Strain (chemistry)

In chemistry, strain describes how chemical compounds can become more unstable based on their shape.

According to VSEPR theory, chemical bonds around an atom are most stable when they (and any lone pairs) are as far apart as possible. This means certain arrangements and angles give more stable, "unstrained" molecules.

Steric strain happens in molecules with large overlapping pieces. It is also called van der Waals strain, because it happens when atoms are closer than their van der Waals radius.

Ring strain happens in cyclic compounds. These compounds have rings where atoms are connected in a loop. Ring strain is complicated because there are many different interactions that add to it.

Cyclic alkanes are one of the simplest cases of ring strain. These chemicals have only two kinds of bonds, C−H and C−C, centred on carbon atoms with sp³ orbital hybridization. These kinds of bonds like to have angles of about 109.5°. The further the angles in the ring are from that value, the more strain there is. This is what makes cyclohexane have bent shapes instead of being a flat hexagon.

Cyclopentane is a good example of the complex nature of ring strain. While it would have very little angle strain if it were a flat pentagon, this would increase the steric strain from the hydrogen atoms around the ring. Instead of a flat pentagon, it takes a bent shape that reduces overall strain.