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Properties of Alkanes and Alkenes

 
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Low molecular weight alkanes and alkenes are gases at room temperatures and pressures; as the molecular weight increases, these hydrocarbons become liquids and then solids.

Where do the attractive forces between hydrocarbon molecules come from?

A molecule of a hydrocarbon is a very different thing from a lump of hydrocarbon visible to the naked eye. While the lump is exactly the same from one moment to another, the molecule is constantly changing: its electrons are in constant motion. This means that at any one time, there will be small regions of positive and negative charge on an uncharged molecule. Attraction of oppositely charged regions gives the cohesive force that makes some hydrocarbons solids at room temperature. The larger the "surface area" of the molecule, the larger this cohesive force will be.

A useful analogy is to think of stitching pieces of cloth together. Two stiches along a seam a few centimetres long will be easy to pull apart, while twenty stiches along a seam ten times as long will be much more difficult.

These intermolecular attractions are called "van der Waals forces", "London forces" or "dispersion forces". The melting point, boiling point, and density of a hydrocarbon are related to the strength of these forces.


The experimentally observed melting and boiling points of the alkanes. (Temperatures in kelvin)

The experimentally determined densities of the alkanes.

For alkenes and branched hydrocarbons, the geometry of the molecule can be more important than the number of carbons. This is because of the effect the precise arrangement of the atoms has on the surface area of the molecule and the ease with which they can pack together.

Geometry and physical properties: three pentanes
boiling
point
(°C)
melting
point
(°C)
density
(g/cm3)
  structure
n-pentane 36.1 -129.7 0.626
i-pentane 27.9 -159.9 0.620
neo-pentane 9.5 -16.5 0.614

The more branched a molecule is, the more "spherical" it is, and the smaller surface area it will have - since dispersion forces increase with surface area, the more branched molecules will have a lower boiling point. You may notice that the trend in the melting points is rather different - this is because the longer a branch is, the more potential it has to flop around and disrupt the regular crystalline lattice of frozen pentane, giving a liquid state.

Geometry and physical properties: three pentenes
boiling
point
(°C)
melting
point
(°C)
density
(g/cm3)
  structure
1-pentene 30.0 -138 0.641
cis-2-pentene 36.9 -151 0.656
trans-2-pentene 36.3 -136 0.648

The different isomers of alkenes have a lower effect on the molecular properties. These three pentenes have much the same boiling point. However, the melting point of the cis isomer is considerably lower, since the molecule is held in a "bent" shape and cannot pack into a crystal as easily as the trans isomer or the 1-pentene.

Although water is a very small molecule, comparable to methane or ethane, it has much higher boiling and melting points - this is an indication that forces other than dispersion forces are important in the intermolecular bonding of water.

If all these names are a little strange to you, then maybe you should have a look at our nomenclature trail (open in new window).