steric hindrance. The bulky tert-butyl group hinders the nucleophilic attack.
- **Alkyl Halide 2: `ClCH₂CH=CH₂` (vinyl chloride)**
- This is a vinyl halide. Vinyl halides can stabilize the transition state due to resonance, making them more reactive in SN2 reactions despite being primary.
- **Alkyl Halide 3: `ClCH₂CH₂CH₃` (1-chloropropane)**
- This is a primary halide. Primary halides are generally more reactive in SN2 reactions compared to tertiary halides but less reactive than vinyl halides.
- **Alkyl Halide 4: `BrCH₂CH₂CH₃` (1-bromopropane)**
- This is also a primary halide, but bromine is a better leaving group than chlorine, which enhances its reactivity.

4. **Rank the Reactivity:**
- Based on the above analysis:
- **Most Reactive:** Vinyl chloride (2) - due to resonance stabilization.
- **Next:** 1-bromopropane (4) - primary halide with bromine as a good leaving group.
- **Then:** 1-chloropropane (3) - primary halide but with chlorine as a leaving group.
- **Least Reactive:** tert-butyl bromide (1) - tertiary halide with steric hindrance.

5. **Final Order of Reactivity:**
- The order of reactivity in the Williamson reaction is:
- **2 > 4 > 3 > 1**