Ring strain

In organic chemistry, ring strain is a kind of instability that happens when bonds in a molecule form angles that are different from their normal sizes because they are part of a ring. This is especially important for small rings like cyclopropanes and cyclobutanes, where the angles inside are much smaller than the usual angle of about 109°. Because of this extra strain, these small rings need more energy when they burn, which makes their heat of combustion higher.

Ring strain comes from several things working together: angle strain, conformational strain or Pitzer strain (which is about parts of the molecule bumping into each other), and transannular strain, also called van der Waals strain or Prelog strain. The easiest way to see angle strain is in small cycloalkanes like cyclopropane and cyclobutane.

The energy from ring strain is the extra energy needed to bend bonds and angles to close a ring. Trying to avoid or reduce ring strain can make chemical reactions happen faster or in a certain way.

History

Ring strain theory was first created by a German chemist named Adolf von Baeyer in 1890. Before this, people only thought there were two types of strain in molecules. Baeyer’s idea looked at how these two types of strain worked together.

At the same time, another scientist named Hermann Sachse suggested that rings in molecules aren’t flat and can be folded or twisted into different shapes. Later, Ernst Mohr mixed these ideas to explain why six-membered rings are stable and common in nature, and to understand the energy in other ring shapes.

Angle strain (Baeyer strain)

Alkanes

In molecules shaped like rings, the bonds often have to bend in ways that are not natural. The best shape for bonds in straight chains is about 109.5 degrees. When bonds are forced into tighter angles, the molecule becomes less stable and more reactive. This is called angle strain.

Angle strain makes molecules less stable. It can be measured by how much extra energy is released when the molecule burns. Small rings, like those with three or four carbon atoms, have the most angle strain because their bond angles are much smaller than the natural angle.

Angle strain in alkenes

In molecules with double bonds shaped like rings, the bonds can also be forced into tight angles. This makes these molecules more reactive. Some very small rings with double bonds cannot stay stable for long. For example, the smallest ring with a trans double bond that can be kept is one with eight carbon atoms. Smaller rings with double bonds exist only for very short times or as middle steps in reactions. Having more double bonds in a ring usually makes the ring strain worse.

Torsional strain (Pitzer strain)

In some molecules, there is extra strain called torsional strain that adds to ring strain. For example, in cyclopropane, the bonds between carbon atoms form angles of 60°, which is much smaller than the usual angle of 109.5° in other similar molecules. This small angle causes most of the ring strain. But, as we can see in the Newman projection of the molecule, the hydrogen atoms are also lined up in a way that adds a little more strain, called torsional strain.

Examples

In molecules called cycloalkanes, each carbon atom is connected to two other carbons and two hydrogen atoms. These carbons normally prefer bond angles of about 109.5°, but in rings this can be hard to achieve.

Small rings like cyclopropanes and cyclobutanes have bond angles much smaller than 109.5°. Because of this, these molecules are unstable and have higher energy. For example, cyclopropane has bond angles of just 60°, making it very reactive. Cyclobutane has angles close to 88°, which is still far from the ideal. Even cyclopentane and cyclohexane show some of these effects, though they are less strained.

Applications

Molecules with ring strain have extra energy that can help make chemical reactions happen. This is useful in making new materials and medicines. Some examples include special reactions that open up ring-shaped molecules.

Ring strain can also make explosives release more energy or react more easily to shocks. For instance, the explosive 1,3,3-Trinitroazetidine behaves this way partly because of its ring strain.