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Q.
The halogen which is most reactive in the halogenation of alkanes under sunlight is
Haloalkanes and Haloarenes
Solution:
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.