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Conformations of Cycloalkanes

Cyclopropane is a 3-carbon ring structure. The bond angles form an equilateral triangle with bond angles of 60 degrees. The bond angles between the carbon atoms would prefer to be 109.5 degrees but because of geometrical constraints of the 3 carbon ring, this is not possible. Thus, there is considerable angle strain in cyclopropane. There is no non-bonded interaction strain in cyclopropane because no atoms crash into one another. but there is significant torsional strain in cyclopropane because the bonds are eclipsed, causing electron repulsion. You can see the torsional strain if you draw Newman projections for any of the C-C bond in cyclopropane.

Cyclobutane is more stable than cyclopropane. Cyclobutane has considerable angle strain, but not as much as in cyclopropane. Unlike cyclopropane, which is flat, cyclobutane puckers to lessen somewhat (not eliminate, however) torsional strain. Puckering allows the bonds to remain only partially eclipsed. There is no evidence of non-bonded interaction strain for cyclobutane, as can be seen by inspecting the space-filling model.

Cyclopentane has little angle strain because the interior angles of a pentagon are 108 °. Like cyclobutane, cyclopentane is able to pucker, making the bonds only partially eclipsed, and thereby relieving some, but not all, of its torsional strain. There is no evidence of non-bonded interaction strain for cyclopentane, as can be seen by inspecting the space-filling model.

Chair Cyclohexane
This is cyclohexane in the chair conformation. This is the most stable conformation for cyclohexane. Chair cyclohexane has no angle strain, as the bond angles around carbon are the ideal 109.5°. Notice also how all the bonds are perfectly staggered, thereby eliminating torsional strain. As can be seen in the space filling model, there is no non-bonded interaction strain in unsubstituted chair cyclohexane. Non-bonded interactions strain is only introduced when an axial hydrogen atom is replaced by a larger atom or group.
Side (Above) and Top (Below) views of chair cyclohexane. Chair cyclohexane has two distinct types of hydrogen atoms, axial (red) and equitorial (purple). The axial positions point in a direction that is normal to the mean plane of the cyclohexane ring, while the equatorial positions are splayed out around the periphery of the ring. You need to be able to recognize the difference between axial and equatorial positions, and you need to be able to draw chair cyclohexane on a piece of paper.
Here are the two chair conformations of cyclohexane, with the hydrogen atoms colored to indicate the axial and equatorial positions. For the chair on the left, the axial hydrogens are drawn red, while the equatorial hydrogens are drawn purple. After the ring flips to the other chair, shown on the right, notice how the axial and equatorial hydrogens have exchanged.
Steric Strain (Also Called Non-bonded Interaction Strain) in Methyl Cyclohexane: 1,3 Diaxial Strain
Groups larger than hydrogen such as the methyl group shown here experience more non-bonded interaction strain (atom crunching) in the axial position (1,3-diaxial interactions) than in the equatorial position. As a result, the chair on the right with the methyl group equatorial predominates at equilibrium. The larger the group, the greater its preference for being equatorial.