A guide to naming organic compounds : Unsaturated hydrocarbons

From A guide to naming organic compounds. This is Section 3:

So far all we have been dealing with have been saturated hydrocarbons, that is, a carbon chain that has only single bonds. Carbons need four covalent bonds in order to be stable, and thus far each covalent bond has been going to a different atom, but what if a carbon has two covalent bonds with one carbon atom?

If a carbon chain contains 'double bonds' between the carbons it is considered to be unsaturated (there are more factors involved that can give a chain the name 'unsaturated' and they'll be covered later). Double bonds and triple bonds are an important feature of a carbon chain because they affect so many properties of the substance.

Geometric Isomerism: Double bonds, unlike single and triple bonds, cannot rotate around their axis. Because of this, the following two substances are not the same:

            CH3   CH3
              \   /
               C=C
              /   \
            H      H

            H     CH3
             \   /
              C=C
             /   \
            CH3   H

If the bond had been a single bond or a triple bond, the above two substances would have been the same, but because double bonds cannot rotate they are different. According to the UIPAC method of naming organic compounds, the top molecule cis-2-butene and the bottom is trans-2-butene.

We use the prefixes 'cis' and 'trans' to denote how the atoms are organised about a double bond. 'cis' says that the important group that is bonded to the carbon is in a 'C' shape:

            CH3   CH3
              \   /
               C=C
              /   \
            H      H

And 'trans' is for an 'S' shape:

            H     CH3
             \   /
              C=C
             /   \
            CH3   H

But what do we do to say that there is a double or a triple bond in a molecule?

You remember back in the first section when I only gave you the prefixes? Well that's because the suffixes usually allude to the types of bonding within the molecule. So, as you probably guessed, "-ane" means that there are only single bonds within the molecule. The other two are "-ene" for a double bond and "-yne" for a triple bond. But simply putting the suffix in is not enough. It doens't allow us to determine where the bond is.

        H H H H
        | | | |
      H-C=C-C-C-H
            | |
            H H    

The above example would be called cis-1,propene OR cis-prop-1-ene.

And remember that the carbon chain is numbers so as to make the double or triple bonds have the lowest possible numbers.

So that's pretty much everything on unsaturated hydrocarbons of this sort.

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