The Alkene Homologous Series:
Structures and Isomers
An Alkene is a Hydrocarbon which means that it is a molecule made up entirely of Carbon and Hydrogen atoms. The Alkene series is similar to the Alkane series except that each carbon chain has one, and only one, double bond in it. Since the element Carbon bonds four ways and the element Hydrogen bonds only one way we can easily construct the members of this series.
It is important to note that due to the properties of Carbon and Hydrogen, the double bond can ONLY be between two carbon atoms. Hydrogen is incapable of forming anything other than one single bond.
Alkenes are named in the same way as the alkanes except the “-ane” is replaced with “-ene” at the end.
Ethene: The Simplest Alkene
Since membership of this series requires one double bond somewhere in the Carbon chain, it is impossible to have a molecule with just one Carbon atom. Therefore the first member of the series is Ethene, which has the chemical formula C2H4. This is also known as Ethylene. The double bond is located between the two Carbon atoms, which leaves two unbonded electrons on each Carbon atom. Each of these unbonded electrons forms a single bond with one Hydrogen atom to give the final structure.
The diagrams below use the electron dot diagram for each of Carbon and Hydrogen. For convenience all structures are drawn flat, but they of course are really 3 dimensional and follow the vsepr rules for molecule structure.
Note that there is only this one possible location of the double bond in this molecule, and that no matter which way up or around you rotate it, the structure looks the same. This means that there is only this one structural isomer for Ethane. For an exploration of structural isomers, see the page What is Octane?.
The Importance of Ethene
Ethene is a vital component of the plastics industry and is also used to mass produce commercial ethanol, detergents and a large range of other compounds. It is produced on a massive scale around the world and is manufactured from crude oil. Ethene from this source is essentially a limited resource.
Propene is the next compound in the series. It has 3 Carbon atoms in its Carbon skeleton, and two of these share a double bond. As we can see from the diagram below, the Propene molecule has three Carbon atoms and 6 Hydrogens fill up the unused bonding sites on the Carbon atoms.
The Carbon to Carbon double bond is between the first and second Carbon atoms. This is true no matter which way you arrange the Carbon atoms and therefore there is only one isomer of Propene, the one shown above. The formula for Propene is thus C3H6. We can now see that Alkene molecules have the general formula CnH2n where n is the number of Carbon atoms in the molecule.
Next comes Butene, with a skeleton of four Carbon atoms making up the backbone of the molecule. The chemical formula for Butene is C3H6, following the pattern for all the molecules in this series. Four Carbon atoms allows for two different possible structures for Butene. The double bond can be either between the first and second Carbons, or between the second and third. These two isomers, or variants, are shown in the diagram below.
Naming the Butene Isomers
Because of the different structures possible in Butene, we need a system of naming them so that they can be distinguished from one another. The standard method is to find the end of the Carbon chain the double bond is closest to. The Carbon at that end is labeled 1, then the next is 2 and so on until the end of the chain is reached. In the case of the right most isomer, the C=C bond is in the middle of the molecule so we can start the numbering from either end.
We then identify the variant of Butene by looking at the Carbon atom where the double bond starts. In the above diagram the isomer on the left has the double bond starting at the first Carbon. This is given the name But-1-ene. The second form has the double bond starting at the second Carbon and so this is called But-2-ene.
There are other versions of the naming system in use, but the only differences are the placement of the 1 or 2; the labeling of the carbon chains is the same in all systems.
There is no But-3-ene
If we draw Butene with the double bond between the third and fourth Carbons, this is exactly the same structure as But-1-ene. Since the numbering of the Carbons goes from the end closest to the double bond the name But-3-ene is incorrect.
The rules for drawing the structures of Pentene are exactly the same as for all the other Alkenes. Pentene, like its Alkane cousin, has five Carbon atoms making up the skeleton of the molecule. It quickly becomes apparent that Pentene, like Butene, has only two structural isomers which are called Pent-1-ene and Pent-2-ene, as shown in the diagram below. This diagram assumes all the steps taken in the previous diagrams.
The last example of the series is the 6 Carbon molecule Hexene which has the formula C6H12. With 6 Carbon atoms, there are three possible locations for the double bond. The three possible isomers of Hexene are Hex-1-ene, Hex-2-ene and Hex-3-ene as shown below. The names Hex-4-ene, Hex-5-ene or Hex-6-ene are incorrect for the same reasons given in the Butene examples given above.
And So On…
The alkene series continues on from there. As the molecules get larger, there are more variations in the form available. If we take into account possible branching of the Carbon skeleton, the number of possibilities increases far more quickly. Again, the example on the what is Octane? page demonstrates this clearly.
Triple and Quadruple Carbon bonds?
It appears from the behavior of Carbon that it would also logically be able to form triple and quadruple bonds. The former is certainly true, and the series of Hydrocarbons that contain a triple bond rather than a double bond is called the Alkyne series.
Quadruple bonds however do not exist since there is not enough room for all four pairs of electrons to occupy the same region of space between the two atoms involved.
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