Basic Organic Nomenclature

With so many types of organic molecules, we have to figure out how we can talk about long molecules without resorting to complicated chemical formulas.

In order to do so, we resort to nomenclature based on the structure of the molecules. This section will cover a simple introduction to the IUPAC nomenclature that is the common choice in organic chemistry.

Our first step is to talk about the simple saturated carbon chain. As we discussed before, the saturated hydrocarbons have as much hydrogens as possible. A simple hydrocarbon chain looks like this:

This family of molecules is known as the alkanes. They are named based on the number of carbon atoms in the molecule. The prefix is dependent on the number of carbon atoms, and the suffix will always be “-ane” for the alkanes. This lets us simplify $CH_3(CH_2)_4CH_3$ into $Hexane$ .

The prefixes for various numbers of carbon atoms is:

$\begin{matrix} \text{Number of Carbon}&\text{Prefix}\\ 1&meth-\\ 2&eth-\\ 3&prop-\\ 4&but-\\ 5&pent-\\ 6&hex-\\ 7&hept-\\ 8&oct-\\ 9&non-\\ 10&dec-\\ 11&undec-\\ 12&dodec- \end{matrix}$

Most of these prefixes are just like the polygon name you are familiar with from geometry. The early few, though ($\le4$) have unfamiliar prefixes. This means that you need to remember these prefixes well.

Another new concept in organic chemistry is modified Lewis Diagrams. Most of the time in the Lewis Diagram of an organic molecule, the hydrogens are implied, as are the carbons. The hydrogens are not shown at all, and the carbons are only shown as vertexes in the bond structure. This means that a complex Lewis Diagram, such as in pentane, can be simplified greatly.

Normal Lewis Structure of $Pentane$ :

Modified Lewis Structure:

Normal Lewis Structure of $Octane$ :

Modified Lewis Structure:

Gallery 10.2.1 covers the Lewis Diagrams for the first 12 alkanes.

The next set of organic molecules is the alkenes. These are the simplest unsaturated hydrocarbons, as they replace hydrogens with double bonds between carbon atoms.

For an alkene, the name depends on the number of double bonds in the chain. The suffix is going to be $-ene$, but if there’s more than one, it is prefaced depending on the number. The prefix is the same as an alkane, dependent on the number of carbons.

$\begin{matrix} \text{Number of Double Bonds}&\text{Suffix}\\ 1&-ene\\ 2&-diene\\ 3&-triene\\ 4&-tetraene\\ 5&-pentaene\\\end{matrix}$

We also have to introduce a new concept of numbering the carbon atoms, to place the double bond.

To figure out why, take a look at $hexene$:

Now look at $hexene$:

Both of these molecules have six carbon atoms, and one double bond, but the double bond is in a different spot. So how do we distinguish these bonds? For now, let’s call them Hexene A and Hexene B, respectively.

To number the carbons, we first identify the base chain, which is the longest unbroken chain of carbon-carbon bonds. Gallery 10.2.2 shows the base chain of various complex compounds.

We will get to naming those complicated molecules later. For now, we have only a few things on our main chain. Anyway, we now need to number the carbon atoms from one side of the chain to the other. We always number them starting on the side closest to the first abnormality. So in our two hexene molecules, the ordering is:

Hexene A:

Hexene B:

Perfect! We can number our carbons. Now we need to identify the carbon where our bond is, and label the molecule accordingly. We place the bond on the first carbon atom involved, and put that number in front of the atom’s name. For example, if we had a double bond between the 2nd and 3rd carbon atoms in pentene, then our molecule would actually be named $2\text-pentene$.

So for our hexenes, Hexene A is $2\text-hexene$, and Hexene B is $3\text-hexene$.

If we have multiple double bonds, we label them in increasing carbon number.

Example Problem 10.2.1

Another form of base chain you can have is an alkyne. These are compounds which include a triple bond between the carbon atoms. These types of compounds are named similarly to alkenes, just with a y instead of an e, making its chart:

$\begin{matrix} \text{Number of Triple Bonds}&\text{Prefix}\\ 1&-yne\\ 2&-diyne\\ 3&-triyne\\ 4&-tetrayne\\ 5&-pentayne\\\end{matrix}$

Another situation you might encounter is a molecule that is not a straight line, but instead circles upon itself. In this case, all the rules still apply, you just append $cyclo-$ to the beginning of the molecule’s name. For example:

$1,3\text{-} cyclopentene$:

Here is a full list of the prefixes for naming compounds with multiple of the same functional group.

$\begin{matrix} \text{Number}&\text{Counting Prefix}\\ 2&di-\\ 3&tri-\\ 4&tetra-\\ 5&penta-\\ 6&hexa-\\ 7&hepta-\\ 8&octa-\\ 9&nona-\\ 10&deca-\\ 11&undeca-\\ 12&dodeca-\\ 13&trideca-\\ 14&tetradeca-\\ 15&pentadeca-\\ 16&hexadeca-\\ 17&heptadeca-\\ 18&octadeca-\\ 19&nonadeca-\\ 20&eicosa-\\ \end{matrix}$