Molecular Fragments: Cyclohexane Conformations

Site and models created by Dr. Dave Woodcock.
Associate Professor Emeritus, UBC (Kelowna).

(email)
Copyright 1996, 1997, 2008 Dave Woodcock

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I. Cyclohexane conformations

Cyclohexane has two major conformations:

Chair conformation

In this conformation, all the hydrogens (white) are staggered. You can see this by rotating the model to look down any one of the C-C bonds. This is the lowest energy conformation.

All the models below this section are shown in their chair conformations

Boat conformation

In this conformation, some of the hydrogens on neighbouring carbons are eclipsed (on top of each other). You can see this by rotating the model and looking down the C-C bonds making up the 'body' of the boat. Additionally, two hydrogens on the apex of the boat (called 'flagpole' hydrogens) are pointed towards each other. (Change to a space-filling model to see the interaction between these two hydrogens.). Both these cases lead to physical interactions which give this conformation a higher energy than the chair.

In fact, a slightly lower energy can be attained if the boat twists slightly, a conformation usually called the 'twist boat' conformation, shown below. See how the non-bonding conflict between hydrogens has been relieved a little.

II. Bromocyclohexane: axial and equatorial chair conformations


Equatorial In the equatorial position, the bromine atom is positioned away from the main ring structure and into the most open position. This is the lower energy conformation.	Axial Note that in the axial position, the bromine atom is not clearing the molecule and is in fact close to two other axial atoms, in this case, two hydrogens. This leads to a physical non-bonding interference (steric) which gives this conformation a higher energy that the equatorial.
Switch each to the space-filling models to see this feature more clearly.

III. t-Butylcyclohexane

t-Butylcyclohexane
In the equatorial position, the t-butyl group is positioned away from the main ring structure and into the most open position. The axial conformation of this molecule is so much higher in energy than the equatorial that the molecule is essentially locked with the t-butyl group equatorial.

IV. Two natural products: menthol and glucose


Menthol The three substituent groups (isopropyl, methyl and OH) are all in the most favourable equatorial position in this monoterpene with a typical odour.	beta-D-Glucopyranose Glucose exists mainly (~65%) in the cyclic oxacyclohexane (one oxygen replacing a carbon in the cyclohexane ring) form with all substituent groups in the favoured equatorial positions. The other 35% is a cyclic oxacyclohexane form with one of the OH groups in an axial position. This epimer is known as alpha-D-glucopyranose

V. 1,2,3,4,5,6-hexaisopropylcyclohexane (all trans)

1,2,3,4,5,6-Hexaisopropylcyclohexane (all trans)

In this case, contrary to the all equatorial conformation, which might be expected from the preceding arguments for bromo- and t-butylcyclohexanes, the molecule adopts the all axial configuration. Apparently, the shape of the isopropyl group is such that it fits more readily, and consequently with less steric interaction, when all the groups are axial. 'A deliciously counter-intuitive' example (M. Jones, Organic Chemistry, Norton, 1997)

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