Question

why cyclopropane is more reactive than propane?​

Answer 1

The triangular structure of cyclopropanerequires the bond angles between carbon-carbon covalent bonds to be 60°. This is far less than the thermodynamically most stable angle of 109.5° (for bonds between atoms with sp3 hybridised orbitals) and leads to significant ring strain.

Answer 2

The greater reactivity of cyclopropane derives from what we call bond-strain. ... The angle between the bonds is 109,5 degrees. In cyclopropane the three carbon ring forms an equilateral triangle, and the resulting bond angle will be 60 degrees: the bonds are heavily compressed, resulting in a lot of bond strain.