Which step is chain propagation step in the following mechanism ?
(i) `Cloverset(hv)rarrCl^(.) +Cl^(.)`
(ii) `Cl^(.)+ CH_4rarr^(.)CH_3+HCl`
(iii) `Cl^(.)+ Cl^(.)rarrCl_2`
(iv) `CH_3 +^(.)ClrarrCH_3Cl`

Options

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

Answer

To determine which step is the chain propagation step in the given mechanism, let's analyze each step carefully.

1. **Step (i):** `Cl2 (hv) → Cl· + Cl·`
- This step involves the dissociation of chlorine gas into two chlorine radicals in the presence of light. This is known as the chain initiation step because it generates the free radicals needed for the subsequent reactions.

2. **Step (ii):** `Cl· + CH4 → CH3· + HCl`
- In this step, a chlorine radical reacts with methane (CH4) to produce a methyl radical (CH3·) and hydrochloric acid (HCl). This step is characteristic of chain propagation because it involves the reaction of a free radical (Cl·) with a stable molecule (CH4) to generate another free radical (CH3·). The chain continues as the newly formed radical can participate in further reactions.

3. **Step (iii):** `Cl· + Cl· → Cl2`
- This step involves the recombination of two chlorine radicals to form chlorine gas (Cl2). This is a termination step because it removes free radicals from the reaction and stops the chain reaction.

4. **Step (iv):** `CH3· + Cl· → CH3Cl`
- In this step, the methyl radical (CH3·) reacts with a chlorine radical (Cl·) to produce methyl chloride (CH3Cl). This is also a chain propagation step as it continues the chain reaction by using free radicals to generate more stable products.

**Conclusion:**
The chain propagation steps in this mechanism are steps (ii) and (iv). However, if we are to select only one option as the primary chain propagation step, step (ii) is the most significant as it initiates the formation of the methyl radical which can then propagate further.

**Final Answer:**
The chain propagation step is **(ii) `Cl· + CH4 → CH3· + HCl`**.

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