The Relative Stabilities of Cycloalkanes Ring Strain

Cycloalkanes do not all have the same relative stability. Experiments have shown that cyclohexane is the most stable cycloalkane and that, in comparison, cyclopropane and cyclobutane are much less stable. This difference in relative stability is due to ring strain, which comprises angle strain and torsional strain. 

The carbon atoms of alkanes are sp hybridized. The normal tetrahedral bond angle of an -hybridized atom is 109.5°. In cyclopropane (a molecule with the shape of a regular triangle), the internal angles must be 60° and therefore they must depart from this ideal value by a very large amount— by 49.5°  

Angle strain exists in a cyclopropane ring because the sp orbitals of the carbon atoms cannot overlap as effectively (Fig. 4.9a) as they do in alkanes (where perfect end-on overlap is possible). The carbon-carbon bonds of cyclopropane are often described as being bent. Orbital overlap is less effective. (The orbitals used for these bonds are not purely sp they contain more p character.) The carbon-carbon bonds of cyclopropane are weaker, and as a result the molecule has greater potential energy. 

While angle strain accounts for most of the ring strain in cyclopropane, it does not account for it all. Because the ring is (of necessity) planar, the C—H bonds of the ring are aU eclipsed (Figs. 4.9b,c), and the molecule has torsional strain from repulsive dispersion forces as well. 

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