Why does cyclohexene have a lower melting point than cylcohexane?

1 Answer
Jul 6, 2016

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).