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The Dilemma of Cyclohexane

One early explanation given for the relative lack of ring strain in cyclopentane and cyclohexane invokes the geometry of sp3-hybridized carbons. The natural bond angle at sp3-hybridized carbons is 109.5 degrees. However, in order to accommodate the geometry of cycloalkanes these bond angles are forced into other angles, resulting in angle strain. For instance, the large amount of ring strain in cyclopropane can be explain by the large deviance of the required 60 degree bond angle from 109.5. Instead of forming direct head-on overlaps, the C-C σ bonds of cyclopropane are bent out of linearity, resulting in less stable interactions.

Figure %: Angle strain in cyclopropane

While this simple model of angle strain explains some of the trends in ring strain, it fails to address others. For instance, the 90 degree bond angles of cyclobutane are much closer to 109.5 than the 60 degree bond angles of cyclopropane, yet its ring strain is smaller by a mere 1 kcal/mol. Most important of all, however, is that this model predicts that cyclopentane, with bond angles of 108 degrees, should be the most stable of the cycloalkanes. This is not the case. In fact, cyclohexane is the most stable of the series with no ring strain. As we'll see in the next section, the dilemma of cyclohexane can be resolved using conformational analysis.

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