Which of the following is a free radical substitution reaction?

Options

A. Benzene with `Br_2/AlCl_3`
B. Acetylene with Hbr
C. Methane with `Br_2/hv`
D. Propene with `HBr/(C_6H_5COO)_2`

Answer

To determine which of the given reactions is a free radical substitution reaction, we need to analyze each option and identify the mechanism involved. A free radical substitution reaction typically involves the formation of free radicals and the substitution of one atom or group by another.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze the mechanisms of the reactions provided in the options. The reactions involve various compounds and conditions.

2. **Option 1: Benzene with Br2 and AlCl3**:
- In this reaction, benzene reacts with bromine in the presence of AlCl3.
- The mechanism involves the formation of a bromonium ion (a carbocation), not free radicals.
- Therefore, this is not a free radical substitution reaction.

3. **Option 2: Acetylene with HBr**:
- Acetylene (C2H2) reacts with HBr.
- The mechanism involves the formation of a carbocation intermediate when HBr adds across the triple bond.
- Again, no free radicals are formed in this reaction, so it is not a free radical substitution reaction.

4. **Option 3: Methane with Br2 in the presence of light**:
- Here, methane (CH4) reacts with bromine (Br2) in the presence of light.
- The light causes the Br2 to dissociate into two bromine free radicals (Br•).
- These bromine free radicals can then abstract a hydrogen atom from methane, forming a methyl free radical (CH3•) and HBr.
- The methyl free radical can then react with another Br2 molecule to form bromomethane (CH3Br) and regenerate another bromine free radical.
- This process clearly involves the formation of free radicals and is a classic example of a free radical substitution reaction.

5. **Option 4: Propene with HBr**:
- Propene (C3H6) reacts with HBr.
- The mechanism involves the formation of a carbocation when HBr adds across the double bond.
- No free radicals are formed in this reaction, so it is not a free radical substitution reaction.

### Conclusion:
After analyzing all the options, we find that **Option 3** (Methane with Br2 in the presence of light) is the only reaction that involves free radicals and is classified as a free radical substitution reaction.

### Final Answer:
**Option 3: Methane with Br2 in the presence of light is a free radical substitution reaction.**

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