Question

Sort the following compounds according to stability.

• A. Cyclopropane
• B. Cyclobutane
• C. Cyclohexane
• D. Cyclopentane

Answer 1

Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect.

The reason has to do with the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are about ${109.5}^{\circ }$ . In cyclopropane, they are ${60}^{\circ }$.

With the electron pairs this close together, there is a lot of repulsion between the bonding pairs joining the carbon atoms. That makes the bonds easier to break.

Similarly, cyclobutane and cyclopentane have bond angles ${90}^{\circ }$ and ${108}^{\circ }$ respectively. Hence, the order of stability is,

cyclohexane > cyclopentane > cyclobutane > cyclopropane

For cyclohexane,the most stable conformation of cyclohexane is the chair form. The C-C-C bond angles are very close to ${109.5}^{\circ }$, so it is almost free of angle strain. It is also a fully staggered conformation and so is free of torsional strain.
Therefore, cyclohexane is the most stable compound among these.