Skip to main content
Chemistry LibreTexts

11.3: Physical Properties of Alkenes

  • Page ID
    201192
  • Physical state

    Ethene, Propene, and Butene exists as colorless gases. Members of the 5 or more carbons such as Pentene, Hexene, and Heptene are liquid, and members of the 15 carbons or more are solids.

    Density

    Alkenes are lighter than water, therefore, are insoluble in water. Alkenes are only soluble in nonpolar solvent.

    Solubility

    Alkenes are virtually insoluble in water, but dissolve in organic solvents. The reasons for this are exactly the same as for the alkanes.

    Boiling Points

    The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.

    Boiling points of alkenes depends on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes gets stronger with increase in the size of the molecules.

    Compound Boiling points (oC)
    Ethene -104
    Propene -47
    Trans-2-Butene 0.9
    Cis-2-butene 3.7
    Trans 1,2-dichlorobutene 155
    Cis 1,2-dichlorobutene 152
    1-Pentene 30
    Trans-2-Pentene 36
    Cis-2-Pentene 37
    1-Heptene 115
    3-Octene 122
    3-Nonene 147
    5-Decene 170

    In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.

    Melting Points

    Melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers.

    Compound Melting Points (0C)
    Ethene -169
    Propene -185
    Butene -138
    1-Pentene -165
    Trans-2-Pentene -135
    Cis-2-Pentene -180
    1-Heptene -119
    3-Octene -101.9
    3-Nonene -81.4
    5-Decene -66.3

    Polarity

    Chemical structure and fuctional groups can affect the polarity of alkenes compounds. The sp2 carbon is much more electron-withdrawing than the sp3 hybridize orbitals, therefore, creates a weak dipole along the substituent weak alkenly carbon bond. The two individual dipoles together form a net molecular dipole. In trans-subsituted alkenes, the dipole cancel each other out. In cis-subsituted alkenes there is a net dipole, therefore contributing to higher boiling in cis-isomers than trans-isomers.

    dipole dipole (8).bmp

    dipole 3.bmp

    dipole 5 (1).bmp

    dipole 7 (1).bmp

    ContributorsEdit section

    • Trung Nguyen