Question

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

• A. fluorine
• B. chlorine
• C. bromine
• D. iodine

Answer 1

The correct option is A fluorine

Fluorination ($155$ kJ/mol) seems to have relatively high activation energy. The initial reaction (chain initiation) - that is, the homolytic cleavage of a halogen molecule - must, however, occur only a few times.

The subsequent reactions (chain propagation) between a halogen radical and methane, and then between a methyl radical and a halogen molecule, yield another halogen radical.

Therefore, one start reaction may initiate thousands of fluorination reactions. In addition, fluorination is very exothermic, the reaction enthalpy is $-431$ kJ/mol. As a result, the reaction itself provides enough energy for additional initiation reactions.

Hence, the halogenation of alkanes is violent and very fast in Fluorine.

So correct option is B