Physical Properties of Alkenes

Alkenes are almost non-polar: C and H have very similar electronegativities, and the C=C bond is between two identical atoms. So the only forces between alkene molecules are weak van der Waals forces (induced dipole-dipole forces).Boiling point increases as the carbon chain gets longer. Bigger molecules have more electrons and a larger surface area, so the van der Waals forces between them are stronger and more energy is needed to separate the molecules:ethene –104 °C   propene –47 °C   but-1-ene –6 °C   pent-1-ene 30 °CSo ethene, propene and the butenes are gases at room temperature, and alkenes from C₅ upwards are liquids at first, then solids for long chains. These values are close to those of the alkanes with the same number of carbons, because both series have the same kind of intermolecular force.Branching lowers the boiling point. Branched molecules are more compact, so there is less surface contact between neighbouring molecules and the van der Waals forces are weaker.Alkenes are insoluble in water. Water molecules are held together by hydrogen bonds, and non-polar alkene molecules cannot form hydrogen bonds with water to replace them. Alkenes mix with other non-polar solvents instead, and liquid alkenes are less dense than water, so they form a separate layer on top.