Figure 9
The compounds considered so far are linear compounds and thus rotation about carbon-carbon bond is easily possible. However, it is expected that this rotation will be hindered in cyclic compounds. Also, the hindrance will large in small rings compared to large rings. This brings to a new concept- Bayer’s ring strain.
In a alicyclic compounds, all the carbons are sp3 hybridized and thus the bond angle should be 109° ideally. But in a small planar ring, like that of cyclopropane it is not possible to achieve this bond angle. As such the actual bond angle in cyclopropane is 60° instead of 109°. This thus introduces a strain in the molecule known as ring strain. According to Bayer, this strain would increase as rings grow larger and smaller than cyclopentane, they should show increasing angular strain and increasing strain energy.
In reality, a different scenario emerges as it is observed that, the cyclopropane ring is highly strained, the ring strain decreases with ring size and reaches a minimum for cyclohexane and not cyclopropane. The ring strain then increases but not as rapidly as is expected by Bayer’s theory and reaches a maximum at cyclononane and then decreases again. As the number of ring carbons increase beyond 14, the ring strain remains roughly constant.
This apparent deviation between theory and observed fact could be explained by the erroneous assumption that the rings are planar. In 1890 Hermann Sachse argued that, cyclohexane exist as non-planar chair and boat conformations which could rapidly interconvert into one other. This was experimentally observed by O. Hassel and D. Barton, who using X-ray crystallography one half of the twelve bonds of cyclohexane in the chair conformation of cyclohexane were arranged parallel to 3-fold rotational axis (C3 axis) while the remaining half were close to the imaginary equator plane of the ring system. The former group of hydrogens are called axial hydrogen while the later are called equatorial hydrogens (Figure 10).
Figure 10