Alkanes are organic compounds that consist of single-bonded carbons. Alkanes are the simplest hydrocarbons and are the least reactive. In this lesson, we will use the IUPAC naming system to name alkanes. IUPAC is a system of naming organic chemical compounds based on the International Union of Pure and Applied Chemistry. Below this lesson will consist of various examples with explanations.
Naming Alkanes in 3 steps:
- Find the longest carbon chain
- Identify and correctly name the substituents
- Complete the name using IUPAC protocol
Find the longest carbon chain
The longest carbon chain of uninterrupted carbons in a molecule is called the parent chain or carbon backbone. To find the parent chain, count the number of carbons in that chain and find the corresponding prefix in the table below. The prefix corresponds to the number of carbons in the parent chain.
The suffix indicates the functional group that takes priority in the compound. In this lesson, we are strictly practicing with alkane molecules that have the ending of -ane. However, some molecules have multiple functional groups and priority will have to be established. Compounds with multiple functional groups will be discussed in a future lesson.
Useful Prefixes (Table 1.1)
| Number of Carbon Atoms | Prefix Assigned | IUPAC NAME | Molecular Formula |
| 1 | Meth- | Methane | CH4 |
| 2 | Eth- | Ethane | C2H6 |
| 3 | Prop- | Propane | C3H8 |
| 4 | But- | Butane | C4H10 |
| 5 | Pent- | Pentane | C5H12 |
| 6 | Hex- | Hexane | C6H14 |
| 7 | Hept- | Heptane | C7H16 |
| 8 | Oct- | Octane | C8H18 |
| 9 | Non- | Nonane | C9H20 |
| 10 | Dec- | Decane | C10H22 |
Let’s look at some examples of naming alkanes.
Above, the parent chain is the entire molecule. This chain has no substituent groups attached and the final name is pentane. The compound is composed of 5 carbons.
Let’s look at another example:
Above the identifying, the parent chain is not as easy to see as the previous example. The chain highlighted in red is the parent chain in this example. In this example, the parent chain is heptane (we will discuss how to name the other parts of this molecule in the upcoming sections).
Note: The parent chain may not always be the chain that goes horizontal across the page. The best way to find the chain is to count using a different colored pen/highlighter to determine the parent chain.
Identifying and correctly naming substituents
Substituents are other groups attached to the parent chain (longest carbon chain). In other words, any part of the molecule that is not included in the parent chain. When naming a substituent group, the suffix -yl is applied.
For example, let’s compare two ways of numbering the parent chain to minimize the number given to the substituents.
In the example above the compound that has substituents at carbon 3 and 4 are the correct way to number the parent chain. Another way to think of this is to add the locations to compare. For example, the compound that has substituent groups are carbon 4,4, and 5 can be mathematically set up like this 4 + 4 +5 = 13. The other compound has substituents at carbon locations 3,4, and 4. Mathematically this can be set up as 3 + 4 + 4 = 11. Since 11 is smaller than 13 the molecule with groups at 3, and 4 is the correct way to number the molecule.
Complete the name using IUPAC.
A few more rules about naming using IUPAC are listed below:
- Dashes (-) always go between numbers and letters (i.e., 2-methyloctane)
- If you have more than one of the same substituent groups at the same carbon on the parent chain (i.e., multiple ethyl groups) a prefix needs to be added.
- Substituent groups are arranged in alphabetical order and the parent name is placed last.
| Numbers of same substituents | Prefix |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
Hyphenated prefixes such as di-, tri-, tetra-, n-, and t- are ignored when alphabetizing. Other modifies such as iso-, neo-, or cyclo- are not ignored.
We have identified the correct numbering of the parent chain and the substituents. Let’s complete the name using IUPAC.
Example 1: Name the following alkane
Example 2: Name the following alkane
Example 3: Name the following alkane
Answers
- 4-ethyl-2,4-dimethylheptane
- 4-propylnonane
- 2,5-dimethylheptane
- 2-methylpentane
