Substituted Cyclohexanes Axial and Equatorial Hydrogen Groups

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes also exist in nonplanar conformations. The small instabilities of these higher cycloalkanes appear to be caused primarily by torsional strain and repulsive dispersion forces between hydrogen atoms across rings, called transannular strain. The nonplanar conformations of these rings, however, are essentially free of angle strain. 

X-Ray crystallographic studies of cyclodecane reveal that the most stable conformation has carbon-carbon-carbon bond angles of 117°. This indicates some angle strain. The wide bond angles apparently allow the molecules to expand and thereby minimize unfavorable repulsions between hydrogen atoms across the ring. 

In 1994 J. F. Stoddart and co-workers, then at the University of Birmingham (England), achieved a remarkable synthesis of a catenane containing a linear array of five interlocked rings. Because the rings are interlocked in the same way as those of the Olympic symbol, they named the compound olympiadane. 

12 Substituted Cyclohexanes Axial and Equatorial Hydrogen Groups 

The six-membered ring is the most common ring found among nature s organic molecules. For this reason, we shall give it special attention. We have already seen that the chair conformation of cyclohexane is the most stable one and that it is the predominant conformation of the molecules in a sample of cyclohexane. 

---

SUBSTITUTED CYCLOHEXANES AXIAL AND EQUATORIAL HYDROGEN GROUPS... 

The case becomes interesting when one studies mono substituted cyclohexanes. For example, methyl cyclohexane exists in two forms, one in which the methyl group is axial and the other in which it is equatorial. The latter is more stable than the former by 1.6 K cals/mole and in an equilibrium mixture, it is present to the extent of 98%. This is because that when the CH3 group is axial, it is so close to the two axial hydrogens on the same side of the molecule, that the van der Waals, forces between them are repulsive ... 

If this principle is taken to its ultimate conclusion, there should be halo-cyclohexanes for which an E2 reaction is impossible. 2,6-Dimethyl-l-bromocy-clohexane (34) is such a case. To satisfy the relative stereochemistry of the two methyl groups (cis to each other) and the bromine anti to the two methyl groups), the bromine atom must be axial in one chair conformation with two axial methyl groups (34A), but equatorial in the other chair conformation that has two equatorial methyl groups (34B). Only conformation 34A has an axial bromine atom required for an E2 reaction, but both P-hydrogen atoms (Hg and Hb) are equatorial. No P-hydrogen atoms are trans, diaxial to an axial bromine, so there is no E2 reaction. When 34 is heated with ethanol KOH, there is no E2 reaction. The carbon bearing the bromide in 34 is very sterically hindered, so an Sn2 reaction is very unlikely certainly the substitution will be very slow. 

---