Organic Chemistry - Alkenes

This is a family of hydrocarbons which is related to alkanes (as you can tell from the name), but with one difference. Two of the atoms in each molecule of an alkene have a double bond between them. This is the same as a single covalent bond, except that each carbon atom contributes two electrons, resulting in four electrons in total passing between them. A double bond is represented by two lines between the atoms.

The simplest alkene is ethene which has two carbon atoms. Note that each carbon atom is joined to two hydrogrens (not 3 as in ethane) and that each carbon still has four bonds in total (counting the double bond as two).

The formula for ethene is C₂H₄. In fact, the general formula for any alkene is C_nH_2n, where n is any whole number from 2 upwards. Here are the next two members of the family:

You won't be surprised to hear that alkenes burn just as well as their alkane cousins. Again, they will produce carbon dioxide and steam if there is plenty of oxygen available, or carbon monoxide plus steam if the oxygen supply is limited:

However, alkenes also react in other ways, differently from alkanes. The two most important reactions they have are addition and polymerisation

Addition reactions

Both addition and polymerisation reactions occur because the double bond behaves differently from a single-bond. In a single bond, the two electrons (one from each carbon atom) bind the two carbon atoms together strongly. This makes the single bond relatively difficult to break. In a double bond, however, while two electrons bind the carbon atoms strongly together, the other two electrons (again, one from each atom) form a relatively weak bond. Effectively, this means that a double bond consists of two bonds, one of which is strong, the other of which is weak.

The upshot of this is that a double bond tends to open out, to form a strong single bond and two "half bonds" (unpaired electrons), one on each of the carbon atoms. This doesn't happen at random, of course, but only when there are other atoms in the neighbourhood for the half bonds to join with. This process is illustrated below:

This happens in the presence of small molecules such as water, or halogens or hydro-halogens (such as hydrogen chloride or hydrogren bromide). These smaller molecules also split up and one half joins to each flapping half-bond on the alkene. In each of the following reactions, see if you can spot where the two halves of the small molecule have gone.

(In that last one I should point out that water generally only adds to an alkene in the presence of a sulphuric acid catalyst. Also, note that we usually write the formula of butanol as C₄H₉OH rather than C₄H₁₀O in order to give some idea of its structure to the reader. There is another compound which has the same number and types of atoms as butanol but arranged differently - butanone - and just writing C₄H₁₀O might be confusing).

Polymerisation reactions

Since one of the parts of a double bond is particularly weak, it doesn't take much for the double bonds to open on a large number of alkene molecules at the same time. If there are no small molecules to add to, these bonds can either close again (which sometimes happens) or join to the open bonds on other alkene molecules.

In the diagram below, I have shown various molecules of a variation on ethene called chloro-ethene, in which one of the hydrogen atoms has been replaced by a chlorine atom.

Next, we see those same molecules with the double bonds opened out.

And lastly, the molecules join together to form a very large one. I have left the end of the molecule undefined (as dots) to show that it continues indefinitely in both directions. Obviously the molecule has to come to an end at some point, and you might ask what happens at the very end. The frank answer is, I don't know! Perhaps some stray hydrogen molecule obliges by joining on. Perhaps the ends of the molecule might join to each other, forming a large loop, in which case there are no ends.

This large molecule is a called a polymer and the small molecules that make it up are referred to as monomers (to indicate that the polymer is made up of a large number of repeating units). In this case, the polymer is a type of plastic called PVC (the plastic out of which supermarket carrier bags are made), standing for "poly vinyl chloride" (vinyl being the old word for ethene). We can't really give it a formula, since it consists of an indefinite number of monomers, but the closest we can get is to call it (C₂H₃Cl)_n where n stands for a very large number.

Polymers are very common in industry, mainly forming types of plastic. Other polymers you've almost certainly heard of are

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