First, the answers to the questions on the previous page. The two interconvertible chair-conformations of cis-1,2-dimethylcyclohexane are mirror images of each other. They are, therefore, conformational enantiomers. Because they are readily interconvertible, they are not resolvable.
While looking at the pages about the ring-flip in trans-1,2-dimethylcyclohexane and the cis-1,2-dimethylcyclohexane, you might have noticed that the energy profiles of the two structures differ significantly. Just to refresh your memory, we show you the energy profiles again, this time with absolute energies instead of relative energies:
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| trans-1,2-dimethylcyclohexane | cis-1,2-dimethylcyclohexane |
The difference in energy maxima is not really important at this point; however, what is important is the energy of the four chair-conformations (the starting and ending structures, i.e.). The differences between these energies are indicative of the relative stabilities of the four chair-conformations.
A methyl group in an axial position results in crowding by means of what is called a 1,3-diaxial interaction. In both chair-conformations of cis-1,2-dimethylcyclohexane, there is one methyl group in an axial position and one in an equatorial position. Therefore, the energies of these conformations are the same.
In the chair-conformations of trans-1,2-dimethylcyclohexane, however, the methyl groups are either both axial, or both equatorial. This results in an energy difference of almost 10 kJ/mole for these two conformations. The diequatorial conformation has the lowest energy; it is even lower than in the cis structures. The diaxial conformation has four repulsive 1,3-diaxial interaction, which gives it the highest energy of the four structures discussed here.
The next page discusses stereochemistry in reactions of cyclic compounds.
