4.1 Cyclohexane; chair form and boat form
4.1 Cyclohexane; chair form and boat form
In the past, chemists believed that cycloalkanes, including cyclohexane, were planar, like benzene (see below). Since the ∠C-C-C bond angle of sp3 hybridized carbon chain is 109o28', cycloalkanes will suffer from some strain if these are to be planar. All cycloalkanes, except for cyclopentane, become unstable because of this strain (Baeyer's strain theory). According to this theory, the most stable cycloalkane is cyclopentane, and cyclohexane is the next.
Although this theory could explain the high reactivity of cyclopropane, it gradually turned out that cyclohexane could be more stable if the nonplanar structure was assumed. By taking the nonplanar structure, the ∠C-C-C bond angle of cyclohexane could maintain the tetrahedral angle. Because of the cyclic nature, the number of structures cyclohexane can take is much less as compared with its acyclic analog hexane. There is, however, flip-flop of the ring as the rotation about the C-C bond of cyclohexane takes place. The movement of the ring can most conveniently revealed by the torsion angle made by four successive carbon atoms (butane unit) arbitrarily chosen. If we choose C1~C4 (1) as the butane unit, the torsion angle can be defined as is shown in 2. You should confirm the discussion above by using a molecular model.
If the torsion angle-energy diagram for C1～C4"butane unit" of cyclohexane is essentially identical with that of butane (Fig. 2.4), the staggered form 3(which is referred to as "butane gauche form") should be more stable than the eclipsed form 4. If a given cyclohexane contains six butane-gauche units, it should be the most stable conformer in which all bond angles are tetrahedral.
The conformer of cyclohexane 5or 5’is named chair form,which is the most stable one. There is another conformer of cyclohexane 6 or 6’in which all bond angles are tetrahedral. This conformer is named boat form.
▶go to S4.1 Cyclohexane: chair form and boat form.
Now let us pay attention to twelve hydrogen atoms of cyclohexane. Of the two C-H bonds from each carbon atom, one is perpendicular to the pseudo-plane made by six carbon atoms, and the other parallel to it. If we admit that cyclohexane resembles the earth, the chain of six carbon atoms may be accepted as the equator. Based on this analogy, the C-H bond perpendicular to the equator (hence parallel to the axis of the earth) is named an axial bond, and that parallel to the equator to an equatorial bond. An atom or a group of atoms bonded to the these bonds are referred to an axial atom (group) or an equatorial atom (group), respectively.
there are two way of writing a cyclohexane in the perspective view. In one way, the C2-C3-C5-C6 plane is horizontal (as 5). This way is advantageous when comparison with a boat form is attempted. In the other way, the C2-C3-C5-C6 plane is slightly tilted so that the axial bonds are perpendicular (as 9). This method is convenient when the differentiation between axial and equatorial is important.