4.2 Ring inversion of cyclohexane
4.2 Ring inversion of cyclohexane
The relation between a chair cyclohexane and a boat cyclohexane can most conveniently examined with the aid of a molecular model (Fig. 4.1). If you lift up the leg of 11 (i.e., C1) gently, you will obtain a boat cyclohexane 12 (process i). You will notice that the rotation about C-C bonds takes place at the same time. This chair-boat interconversion is referred to as the ring inversion of cyclohexane. This phenomenon is a type of chemical exchange.
Figure 4.1 Inversion of a cyclohexane ring
In addition to process (i) (11→12), there is another process (v) with equal probability to lead the other boat form (11→14). The boat form 12 can be reconverted to 11 by pulling down the bow (or the stern) (C1) (process viii)or converted to another chair 13 by pulling down the stern (or the bow) (C4) (process ii). The second chair 13 can be converted to the boat 14 by pulling down the support C1 (process iii) or to the boat 12 by lifting up the leg C4 (process vii). The boat form 14 can similarly be converted to two chairs 11 or 13 (processes iv and vi). Since the energy barrier of the boat ・ chair interconversion is low, the process 
In the case of cyclohexane, the two chair forms 11 and 13 obtained by the ring inversion were proved identical as far as the shape of carbon skeleton is concerned. Can we say, however, that these two are strictly identical? Our belief will waver if we make a chair form with six axial hydrogen atoms 15. By inversion you notice that the second chair form 16 has six equatorial hydrogen atoms rather than six axial hydrogen atoms. Similarly, a chair form with six equatorial hydrogen atoms 17 is converted to the second chair form with six axial hydrogen atoms 18.
Figure 4.2 Axial-equatorial exchange by ring inversion
A chair form of cyclohexane is not converted to the exactly identical chair form. If we neglect hydrogen atoms, 11 and 13 are identical. However, 15 or 17 are not identical with 16 or 18 if hydrogen atoms are taken into consideration. If we can differentiate axial hydrogen atoms from equatorial hydrogen atoms, cyclohexane 19 is not identical with cyclohexane 20 which is formed from 19 by ring inversion. This fact will more clearly demonstrate when we learn substituted cyclohexanes.
