Cycloalkanes: Structural Variety
Cyclohexane is one of the most important and and most frequently encountered structural units in organic chemistry. Substituted derivatives are found in nature everywhere. A comparison of calculated and experimental values of the enthalpy of combustion indicates that the cyclohexane ring is nearly devoid of angle and eclipsing strain because of the chair-like ring structure. A planar ring would require C-C-C angles of 120° and twelve eclipsed hydrogen atoms. In the chair conformation of cyclohexane, carbon atoms 1 and 4 are bent out of the plane in opposite directions. In this conformation, all hydrogen atoms are staggered and all C-C-C bond angles are nearly tetrahedral with a value of 111.4°. Other, less stabile conformations are the boat and the twist-boat conformation.
In the boat conformation, carbon atoms 1 and 4 are above the plane resulting in eight eclipsed hydrogens along the sides of the boat and two hydrogen atoms pointing inside. The steric interactions result in an energy difference of 27.2 between the less stable boat conformation and the chair form. If one of the ring C-C bonds in the boat conformation is twisted relative to an adjacent one, stabilization of the conformation occurs because interaction between the two inner hydrogen atoms is eliminated. The difference between twist-boat and chair conformation is now only 20.9 . The boat form cannot be isolated and only represents a higher-energy transition state between the two twist-boat conformations. The chair conformation being the main conformer is separated from the twist-boat conformation by an energy barrier of 45.2 . The two possible chair conformations exist in an equilibrium and by moving from one to the other, axial hydrogen atoms become equatorial and vice versa. This flipping of the conformations requires an activation energy of 45.2 , sufficiently small to allow for interconversion of the conformations at a rate of 100,000 times per second.
Substituents can be positioned equatorially or axially in regard to the plane of the molecule. The former position is energetically favored, because the substituent in this arrangement is the furthest apart from any other parts of the ring. On the other hand, an axial substituent is located very close to both hydrogen atoms on the same side of the ring and, additionally, is gauche to two C-C bonds. Because of the limited rotation of C-C bonds in cycloalkanes, stereochemistry plays an important role. Several substituents in the ring can give rise to cyclic stereoisomers.