an alkane reacts with a halogen to form a halocarbon. what is the role of uv light in this reaction?

it provides energy for the reaction.

it is released by the reaction.

it catalyzes the reaction.

explain your answer

Explanation:

Alkanes are relative stable molecules: saturated chains of carbon and hydrogen atoms, where each carbon atom is hydridized sp³ and is bonded to one or two carbon atoms and 2 or 3 hydrogen atoms.

The C-H bonds are slightly polar (carbon is more electronegative than oxygen).

On the other hand, the diatomic halogen molecule (F₂, Cl₂, Br₂, or I₂) is a non-polar molecule.

The mechanism of reaction between the alkane and the halogen is via free radicals: the halogen molecule must be splitted into two radicals (two atoms each with a free electron). This implies the rupture of the diatomic halogen, and this requires an important amount of energy. The role of the UV ligth is to provide the energy to split the halogen molecule and, in this way, start the reaction.

Then, the free halogen radical may react with the alkane molecule breaking a C - H bond and substituting the leaving hydrogen atom, forming a C - X bond (X represents the halogen atom).

As a demonstration of this role, you may find in the specialized textbooks this typical reaction:

The  energy shown in the reactant side may be supplied as UV light or heat. So, the role of UV light is to provide energy to start the reaction.

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