Why does cyclohexene have a lower melting point than cylcohexane?

Question

Maya Allen · Accepted Answer

Here's my explanation.

Melting points depend, not only on the strength of intermolecular forces, but also on the ability of the molecules to pack into a crystalline array.

This, in turn, depends on their shape.

Cyclohexane has a high degree of symmetry, and this enables the molecules to pack nicely into a crystalline structure with strong London dispersion forces between them..

Cyclohexene exists in a half-chair conformation.

It has less symmetry than hexane, so it does not pack as well into a crystal structure.

Thus, cyclohexene has a much lower melting point than cyclohexane (-103.5 °C vs. 6.5 °C).

Addison Armstrong · Answer

undefined Cyclohexene has a lower melting point than cyclohexane because cyclohexene molecules are capable of forming intermolecular forces, specifically dipole-dipole interactions, due to the presence of the carbon-carbon double bond. These interactions are weaker than the London dispersion forces present in cyclohexane, which has only non-polar C-H bonds. As a result, less energy is required to overcome the intermolecular forces in cyclohexene, leading to a lower melting point compared to cyclohexane.