Bluffton University

Organic Chemistry

Molecular Models:
Conformation and Newman projections
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If cyclohexane were planar, there would be considerable angle strain as the angles in a planar hexagon are 120° (vs. 109.5°). However, when you use your model kit to build cyclohexane you will see that the ring is really rather "floppy." This floppiness allows both angle and torsional strain to be relieved, and cyclohexane has two main conformational types: boat (in which two opposite carbons are bend out-of-plane in the same direction) and chair (in which two opposite carbons are bent out-of-plane in opposite directions).

Boat cyclohexane has no angle strain, but there are several eclipsed interactions between neighboring hydrogens. The actual "boat" conformation is the so-called twist boat, in which the eclipsed interactions are relieved by twisting around some of the carbon-carbon bonds. Nevertheless, the boat conformation is still relatively high in energy because of the unavoidable flagpole interactions between the hydrogens on the insides of the "prow" and "stern." The Newman projection below emphasizes the eclipsed interactions along the "gunwales" of the boat.

boat cyclohexane The flagpole hydrogens are highlighted in red.
Boat cyclohexane Twist Boat cyclohexane

Chair cyclohexane has neither angle strain nor eclipsed interactions! In fact, it has zero strain energy. If you build a model of chair cyclohexane, you will notice that there are two types of hydrogens, depending on whether they point up or down with respect to the ring, or point along the ring "plane." These types are called, respectively, axial and equatorial, and are color-coded below. The Newman projection emphasizes that all hydrogens are staggered; if you examine your model you will see that this is true all the way around the ring.

chair cyclohexane
When you "ring-flip" from one chair form to the other, axial and equatorial hydrogens change places.
chair cyclohexane chair cyclohexane

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Copyright © 1998, 1999, 2000, 2003, 2007 by Daniel J. Berger. This work may be copied without limit if its use is to be for non-profit educational purposes. Such copies may be by any method, present or future. The author requests only that this statement accompany all such copies. All rights to publication for profit are retained by the author.