Compare the orbital overlap in cyclopropane and alkane.

Explanation

As there is maximum overlap of orbitals along the bond axis, the covalent bond formation is strongest. In alkanes, there is perfect end on overlap of `sp^(3)` hybrid orbitals of C atoms. In cyclopropane, head on overlap of `sp^(3)` hybrid orbitals does not lead to ring closure because it requires bond angle of `109^(@)` , 28'. Therefore, in cyclopropane, overlap of `sp^(3)` hybrid orbitals is off the axis giving less effective bent ('banana') bonds.

In oprder to minimum the angle strain, the C atoms develop more p character in the orbitals forming the ring and consequently more s character in the external (C----H) bonds. Since bond angles obtained by overlapping p orbitals are `90^(@)` , additional p charactter, the hybrid orbitals for the external bonds are between `sp^(3)` and `sp^(2)` , leading to an increase in the bond angle. The more is the s character in the hybrid orbital used by C to form a bond, the shorter is the (C----H) bond, and hence the cyclopropane is more acidic than propoane.