Question

What kind of reaction do alkenes normally undergo

Answer 1

alkenes normally go into dehydration reaction.
and due to this reaction the alkenes are converted into alkanes.

Answer 2

"alkenes undergo combustion [burning]in a similar way to alkanes. they also undergo many other chemical reaction that alkanes do -not double bond in alkenes makes them more reactive than alkanes .
for example: bromine​ can be added to the alkene double bond