Answer and explanation:
The relative rates of free radical halogenation is in the order of,
F₂ (10⁸) > Cl₂ (1) > Br₂ (10⁻¹¹) > I₂ (10⁻²²)
The above order also show a decreasing reactivity from left to right
Hence, reaction of fluorine with alkanes is highly reactive and it is too difficult to control.
The reaction with Cl₂ is moderately fast, while with Br₂ is slow and with I₂ is too slow reaches to equilibrium.
Hence in general we don't prefer radical fluorination of alkanes.
