Ring Strain in Cycloalkanes

Ring Strain occurs because the carbons in cycloalkanes are sp³ hybridized, which means that they do not have the expected ideal bond angle of 109.5^o ; this causes an increase in the potential energy because of the desire for the carbons to be at an ideal 109.5^o. An example of ring strain can be seen in the diagram of cyclopropane below in which the bond angle is 60^o between the carbons.

The reason for ring strain can be seen through the tetrahedral carbon model. The C-C-C bond angles in cyclopropane (diagram above) (60^o) and cyclobutane (90^o) are much different than the ideal bond angle of 109.5^o. This bond angle causes cyclopropane and cyclobutane to have a high ring strain. However, molecules, such as cyclohexane and cyclopentane, would have a much lower ring strain because the bond angle between the carbons is much closer to 109.5^o.

Below are some examples of cycloalkanes. Ring strain can be seen more prevalently in the cyclopropane and cyclobutane models.

Below is a chart of cycloalkanes and their respective heats of combustion ( ΔH_comb). The ΔH_comb value increases as the number of carbons in the cycloalkane increases (higher membered ring), and the ΔH_comb/CH₂ ratio decreases. The increase in ΔH_comb can be attributed to the greater amount of London Dispersion forces. However, the decrease in ΔH_comb/CH₂can be attributed to a decrease in the ring strain.

Certain cycloalkanes, such as cyclohexane, deal with ring strain by forming conformers. A conformer is a stereoisomer in which molecules of the same connectivity and formula exist as different isomers, in this case, to reduce ring strain. The ring strain is reduced in conformers due to the rotations around the sigma bonds. More about cyclohe