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Physical Properties of Alkenes

1. Physical State. The first three members of alkenes are gases, the next fourteen are liquids while the higher members are waxy solids. Ethene is a colourless gas with faint sweet smell. Higher alkenes are colourless and odourless.

2. Melting and Boiling Points. The melting and boiling points of alkenes are also quite low and increase regularly with increase in molecular mass. For every -CH2 added, the boiling point increases by 20-30 K. Comparison of melting and boiling points of geometric isomers indicates that cis-isomer bas higher boiling point than the corresponding trans-isomer on the other hand the melting point of transisomer is generally higher.

 

Table 41.1. Physical Properties of Some Alkenes

For example, melting and boiling points of cis and trans isomers of but-2-ene are given below:

Melting point:                         134K              167 K

Boiling point:                          277 K              274 K

 

3. Solubility. Like alkanes, alkenes are also insoluble in water. They are soluble in non-polar solvents such as benzene, hexane, CC14, etc.

4. Density. Densities of alkenes show regular increase with increase in molecular mass. However, the maximum density of alkenes is about 0.8.Thus, all alkenes are lighter than water.