IUPAC Names
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IUPAC Names

This other method is called the IUPAC method (from the International Union of Pure and Applied Chemistry) and it involves picking out the longest continuous chain of carbon atoms, then identifying what groups are attached to that chain and indicating where they are attached. Let's look at a few examples.

Example 1

Let's go back to the two butane compounds. This compound has four carbon atoms lined up one after the other, so its IUPAC name is butane.

C - C - C - C

butane
The IUPAC name for the other structural formula is methylpropane (or 2-methylpropane). We come up with this name by looking at the fact that we have three carbon atoms in a row. That gives us propane. Then we have one carbon (with three hydrogens) attached to the side of propane. Meth- indicates that there is one carbon atom in the attached group. To show that it is a group of atoms attached to something else, we use a -yl ending. So this group of atoms is called a methyl group. The IUPAC name is methylpropane. The common name is isobutane.

C - C - C
|
C

methylpropane
2-methylpropane
As you will soon see, it can be necessary to indicate where a side group is attached to the main chain. This is done using a number that indicates to which carbon in the main chain a side group is attached. In this case, the methyl group is attached to the 2nd carbon of the propane, thus the full name is 2-methylpropane. The 2- is optional in this case because that is the only place the methyl group could be attached.

Example 2

As another example, let's use the structural formula for isopentane (which is shown in example 10 in your workbook). We can ignore the hydrogens and focus on the carbons by using the skeletal structural formula. The longest continuous chain of carbon atoms in that molecule is four carbons long. We will use the word butane to represent that four carbon chain.

The -CH3 group is attached to the four carbon chain, so we will attach the name in the same way to come up with methylbutane.

Now where is this methyl group attached? Depending on which end you start counting from, you could think of it either going on the third carbon or the second carbon. To make sure everyone gets the same name for the compound, we start counting on the carbon chain from the end that is closest to where the side group is attached. This will give us the lowest number possible. So, the methyl group is attached to the second carbon atom in the butane chain. This compound would be called 2-methylbutane.
C - C - C - C
|     
C     
C - C - C - C
|     
C     
-butane
C - C - C - C
|     
C     
methylbutane
C1 - C2 - C3 - C4
|      
C      
2-methylbutane
We didn't have to worry about a location number in methylpropane because there was only one place to put the methyl group. In a sense that is also true for 2-methylbutane. If you can see why, good. If not, let's not worry about it.

Example 3

Let's try naming this compound. (It is in example 12 in your workbook.) When looking at this and other structural formulas, ignore the different lengths of lines used to represent the bonds. Some of them have been extended to keep the hydrogens in the formula from bumping into one another.
The compound consists of carbon, hydrogen, and single bonds. Therefore, the name has an -ane ending.
C, H, and  ¾ -ane
The longest continuous chain of carbon atoms is four carbons long. Therefore, we add the stem but- to get butane.
4 Cs in a continuous chain butane
Attached to this butane chain there are two methyl groups, so we use the prefix di- to indicate the presence of two methyl groups; and we have dimethylbutane.
two methyl groups attached dimethylbutane
Where are those two methyl groups attached? They are attached to the second and third carbons. So, we put down a 2- and a 3- to indicate the second and third carbons for each of the methyl groups. The name of this compound is 2,3-dimethylbutane. Note that the name is written continuously with no spaces, with a comma between the numbers, and with a hyphen between the numbers and letters.
location of attached groups 2,3-dimethylbutane

 

Before continuing with additional naming challenges and practice problems, let's consider other side groups. Methyl groups are not the only kind of groups that are going to be added to chains. Let's spend some time naming other similar groups in the next section.

 

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