Cyclic alkanes

In a cyclic alkane, we remove one of the hydrogens in each of the terminating methyl groups of a linear alkane chain and then use the dangling $sp^3$ orbitals that result to form an additional $\sigma$ bond between the ends to form a cyclic molecule. The general formla is C$_{2n}$H$_{2n}$. Examples of cyclopropane, cyclobutane and cyclohexane are shown in figure below:

Figure: cyclopropane, cyclobutane, cyclohexane.

Although the bonding is based on the $sp^3$ hybrid orbitals as in the linear alkanes, the fact that the angle between the different $sp^3$ hybrids is 109.5$^{\circ}$ is somewhat at odds with the cyclic geometry of the cyclic alkanes. As a result, the ring possesses some amount of strain energy that needs to be minimized. This fact determines the stable conformations of cyclic alkanes, which are not generally planar like benzene. Consider, for example, cyclohexane. Two stable conformations of this molecule, known as the ``chair'' and ``boat'' conformations exist and are shown in the figure below (or as an animated gif in the html version of the lecture):

Figure: Chair and boat conformations of cyclohexane.

Such conformations exist because of the conformational flexibility of the molecule combined with the need to minimize strain energy. In the chair and boat conformations of cyclohexane, the chair is lower in energy. However, there is only a small barrier to convert from the chair to the boat, and at room temperature, the molecule undergoes an isomerization process, in which the chair and boat conformations interconvert (see animated gif in the html version of the lecture).