Which of the following reaction pairs constitutes the chain propagation step in chlorination of methyl chloride ?

Options

A. `.^(•)CH_(3) + Cl_(2) rarr CH_(3)Cl + .^(•)Cl`
`CH_(3)Cl + .^(•)Cl rarr .^(•)CH_(2)Cl + HCl`
B. `CH_(3)Cl + .^(•)Cl rarr .^(•)CH_(2)Cl_(2) + .^(•)H`
`.^(•)CH_(2)Cl + Cl_(2) rarr CH_(2)Cl_(2) + .^(•)Cl`
C. `CH_(3)Cl+.^(•)Clrarr.^(•)CH_(2)Cl+HCl`
`.^(•)CH_(2)Cl+Cl_(2)rarrCH_(2)Cl_(2)+.^(•)Cl`
D. `.^(•)CH_(2)Cl+.^(•)CH_(2)ClrarrCH_(2)Cl-CH_(2)Cl.^(•)CH_(2)Cl+.^(•)ClrarrCH_(2)Cl_(2)`

Answer

To determine which reaction pairs constitute the chain propagation step in the chlorination of methyl chloride, we need to understand the chlorination process, which occurs in three main steps: initiation, chain propagation, and termination.

### Step-by-Step Solution:

1. **Understanding Chlorination**:
- Chlorination is the process of introducing chlorine into a compound, in this case, methyl chloride (CH₃Cl).
- The overall reaction involves the substitution of hydrogen atoms in methyl chloride with chlorine atoms.

2. **Steps of the Reaction**:
- **Initiation**: This step involves the formation of chlorine radicals. When chlorine gas (Cl₂) is exposed to light, it undergoes homolytic bond fission to produce two chlorine radicals:
\[ \text{Cl}_2 \xrightarrow{\text{light}} 2 \text{Cl}^\bullet \]

3. **Chain Propagation**:
- In the chain propagation step, the chlorine radicals react with methyl chloride to form new radicals and products. This involves two main reactions:
- The first reaction involves the chlorine radical attacking methyl chloride, leading to the formation of methyl radical (CH₃˙) and hydrochloric acid (HCl):
\[ \text{Cl}^\bullet + \text{CH}_3\text{Cl} \rightarrow \text{CH}_3^\bullet + \text{HCl} \]
- The second reaction involves the newly formed methyl radical reacting with another chlorine molecule to produce chloromethane (CH₂Cl₂) and another chlorine radical:
\[ \text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_2\text{Cl}_2 + \text{Cl}^\bullet \]

4. **Chain Termination**:
- Eventually, the reaction will reach a point where two radicals combine to terminate the chain reaction, but this is not part of the propagation step.

5. **Identifying the Correct Reaction Pair**:
- From the above reactions, we can see that the chain propagation step consists of the two reactions:
1. \(\text{Cl}^\bullet + \text{CH}_3\text{Cl} \rightarrow \text{CH}_3^\bullet + \text{HCl}\)
2. \(\text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_2\text{Cl}_2 + \text{Cl}^\bullet\)

6. **Conclusion**:
- The correct reaction pair that constitutes the chain propagation step in the chlorination of methyl chloride is the first reaction (formation of the methyl radical and HCl) and the second reaction (formation of chloromethane and chlorine radical).

### Answer:
The reaction pair that constitutes the chain propagation step in the chlorination of methyl chloride is:
1. \(\text{Cl}^\bullet + \text{CH}_3\text{Cl} \rightarrow \text{CH}_3^\bullet + \text{HCl}\)
2. \(\text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_2\text{Cl}_2 + \text{Cl}^\bullet\)