Cycloalkanes: Structural Variety
Cyclopropane exists in the form of a planar, equilateral triangle, a regular polygon with the C-C-C angles measuring 60° which is a clear deviation from a tetrahedral angle of 109.5°. Additionally, all hydrogen atoms are eclipsed resulting in greater instability and a more positive enthalpy than is observed for other cycloalkanes. Because of the bent bonds in cyclopropane, the orbitals assume an angle of 104° which allows for sufficient overlap to form a bond. This is the reason that such a deformed bond structure is even possible.
The C-C bonds in cyclopropane are relatively weak because of the ring strain. Ring opening requires an energy of only 272 as compared to 377 for cleavage of the C-C bond in ethane. The low energy required accounts for unusual reactions of cyclopropane.