How do the boiling and melting points of alkenes change as the chain length increases?

As the chain length of alkenes increases, the boiling and melting points generally increase. This trend can be attributed to the fact that longer hydrocarbon chains have greater surface area, which enhances the London dispersion forces (a type of van der Waals force) acting between the molecules. As the chain length increases, there are more electrons available, leading to stronger temporary dipoles and thus stronger intermolecular attractions. This increased strength in intermolecular forces necessitates more energy to overcome them, resulting in higher boiling and melting points.

In contrast, shorter chain alkenes typically have weaker van der Waals forces due to their smaller surface area, leading to lower boiling and melting points. The presence of double bonds in alkenes does influence their physical properties, but the primary factor for boiling and melting point trends as chain length increases is the increased molecular weight and the enhanced intermolecular forces associated with larger molecules.