The halogen which is most reactive , in the halogenation of alkanes under sunlight is .........

Explanation

To determine which halogen is the most reactive in the halogenation of alkanes under sunlight, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Halogens**: The halogens we are considering are fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2).

2. **Understand the Mechanism**: The halogenation of alkanes occurs via a free radical mechanism. This process involves the homolytic fission of the halogen bond (X-X) in the presence of sunlight, which provides the necessary energy to break the bond.

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