Naming Guidelines
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Naming Guidelines

Let's review the IUPAC process for naming alkanes and then give you some practice using that process.

Guidelines

The following guidelines will help you create the IUPAC names for alkane molecules.

  1. Identify the longest continuous chain of carbon atoms and name it using both the proper root to show the number of carbon atoms and the ending -ane to show that it is alkane (having only single bonds).
  2. Number the atoms in the chain, starting at the end that is closest to an attached side chain. If each end has an equally close side group, start with the end that gives the lowest series of numbers for the groups.
  3. Identify each side group and name it using both the proper root to show the number of carbon atoms and the ending -yl to show that it is a side group.
  4. If there is more than one of a particular side group, use prefixes (di-, tri-, etc.) to show how many there are.
  5. Identify the location of each side group by using the number of the carbon atom to which it is attached.
  6. Put the names of the side groups (along with numbers and prefixes) in front of the name of the main chain in alphabetical order (using the group name, not the prefix).
  7. Write the name with no spaces, with commas between numbers, and with hyphens between numbers and letters.

Practice

The following practice problems are each intended to focus on one or more of the guidelines listed above. The answers shown below may elaborate on the application of the appropriate guideline.

Set 1

This set of structural formulas (which are also shown in example 13 in your workbook) focus on the first guideline.

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

Answers

H H H H H
| | | | |
H-C-C-C-C-C-H
| | | | |
H H H H H		 This first formula has five carbons in a straight line, all with single bonds and hydrogens. This is called pentane. No groups are attached to the carbon chain, therefore the IUPAC name is simply pentane. Sometimes this is also called n-pentane or normal-pentane to emphasize that all five carbon atoms are in a continuous line. We call this a straight chain.
H   H   H   H
|   |   |   |
H - C - C - C - C - H
|   |   |   |
  H   H   H H-C-H
            |
            H		 Here there are 5 carbon atoms in the longest chain, and there is nothing else added to that. Therefore, it is also called pentane. It is the same compound as the one above. Notice that this is true even though the carbons are not all written in a straight line. When you are picking out the longest carbon chain in a molecule, you have to make sure that you consider going around corners to find the chain of carbon atoms that is actually the longest. This is still called a straight chain even though the C's are not written in a straight line.
In this one there are again 5 carbon atoms in a continuous chain. There are two right angles in the way it is drawn, but that makes no difference to the molecule itself. This also is pentane.

The point of these three examples is to show that you cannot just pick the longest chain that is written straight on a piece of paper. You have to find the longest continuous chain. Sometimes they will be written in a straight line; sometimes they won't. You have to be aware of that.

If you have any doubts that each of these three diagrams represents the same molecule, the identical molecule, you should take some time to use the model kits and construct each one of these diagrams. When you have done that, compare the three models that you have made and you will find that they all are identical to one another.

Set 2

This set requires attention to guideline 1 and the others as well. these skeletal structural formulas are also shown in exercise 14 in your workbook.


C-C-C-C-C
    |
      C-C
    |
    C

 

Answers


C-C-C-C-C
    |
      C-C
    |
    C

2,3-dimethylhexane



4-ethyl-3,5-dimethylheptane

Hopefully, you were able to determine the names of these compounds without too much difficulty. However, if you need additional practice try the problems at the end of the chapter in your textbook or ask your instructor in the lab for a sheet of additional problems.

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