Organic Chemistry I For Dummies
Book image
Explore Book Buy On Amazon
The first step in drawing the most stable conformation of cyclohexane is to determine — based on whether the substituents are cis or trans to one another, and based on where they're located on the ring — what the choices of axial and equatorial positions are for the substituents. A handy way of determining the substitution alternatives is to use the Haworth projection, as shown here.

The Haworth projection.

The Haworth projection
This projection is easy to make — simply start at some position on the ring and alternate axial (a) and equatorial (e) from carbon to carbon, and from top to bottom. This projection will tell you what the two options for the two-chair conformations are.

For example, if you were asked to draw the most stable conformation of cis-1,3-dimethylcyclohexane, both substituents could be on the top of the ring or both could be on the bottom of the ring, as shown here. (Recall that cis means that both substituents are on the same side of the ring.) As the figure shows, to get the cis stereochemistry, either both of the substituents could be equatorial (both e) or both could be axial (both a).

The possible positions of cis substituents in positions 1 and 3 on cyclohexane.

Putting large groups in the equatorial position to eliminate 1,3-diaxial strain is energetically favorable.

Because large groups prefer to be equatorial, the most stable conformer for cis-1,3-dimethylcyclohexane is the diequatorial conformer, shown here. The diaxial conformer would be higher in energy.

The diequatorial conformation of cis-1,3-dimethylcyclohexane

If cyclohexane has two substituents and one has to be placed axial and one equatorial (as is the case in trans-1,2-disubstituted cyclohexanes), the lowest-energy conformation will be the one in which the bigger group goes in the equatorial position and the smaller group goes in the axial position.

About This Article

This article is from the book:

About the book author:

Arthur Winter, PhD, is the author of the popular Organic Chemistry Help! website and Organic Chemistry I For Dummies. His professional focus is on the chemistry of magneto-organic materials.

This article can be found in the category: