The photochemical chlorination of paraffins occurs by a free radical mechanism. From the following set of reactions pick out the chain propogation steps.

Options

A. `Cl_(2)overset(hv)rarr2Cl`
B. `CH_(4)+ClrarrCH_(3)+HCl`
C. `H_(3)C+Cl_(2)rarrH_(3)C-Cl+Cl`
D. `Cl+ClrarrCl_(2)`

Answer

To identify the chain propagation steps in the photochemical chlorination of paraffins, we need to understand the mechanism involved in this process. The chlorination occurs through a free radical mechanism, which can be broken down into three main stages: initiation, propagation, and termination.

### Step-by-Step Solution:

1. **Initiation Step**:
- The reaction begins with the dissociation of chlorine molecules (Cl₂) into two chlorine free radicals (Cl•) when exposed to light (sunlight). This step is crucial as it generates the free radicals needed for the subsequent reactions.
- **Reaction**: Cl₂ → 2 Cl•

2. **First Propagation Step**:
- The chlorine free radical (Cl•) then reacts with a paraffin (alkane, e.g., methane, CH₄). The chlorine radical abstracts a hydrogen atom from the alkane, resulting in the formation of a methyl free radical (CH₃•) and hydrochloric acid (HCl).
- **Reaction**: Cl• + CH₄ → CH₃• + HCl

3. **Second Propagation Step**:
- The methyl free radical (CH₃•) generated in the previous step can now react with another chlorine molecule (Cl₂). This reaction produces chloromethane (CH₃Cl) and regenerates another chlorine free radical (Cl•), allowing the chain reaction to continue.
- **Reaction**: CH₃• + Cl₂ → CH₃Cl + Cl•

4. **Termination Step**:
- The process continues with the free radicals reacting with each other until they are consumed or combined to form stable products. This is known as the termination step, where two radicals combine to form a stable molecule, effectively stopping the chain reaction.
- **Example Reaction**: Cl• + Cl• → Cl₂ (This is a termination step)

### Summary of Propagation Steps:
- The chain propagation steps identified are:
- **Step 2**: Cl• + CH₄ → CH₃• + HCl (First Propagation)
- **Step 3**: CH₃• + Cl₂ → CH₃Cl + Cl• (Second Propagation)