Physical Properties of Alkenes

Physical State and Odour of Alkenes

The first three members of alkenes — ethene, propene and butene are colourless gases; the next fourteen members are liquids, and the higher ones are solids. Alkenes are colourless and odourless with an exception of ethene which is a colourless gas with pleasant smell.

Boiling Points of Alkenes

The boiling point increases as the size increases. In fact, the boiling point increases by 20-30K with every -CH2 group added (if we leave out first few members). Among isomers, the boiling point decreases with increase in branch. Amongst the alkenes that show geometrical isomerism, cis-alkenes have higher boiling points than the corresponding trans-isomers because cis-isomers being polar have stronger dipole-dipole interactions.

Dipole Moments of Alkenes

In symmetrical trans-alkenes, the dipole moments of individual groups is equal and opposite resulting in zero dipole moment. Unsymmetrical alkenes, however, have some net dipole moment because the individual groups are not equal.

Symmetrical trans alkene

Symmetrical trans alkane has zero dipole moment

Unsymmetrical trans alkene

Unsymmetrical trans alkene has finite dipole moment

In cis-alkenes, the two dipole moments are inclined at an angle of 60° and do not completely cancel out each other; therefore, both symmetrical and unsymmetrical cis alkenes have finite dipole moment.

cis Alkene

cis alkene has finite dipole moment

Solubility of Alkenes

Alkenes being either non-polar or weakly polar molecules are insoluble in water and other polar solvents but are fairly soluble in non-polar solvents such as benzene, CCl4, petroleum ether.

Density of Alkenes

All alkenes are lighter than water.