Which of the following halide is most reactive ?

Question

Accepted Answer

Option 4, $CH_2=CH.CH_2.Cl$, is the most reactive halide among the given options. Here's why:

1. **Explanation for Option 4 ($CH_2=CH.CH_2.Cl$):**
   - This compound is known as allyl chloride. The presence of the double bond (C=C) adjacent to the chlorine atom significantly increases its reactivity. The double bond allows for resonance stabilization of the carbocation intermediate that forms during a reaction. This stabilization makes the allylic position highly reactive, as the positive charge can be delocalized over the π system. This delocalization lowers the activation energy for reactions, making allyl chloride more reactive than other alkyl chlorides.

2. **Explanation for why other options are less reactive:**

- **Option 1 ($CH_3.CH_2.Cl$):**
     - This is ethyl chloride, a simple alkyl halide. It lacks any resonance stabilization because there are no double bonds or other electron-withdrawing groups adjacent to the chlorine. The reactivity of ethyl chloride is primarily determined by the strength of the C-Cl bond, which is relatively strong compared to allylic systems.

- **Option 2 ($CH_2=CH.Cl$):**
     - This is vinyl chloride. Although it contains a double bond, the chlorine is directly attached to the sp2 hybridized carbon of the double bond. This makes the C-Cl bond stronger and less reactive due to the partial double bond character and the lack of resonance stabilization that would facilitate reaction. Vinyl chlorides are generally less reactive than allylic chlorides.

- **Option 3 ($CH_3.CH_2.CH_2.Cl$):**
     - This is propyl chloride, another simple alkyl halide. Similar to ethyl chloride, it does not have any resonance stabilization or electron-withdrawing groups that would enhance its reactivity. The C-Cl bond is relatively strong, and without the presence of a double bond or resonance, it is less reactive than allyl chloride.

In summary, the presence of the allylic position in Option 4 ($CH_2=CH.CH_2.Cl$) allows for resonance stabilization, making it the most reactive halide among the given options. The other options lack this feature, resulting in lower reactivity.