Question
Why are alkenes more reactive than alkanes ?
Solution
Alkenes have a carbon-carbon double bond (>C = C<) in their molecules. The >C = C< bond is made up of one strong σ bond and one weak 
bond. The electrons of the 
bond are more exposed which increase reactivity. In other words, the presence of 
bond (formed by the sideways overlapping of atomic orbital) makes alkenes highly reactive chemically as compared to alkanes.
