Which step is chain propagation step in the following mechanism?
(i) `Cl_(2) overset(hv)to Cl^(**)+Cl^(**)`
(ii) `Cl^(**)+CH_(4) to .^(**)CH_(3)+HCl`
(iii) `Cl^(**)+Cl^(**) to Cl_(2)`
(iv) `.^(**)CH_(3)+^(**)Cl to CH_(3)Cl`

Options

A. (i)
B. (ii)
C. (iii)
D. (iv)

Answer

To determine which step is the chain propagation step in the given mechanism, we need to analyze each step of the reaction mechanism for the monochlorination of methane.

### Step-by-Step Solution:

1. **Identify the Steps**:
- The mechanism consists of four steps:
1. \( \text{Cl}_2 \overset{hv}{\rightarrow} \text{Cl}^{\cdot} + \text{Cl}^{\cdot} \)
2. \( \text{Cl}^{\cdot} + \text{CH}_4 \rightarrow \text{CH}_3^{\cdot} + \text{HCl} \)
3. \( \text{Cl}^{\cdot} + \text{Cl}^{\cdot} \rightarrow \text{Cl}_2 \)
4. \( \text{CH}_3^{\cdot} + \text{Cl}^{\cdot} \rightarrow \text{CH}_3\text{Cl} \)

2. **Analyze Each Step**:
- **Step 1**: This is the chain initiation step where chlorine molecules dissociate into chlorine radicals (\( \text{Cl}^{\cdot} \)) due to the influence of ultraviolet light. This step does not propagate the reaction; it initiates it.
- **Step 2**: In this step, a chlorine radical reacts with methane (\( \text{CH}_4 \)) to produce a methyl radical (\( \text{CH}_3^{\cdot} \)) and hydrochloric acid (HCl). This step is crucial as it generates a new radical that can continue the reaction.
- **Step 3**: This step involves the recombination of two chlorine radicals to regenerate chlorine gas (\( \text{Cl}_2 \)). This is a termination step, as it removes radicals from the reaction mixture.
- **Step 4**: Here, the methyl radical reacts with another chlorine radical to form chloromethane (\( \text{CH}_3\text{Cl} \)). This step also contributes to the continuation of the chain reaction.

3. **Determine the Chain Propagation Step**:
- The chain propagation steps are those that maintain the presence of radicals and allow the reaction to continue.
- In this case, **Step 2** and **Step 4** are both chain propagation steps. However, since the question asks for the chain propagation step specifically, we can identify **Step 2** as the first occurrence of propagation after initiation.

### Conclusion:
The chain propagation step in the mechanism is **Step 2**: \( \text{Cl}^{\cdot} + \text{CH}_4 \rightarrow \text{CH}_3^{\cdot} + \text{HCl} \).

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