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Conformation and Stability of Cycloalkanes

According to Baeyer strain theory, the stability of a cycloalkane is based on how close its angles are to 109.5o. The closer the angle is to 109.5o, the more stable the cycloalkane. Deviation from this angle can cause angle strain. An increase in the angle strain means a decrease in the stability of the molecule.

Conformations of Cyclohexane

Cyclohexane has a non-planar structure that makes it almost free from ring strain. The most important conformations that it can have include chair conformation and boat conformation. The chair conformation is more stable than the boat conformation. The boat conformation can sometimes be more stable than it usually is, by a slight rotation in the C-C bonds and is called the skew boat conformation. Nevertheless, the chair conformation is the most stable cyclohexane form.

image\Ch 15 fig 15-5a.png

image\Ch 15 fig 15-5b.png

In the chair form of cyclohexane, there are two different kinds of carbon-hydrogen bonds. Of the twelve carbon hydrogen bonds in a cyclohexane, six bonds are pointed up or down and are called axial bonds. The remaining six carbon-hydrogen bonds are pointed at an angle out of the ring and are called equatorial bonds. Each carbon atom in the ring is attached to one hydrogen atom by an axial bond, and to the other hydrogen atom by an equatorial bond.

image\Ch 15 sec I, g1.png

When ring flipping occurs between conformers, equatorial groups become axial, and axial groups become equatorial.
Experimental analysis has confirmed that among the two chair conformations of methylcyclohexane, 95% of the molecules have their methyl group in the equatorial position. In other words, the methyl group being in the equatorial position is more stable than methyl group in the axial position. The lower stability of the axial position can be attributed to the fact that there is an increased steric hindrance because of the proximity of the axial methyl group to the axial hydrogens that are attached to carbon atoms 3 and 5. This interaction called 1,3-diaxial interaction results in steric strain. This accounts for the increased relative stability of the conformation when the methyl group is in the equatorial position.

image\Ch 15 sec I, g2.png


The cis-geometric isomer of 1,3-dimethylclohexane is more stable than its trans-isomer. Why?


To understand why the cis-geometric is more stable, let's draw the possible chair forms of 1,3-dimethylclohexane.

image\Ch 15 Example 15-1, g.png

When the two methyl substituents are in 1,3 positions, the cis-isomer can have both substituents in equatorial position. In the trans-isomer, one methyl group must always be axial.

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