Which one of the following is a free - radical substitution reaction ?

Options

A.
B.
C.
D. `CH_3CHO+HCN to CH_3CH(OH)CN`

Answer

To determine which of the given reactions is a free radical substitution reaction, we will analyze each option based on the characteristics of free radical reactions.

### Step-by-Step Solution:

1. **Understanding Free Radical Reactions**:
Free radical substitution reactions typically involve the generation of free radicals, which can occur under specific conditions such as heat, light, or the presence of peroxides. These radicals can then react with other molecules, replacing atoms or groups.

2. **Analyzing the Options**:
- **Option 1**: This option involves boiling, which can generate free radicals. For example, when chlorine (Cl2) is heated, it can dissociate into two chlorine radicals (Cl·), which can then substitute hydrogen atoms in an alkane (e.g., CH4), leading to the formation of chlorinated products. This is a classic example of a free radical substitution reaction.
- **Option 2**: This option describes a reaction involving AlCl3, which is an electrophilic catalyst in Friedel-Crafts alkylation. Here, the reaction is electrophilic, as the electrophile (alkyl group) attacks the aromatic ring, not involving free radicals.
- **Option 3**: This option describes a nucleophilic reaction where a nucleophile attacks a positively charged center. This is characteristic of nucleophilic substitution reactions, not free radical reactions.
- **Option 4**: Similar to option 3, this option describes a nucleophilic attack and is also not a free radical reaction.

3. **Conclusion**:
After analyzing all the options, we conclude that **Option 1** is the only free radical substitution reaction. The other options involve electrophilic or nucleophilic mechanisms.

### Final Answer:
**Option 1** is a free radical substitution reaction.