General Characteristics of Unsaturated Hydrocarbons

Physical properties:-

i) the first three members are gases, the next fourteen members are liquids while alkenes with more than 17 carbon atoms in the molecule are solids at room temperature.

ii) Except ethane, all are odourless. Ethane has pleasant odour. All are colorless. All are colorless.

iii) Like alkenes, alkenes are insoluble in water but dissolve freely in organic solvents.

iv) The boiling points, melting points and specific gravities show a gradual increase with the increase of molecular mass in the homologous series. They are less volatile than alkances. Ie, their boiling and melting points are higher than corresponding alkenes. Cis-isomer has a higher boiling and melting point than trans-isomer due to higher polarity.

v) They are weakly polar. The values of dipole mment for alkenes are pretty close in the range of 0.35 to 0.40 D. However trans-isomers have zero dipole moment and are nonpolar.

vii) The double bond in alkenes consists one sigma and one i-bond. Each carbon atom of a double bond is sp²-hybridized. The sigma bond is formed from the overlap of two sp²-hybrid orbitals and pi-bond from overlap of unhybridized p-orbitals.