cyclohexane conformations

Chemistry 7A

Relationships Between Conformations of Substituted Cyclohexanes

Summary of Relationships

Compound	Conformational Equilibrium*	Relationship Between Conformations	Chirality of Conformers**
cis-1,2	ae === ea	enantiomeric	Each is chiral, but not resolvable due to rapid ring flipping.
trans-1,2	aa === ee	diastereomeric	Both conformers are chiral and therefore resolvable.
cis-1,3	aa === ee	diastereomeric	Both conformers are achiral.
trans-1,3	ea === ae	identical	It is chiral and thus resolvable.
cis-1,4	ae === ea	identical	It is achiral.
trans-1,4	aa === ee	diastereomeric	Both conformers are achiral.

* "a" stands for axial and "e" stands for equatorial.

In those instances where the interconverting conformations are diastereomeric, the more stable of the two has been indicated by length of the double arrow.

**Resolvable refers to a conformer which exists as a pair of enantiomers which are isolable from each other.

Example Discussion: 1,2-dimethylcyclohexanes

The chirality and optical activity of substituted cyclohexanes can be predicted from planar ring drawings. This is true even though the ring is not actually planar but rather has chair conformations. For example, consider the 1,2-dimethyl-cyclohexanes.

A B C

Chiral: B, C Optically Active: B, C

Achiral: A Optically Inactive: A

Enantiomers: B & C Diastereomers: A & B, A & C

From the above planar representations, it can be seen that there are three different isolable stereoisomers corresponding to 1,2-dimethylcyclohexane. Looking at the correct conformations allows one to understand why planar drawings give accurate predictions. Build models and convince yourself that the following information is true.

Cis Diastereomer

The cis diastereomer consists of one pair of enantiomeric conformers. They are not isolable or resolvable since rapid ring flipping interconverts them. Thus, the cis isomer is optically inactive and exists as a non resolvable racemic mixture. There is only one distinct isolable substance from cis-1,2-dimethylcyclohexane.

Trans Diastereomer

The trans diasteriomer consists of two pairs of enantiomeric conformers or alternatively, four sets of diasteriomeric conformers. Two diasteriomeric pairs of conformers cannot be separated into two isomers since they are interconverted by rapid ring flipping. Each enantiomeric pair is isolable and resolvable since they are not interconverted by ring flipping. All conformations of each stereoisomer are chiral and therefore, each stereoisomer is chiral. As a result of all this, trans-1,2-dimethylcyclohexane exists as two optically active, isolable and resolvable substances. They have equal, but opposite specific rotations and thus, consist of an enantiomeric pair.