Ring Strain in Cycloalkanes
The normal bond angle for a tetrahedral is 109.5˚. When we deviate from this angle of 109.5˚, the molecule experiences tension, which we call ring strain or angle strain. Ring strain in cycloalkanes is an important foundational concept to understand.
If we look at a cyclopropane molecule, it is clear that the bond angles are ~60˚.
Because this is so far from the normal 109.5˚, the molecule feels a large amount of strain. Ring strain in cycloalkanes, also known as angle strain, is essentially pent-up energy in the molecule that can be immediately released if the ring is broken.
If we now look at cyclobutane, we see the bond angles are ~90˚. They aren’t exactly 90˚ because the molecule prefers to take a puckered comformation; however, it is clear that the bond angles are closer to 109.5˚ than cyclopropane. They being said, they still are fairly far away from the ideal tetrahedral number.
Cyclopentane is much closer to 109.5˚ than cyclopropane, but it still has some deviation from the perfect value of 109.5˚ as well. Note the bond angle is ~108° as the molecule changes configuration from the pentagon shape shown on the 2-D structure below.
If we look at a six-membered ring, it actually conforms in a way that is strain-free (see next section). This means that its bond angles are indeed the ideal bond angle of 109.5˚.
Just as we had ring strain for angles less that 109.5˚, we can have strain for larger molecules as well. Cycloheptane and cyclooctane are subject to this strain, for example.