What are the trends in boiling points among alkenes?

The boiling points of alkenes increase with increasing molecular size and decrease with increasing branching.

In more detail, the boiling point of alkenes is primarily determined by the size of the molecule and the degree of branching. Alkenes are hydrocarbons, compounds made up of hydrogen and carbon atoms. They are characterised by the presence of a carbon-carbon double bond. The larger the molecule, the more points of contact it can have with neighbouring molecules, and the stronger the intermolecular forces of attraction, specifically the van der Waals forces. This means more energy is required to overcome these forces and convert the substance from a liquid to a gas, resulting in a higher boiling point.

However, the degree of branching in the molecule also plays a significant role. Branching refers to the presence of alkyl groups attached to the main carbon chain. The more branched the molecule, the less surface area it has in contact with neighbouring molecules. This reduces the strength of the van der Waals forces and therefore lowers the boiling point.

For example, compare but-1-ene (CH2=CH-CH2-CH3) and 2-methylpropene (CH3-C(CH3)=CH2). Both have the same molecular formula (C4H8) but different structures. But-1-ene is a straight chain alkene with a boiling point of -6.3°C, while 2-methylpropene is a branched alkene with a boiling point of -6.7°C. Despite the small difference, it clearly shows that the more branched the molecule, the lower the boiling point.

In summary, the boiling point of alkenes is influenced by both the size of the molecule and the degree of branching. Larger, less branched alkenes have higher boiling points, while smaller, more branched alkenes have lower boiling points.