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  Substituted Cyclohexanes Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Substituted Cyclohexane Compounds

Since it is so ordinary among synthetic and natural compounds, and due to its conformational characteristics are rather well understood, we shall get focus on the six-membered cyclohexane ring in further discussion. The two similar chair conformers are present in equal concentration in a sample of cyclohexane, and the hydrogens are all equal (50% equatorial & 50% axial) because of fast interconversions of the conformers. The two chair conformers are not similar when the cyclohexane ring bears a substituent. The substituent is axial in one conformer; it is equatorial in the other. Because of steric hindrance in the axial location, substituent groups prefer to be equatorial and that chair conformer predominates in the equilibrium.
Cycloalkanes having two or more substituents on different ring carbon atoms exist as a pair (sometimes more) of Configurational stereoisomers as we noted earlier. Now we must check the way in which favorable ring conformations affects the properties of the Configurational isomers. Keep in mind, Configurational stereoisomers are do not easily interconvert and stable, where, conformational isomers usually interconvert fastly. It is useful to follow two principles in examining possible structures for substituted cyclohexanes.

(i)   Than the other possibilities Chair conformations are usually more stable.
(ii)   Substituents on chair conformers prefer to occupy equatorial positions because of the increased steric hindrance of axial locations.

The following formulas and equations illustrate how the presence of two or more substituents on a cyclohexane ring perturbs the interconversion of the two chair conformers in ways that may be predicted.

Conformational Structures of Disubstituted Cyclohexanes

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In the case of 1, 1-disubstituted cyclohexanes, one of the substituents should essentially be axial and the other equatorial, in spite of which chair conformer is considered. Because the substituents are similar in 1,1-dimethylcyclohexane, the two conformers are similar and present in equal concentration. In 1-t-butyl-1-methylcyclohexane the t-butyl group is greater than the methyl, and that chair conformer in which the larger group is equatorial will be favored in the equilibrium (> 99%). Subsequently, the methyl group in this compound is approximately entirely axial in its orientation.
The analysis is a bit more complex in the cases of 1, 2-, 1, 3- and 1, 4-disubstituted compounds. All the time it is possible to have both groups equatorial, but whether this requires a trans-relationship or cis-relationship is depends on the relative location of the substituents. As we count around the ring from carbon no.1 to no.6, the uppermost bond on each carbon changes its orientation from axial (or equatorial) to equatorial (or axial) and back. It is significant to keep in mind that the bonds on a given side of a chair ring-conformation always alternate in this fashion. So, it should be understandable that for cis-1,2-disubstitution, one of the substituents must be equatorial and the other axial; in the trans-isomer both may be equatorial. Due to the alternating nature of axial bonds and equatorial, the opposite relationship is right for 1,3-disubstitution (trans is equatorial/axial, cis is all equatorial). At last, 1,4-disubstitution reverts to the 1,2-pattern.

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