In `S_N 2` reactions, the correct order of reactivity for the following compounds `CH_3 Cl, CH_3CH_2 Cl, (CH_3)_2 CHCl and (CH_3)_3 C Cl` is :

Options

A. `CH_3 Cl gt (CH_3)_2 CHCl gt CH_3 CH_2 Cl gt (CH_3)_3 C Cl`
B. `CH_3 Cl gt CH_3 CH_2 Cl gt (CH_3)_2 CHCl gt (CH_3)_3 C Cl`
C. `CH_3 CH_2 Cl gt CH_3 Cl gt (CH_3)_2 CHCl gt (CH_3)_3 C Cl`
D. `(CH_3)_2 CHCl gt CH_3 CH_2 Cl gt CH_3 Cl gt (CH_3)_3 C Cl`

Explanation

To determine the correct order of reactivity for the compounds `CH3Cl`, `CH3CH2Cl`, `(CH3)2CHCl`, and `(CH3)3CCl` in `S_N2` reactions, we need to consider the steric hindrance around the carbon atom that is bonded to the halogen (chlorine in this case).

### Step-by-Step Solution:

1. **Understand the `S_N2` Mechanism**:
- The `S_N2` (Substitution Nucleophilic Bimolecular) reaction involves a nucleophile attacking the carbon atom from the opposite side of the leaving group (halogen). This results in an inversion of configuration at the carbon center.

2. **Identify Steric Hindrance**:
- Steric hindrance refers to the crowding around the reactive center (the carbon atom bonded to the halogen). The more bulky groups attached to the carbon, the harder it is for the nucleophile to approach and attack.

3. **Analyze Each Compound**:
- **`CH3Cl`**: The carbon is bonded to three hydrogens and one chlorine. This is the least sterically hindered compound.
- **`CH3CH2Cl`**: The carbon is bonded to two hydrogens and one ethyl group (CH2). This has slightly more steric hindrance than `CH3Cl`.
- **`(CH3)2CHCl`**: The carbon is bonded to one hydrogen and two methyl groups. This has more steric hindrance than `CH3CH2Cl`.
- **`(CH3)3CCl`**: The carbon is bonded to three methyl groups. This is the most sterically hindered compound.

4. **Determine Order of Reactivity**:
- Since steric hindrance decreases the reactivity in `S_N2` reactions, the order of reactivity will be the reverse of the order of steric hindrance.
- Therefore, the order of reactivity from most reactive to least reactive is:
- `CH3Cl` > `CH3CH2Cl` > `(CH3)2CHCl` > `(CH3)3CCl`

5. **Final Answer**:
- The correct order of reactivity for the given compounds in `S_N2` reactions is:
- `CH3Cl` > `CH3CH2Cl` > `(CH3)2CHCl` > `(CH3)3CCl`