Ring strain is a combination of angle strain (deviation from the ideal tetrahedral angle) and torsional strain (eclipsing groups in ring form) that most rings will have due to forming cyclic structures.
In alkanes (hydrocarbons with single bonds), the carbon bonds adopt a tetrahedral shape, based on sp3hybridization, in which the angle between the bonds is 109.5°. Because the bond angle in cyclic structures often cannot be the optimal 109.5°, cyclic structures often have ring strain due to their angle strain. Ring strain is a combination of angle and torsional strain that determines a molecule's stability. Angle strain is molecular instability resulting from carbon atoms bonded in alkanes at angles other than 109.5°. The bond angles in small cycloalkanes (cyclopropane, cyclobutane, and cyclopentane) deviate from the ideal angle, resulting in angle strain. In some cases, atoms are eclipsed, resulting in torsional strain, which is a resistance to bond rotation due to electronic repulsion that results from nonbonded atoms being forced close to each other. Together, these two effects are called ring strain.
Models Showing 3-D Shape of Cycloalkanes and the Plane of the Carbon Atoms
Cycloalkanes, with the exception of cyclopropane, are nonplanar. Cyclopropane has a large angle strain because the carbon-carbon bonds have an angle of 60°, which is far from the ideal angle of 109.5°. Since rotation around the carbon-carbon bonds is impossible, cyclopropane has torsional strain because of eclipsed hydrogen in the cyclic structure.
Ring Strain in Cyclopropane
Cyclobutane has a large angle strain, but not as large as cyclopropane. Cyclobutane looks like it should have an angle of 90° (like a square), but it actually has an angle of 88°, which is far from the ideal angle of 109.5°. Cyclobutane adopts a nonplanar "puckered" conformation (bond angle of 88°) to reduce the torsional strain that is characteristic of planar conformation (bond angle of 90°). This form has slightly more angle strain because the angle is 88° instead of 90° but has no torsional strain. Cyclobutane is more stable than cyclopropane.
Cyclopentane has a small angle strain because it has an angle of 108°, which is close to the ideal angle of 109.5°. Cyclopentane adopts a conformation with slightly more angle strain to avoid the torsional strain from the eclipsed hydrogen atoms. In the conformation with the ideal angle of 109.5°, there would be torsional strain. So by adding a small amount of angle strain by adopting a conformation of 108°, cyclopentane avoids all torsional strain.
Cyclohexane is the most common cyclic ring and has two main forms, the chair and boat. The chair conformation is a stable, nonplanar conformation seen in cyclohexane rings. Cyclohexane, with bond angles of 111°, is nearly free of angle strain and exists in a nonplanar chair conformation, which is virtually free of torsional strain because all of its bonds are staggered. The chair form of cyclohexane looks like a chair, with four carbons in the center forming the seat and a carbon on each side representing the back of the chair and the foot of the chair. This conformation is most easily visualized in three dimensions from a side perspective, and a Newman projection also clearly shows the staggered conformation.
Newman Projection and Ball-and-Stick Model of Staggered Bonds in Cyclohexane Chair
Cyclohexane can also exist in a less stable nonplanar conformation called a boat conformation, in which the bond angles are almost free of angle strain but torsional strain exists between the eclipsed bonds on four carbons. The boat conformation is a less stable, nonplanar conformation seen in cyclohexane rings. The boat form of the chair looks like a boat, with the with four carbons in the center forming the center of the boat and the two carbon on each side representing the bow and stern of the boat. Additionally, two hydrogens atop (flagpole interactions) the molecule experience van der Waals forces, which can be reduced by rotation around the carbon-carbon bond, resulting in a more stable "twist boat" conformation.
Most cyclohexane molecules exist in the more stable chair conformation. In fact, at room temperature, over 99% of cyclohexane molecules will be in the more stable chair conformation.