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
Unsymmetrical trans alkene
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.
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.