write a equation for the reaction of bromine with an alkane?

Bromine will react with an alkane (any you like) to produce a halogenoalkane.

E.g.

C2H6 + 0.5Br2 ==> C2H5Br

Ethane + Bromine ==> Bromoethane

You could make it more complex by having a secondary or tertiary haloalkane (the above is an example of a primary haloalkane) - but this is the simplest example.

The subject is ORGANIC (Carbon) Chemistry, not inorganic.