Alkanes: Physical Properties

The alkanes can exist as gases, liquids, or solids at room temperature. The unbranched alkanes methane, ethane, propane, and butane are gases; pentane through hexadecane are liquids; the homologues larger than hexadecane are solids.

Branched alkanes normally exhibit lower boiling points than unbranched alkanes of the same carbon content. This occurs because of the greater van der Waals forces that exist between molecules of the unbranched alkanes. These forces can be dipole‐dipole, dipole‐induced dipole, or induced dipole‐induced dipole in nature. The unbranched alkanes have greater van der Waals forces of attraction because of their greater surface areas.

Solid alkanes are normally soft, with low melting points. These characteristics are due to strong repulsive forces generated between electrons on neighboring atoms, which are in close proximity in crystalline solids. The strong repulsive forces counterbalance the weak van der Waals forces of attraction.

Finally, alkanes are almost completely insoluble in water. For alkanes to dissolve in water, the van der Waals forces of attraction between alkane molecules and water molecules would have to be greater than the dipole‐dipole forces that exist between water molecules. This is not the case.