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Assigning Priorities


Determining Stereochemical Relationships


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Three-Dimensional Representations: Chair Conformations

The groups on a Chair are oriented differently from those in the other cyclic conformations. On each carbon, one group is axial (straight up or straight down) and the other group is equatorial (at an angle, up or down). This is shown pictorially below. The structure on the left highlights the axial groups. The groups in pink are axial up, and the groups in red are axial down. The structure on the right highlights the equatorial groups. The groups in pale blue (teal) are equatorial up, and the groups in blue are equatorial down.


It should be noted that the up and down orientations for axial and equatorial groups on the same carbon alternate. Consider the molecules above and focus on the carbon bearing the bromine. The bromine is axial, as it is oriented straight up from the Chair. The remaining hydrogen must therefore be, and is, equatorial. Since the bromine is axial up, the hydrogen must be equatorial down. Study the above molecules to make yourself comfortable with this alternating relationship.

Furthermore, it should be noted that the up and down orientations of the axial groups alternate when going around the ring. This also holds true for the equatorial groups. Again consider the molecules above and focus on the carbon bearing the bromine. Since the bromine is axial up, the axial position on the carbons on either side of the bromine must be down. Since the hydrogen on the carbon bearing the bromine is equatorial down, the equatorial positions on either side of this hydrogen must be up. Study the above molecules to make yourself comfortable with this alternating relationship.

It is important to note that two groups that are both oriented up or both oriented down are cis to one another. Two groups that are oriented one up and one down are trans to one another. The concepts of cis and trans have no direct relationship to the concepts of axial and equatorial.

Consider again the above molecules. Indicate whether each of the main (non-hydrogen) groups equatorial or axial. What is the cis/trans relationship between the groups on the ring?

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