That cyclohexane is virtually strain-free. This must mean that not only is there no angle strain, but there is also no strain from eclipsing interactions either. A model of the chair conformation of cyclohexane including all the hydrogen atoms looks like this.

The view along two of the C–C bonds clearly shows that there are no eclipsing C–H bonds in the chair conformation of cyclohexane—in fact, all the bonds are fully staggered, giving the lowest energy possible. This is why cyclohexane is strain-free. Contrast this with the boat conformation. Now four pairs of C–H bonds are eclipsed, and there is a particularly bad inter action between the ‘flagstaff’ C–H bonds.

This explains why the boat conformation is much less important than the chair conformation. Even though both are free from angle strain, the eclipsing interactions in the boat conformation make it approximately 25 kJ mol−1 higher in energy than the chair conformation. In fact, as we shall see later, the boat conformation represents an energy maximum in cyclohex ane whilst the chair conformation is an energy minimum. Earlier we saw how the eclipsing interactions in planar cyclobutane and cyclopentane could be reduced by distortion of the ring. The same is true for the boat conformation of cyclohexane. The eclipsing interactions can be relieved slightly if the two ‘side’ C–C bonds twist relative to each other.

This twisting gives rise to a slightly different conformation of cyclohexane called the twist boat conformation, which, although not as low in energy as the chair form, is lower in energy (by 4 kJ mol⁻¹) than the boat form and is a local energy minimum, as we shall see later. Cyclohexane has two stable conformers, the chair and the twist-boat. The chair form is approximately 21 kJ mol⁻¹ lower in energy than the twist-boat form.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

Decalins

What happens with more than one substituent on the ring

Substituted cyclohexanes

The ring inversion (flipping) of cyclohexane