Cyclopropane

Cyclopropane

The three carbon atoms of cyclopropane are all coplanar.  This is the only cycloalkane which is planar, all others form non-planar structures upon minimization of their torsional, steric and angle strain.

1. Torsional Strain - All the H atoms are eclipsed.  
   
   Action

   Rotate the structure below so you are looking down a C-C bond and examine the eclipsed H's.
2. Steric Strain - Is minimal in this structure, since no large groups are interacting
3. Angle strain - The carbons in cyclopropane are sp³ hybrid and as such the C-C-C angles theoretically should be 109.5^o.  Because cyclopropane is forced into an equilateral triangle the angles are 60^o.  This results in reduced overlap or sp³ hybrid orbitals and very weak bonds.

Cyclopropane is much less stable than propane.