Organic Chemistry Naming Methodology

If you are like me, you are daunted by the names thrown around in organic chemistry. As it turns out though, there is a very systematic process behind it. I will attempt to explain. First off, let’s start by defining what I will call the root names. The root names are assigned to the largest carbon chain in the molecule.

Root Names

• 1 carbon = meth
• 2 carbons = eth
• 3 carbons = prop
• 4 carbons = but
• 5 carbons = pent
• 6 carbons = hex
• 7 carbons = hept
• 8 carbons = oct
• 9 carbons = non
• 10 carbons = dec
• 11 carbons = undec
• 12 carbons = dodec
• 13 carbons = tridec
• 14 carbons = tetradec
• 15 carbons = pentadec
• 16 carbons = hexadec
• …
• 20 carbons = iso

Largest Chain

What do I mean by largest chain? Take for example the structure of 4-methylnonane:

In organic chemistry, on any point of a line structure that doesn’t have a label, it is assumed to contain a carbon atom, and that there are hydrogen atoms bonded to that carbon atom. Remember, a carbon atom has 4 valence electrons (or electrons in its outer electron orbital), and a Hydrogen atom has 1 valence electron, and that carbon wants to have 8, and really wants them from hydrogen. Anyway, without getting too much into basic chemistry, if you look at 4-methylnonane above, you see that the longest chain is 9 carbons long. At each point in the chain, you can assume a carbon. Count the points, and you get your root name. In this case, consult the chart above, and you see 9 carbons = non.

How many bonds?

Next, you look at the bonds. Are there any double bonds? Double bonds are usually notated by a double line at the bond, instead of a single line. If there are, you would use the suffix
-ene, triple bonds would use the suffix -ine. In our case, 4-methylnonane does not have any double bonds, so we would use the suffix -ane. So we add -ane to the root, non+ane, nonane. Carbon chains without double bonds are called alkanes. All carbon chains that are alkanes, get the suffix -ane. Carbon chains with double bonds are called alkenes, and carbon chains with triple bonds are called alkynes.

alkyl groups

So where does the 4-methyl come from? If you look at the original structure, you’ll notice that there is a branch coming off of the 9-carbon chain. When you have a branch off the longest chain, you count the “branch” chain and use the prefix for that. In our case, the 9-carbon longest chain has a 1-carbon chain branch, so we would use meth, and then you apply the suffix -yl (methyl nonane).

Next, number the carbons on the longest chain, starting from the side of the chain that would be closest to the branch. In our case, the branch is on the 4th carbon, and the name would be 4-methyl nonane, because the methyl group is attached to the 4th carbon on the nonane chain. Makes sense, right?

Let’s take another example.

Now what happens if the chain is a ring? Take buytlcyclopentane:

If a carbon chain forms a ring you add the prefix cyclo- to it. Easy as that.

Common vs. Systematic

Just to make things more complicated, there are two ways of naming. Common naming, and Systematic naming.

Common Name

Common naming uses the additional prefixes sec-, iso-, and tert- to explain where the branch or group is attached to the ring or chain. If the group is attached on the first carbon in the straight chain, there are no prefixes added. For example, in butylcyclopentane, the butyl group attaches to the ring at the point of the first carbon in the chain. For sec-butylcyclopentane, the butyl group is attached on the second carbon atom in the chain where that carbon is bonded with 2 other carbon atoms. For iso-buytlcyclopentane, the butyl group is attached to the ring at one end of the 4 carbon chain, but it also branches off on the other end of the chain. This only tends to happen with groups below 5-6 carbons. For tert-butylcyclopentane, the butyl group would attach to the ring on a carbon that is also bonded with 3 other carbon atoms. The following will try to explain that in image form:

Systematic Naming

For me, systematic naming makes more sense and is easier to use. Systematic naming basically repeats the naming scheme used for the root, when naming the groups attached to the ring or chain. For example, take butylcyclopentane again.

butyl cyclopentane is still butyl cyclopentane

sec-butyl cyclopentane becomes (1-methyl propyl)cyclopentane. The longest chain on the group attached to the ring is 3 carbons long (propyl), and branch off that chain is 1 carbon long (methyl), and that branch is attached on the first carbon (1-methyl).

iso-butyl cyclopentane becomes (2-methyl propyl)cyclopentane. The longest chain on the group attached to the ring is 3 carbons long (propyl), and branch off that chain is 1 carbon long (methyl), and that branch is attached by the 2nd carbon (2-methyl).

tert-butyl cyclopentane becomes (1,1-dimethyl ethyl)cyclopentane. The longest chain on the group attached to the ring is 2 carbons long (ethyl), and branch off that chain is 1 carbon long (methyl), but there are two of them (dimethyl), and the branches are attached by the first carbon (1,1-dimethyl).

Difficult example

How about 5,6 – diethyl – 6-(1-methylethyl) decane?