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Naming Alkanes

Naming alkanes

Core Concepts

In this tutorial on alkane nomenclature, you will learn how to name various alkanes using the IUPAC system. You will also become familiar with identifying various alkyl attachments. We make naming alkanes easy!

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What are alkanes?

Alkanes are single-bonded straight chains of saturated hydrocarbons. This means they are comprised exclusively of single-bonded carbons and hydrogens. The longest continuous chain of an alkane is called the parent chain. Since the parent chain is an alkane, it will end with -ane. (Example: If there are 5 carbons in this chain, the name will end with -pentane). Groups of hydrocarbons branching off of this chain are called substituents and they are alkyls because they have one less hydrogen. Because of this, they will have the suffix -yl. (Example: if there are 5 carbons with a two-carbon substituent, this will become ethylpentane).

What is IUPAC?

To find the name of a given alkane, we will utilize the IUPAC system of naming. This is a set of rules created in 1919 by a group of chemists to guarantee each molecule has a unique name. The reason it is important for each molecule to be given a distinct name is that it tells us exactly how each molecule is built just based on the name alone.

IUPAC for Alkanes

Above: the four parts of an IUPAC name in order.
There are four parts to naming alkanes.
  1. The locant: The number indicating where the substituent is.
  2. The prefix: The substituent attached to the alkane.
    • Ends with -yl.
  3. The Parent: The alkane parent chain.
    • Ends with -ane.
  4. Suffix: The functional group attached to the alkane.
    • Not always present.

How to name an alkane step by step:

In order to show you how to use the IUPAC naming system, we will use the alkane below to demonstrate.

Naming alkanes - examples
Step 1: Find the alkane’s parent chain.

When naming alkanes, an easy way to find the IUPAC name for an alkane is to work backward. This means we will start by finding the parent. To do this we will keep two things in mind:

  1. We want to find the longest parent chain.
  2. We want to have the most substituents possible.

As seen above, the longest parent chain is 7 carbons long. This means the alkane will end with “-heptane”. The table below will help you find the parent name that corresponds to the number of carbons in the chain.

Number of Carbons12345678910
Parent Namemethaneethanepropanebutanepentanehexaneheptaneoctanenonanedecane
Step 2: Number the parent chain

The parent chain can be numbered from left to right or vise versa. In some cases, both ways provide equal results. However, this is not always the case. To determine which way to number, consider the following:

  1. Number the chain towards the closest attachment.
    • If there is a tie, resort to the Point of difference rule:
      • If a tie occurs, consider the locants. We want the substituents on the lowest number locants possible.
      • If there is a tie at the locants, use the alphabetical rule. For example, 1-ethyl would beat 1-methyl because it comes first in the alphabet.
Naming alkanes - 4-methylheptane

Notice this molecule is even and the numbers are mirrored. No matter which way you number this alkane, the methyl group is still on carbon four. This is a situation in which it does not matter which way you number it.

Step 3: Identify the attachments
Naming alkanes - attachment examples
Above: alkyl substituents

Notice the attachments above end in “-yl” instead of “-ane”. This is because they are alkyls which means they have one less hydrogen due to being attached to the parent chain. In the case of our alkane, we have one methyl group and it is located at carbon 4.

Step 4: Put everything in order

Now that we know the locant, prefix, and parent, we can put them together like a puzzle. When doing this, there are three things to remember:

  1. Separate numbers from words with hyphens (-).
  2. Separate numbers from other numbers with commas (,).
  3. Arrange attachments in alphabetical order. (Example: 1-ethyl-7-methyloctane)

We were able to gather the following information from this alkane:

  1. The locant: 4
  2. The prefix: methyl
  3. The parent: heptane

Putting this information together, we will get 4-methylheptane.

Naming Alkanes – Practice Example:

  1. What is the name for this alkane?

Answer: 3-ethyl-2,2,4,5,5-pentamethylhexane

The longest parent chain is 6 carbons long which means the parent name is “-hexane”. Notice you could go straight down the chain and also get a 6 carbon parent chain. The difference is that the highlighted parent chain gives us more substituents.

The numbering should start from the right and go left. This is because if you go from the right and the left, they are even at carbon two. The difference occurs at the third carbon down the line from both ways. The third carbon from the left contains a methyl, whereas the third carbon from the right contains and ethyl. Ethyl will trump methyl and we, therefore, count right to left.

Notice there is one ethyl group and five methyl groups. These will be written starting with ethyl at the front. It will look like this:

3-ethyl-2,2,4,5,5-pentamethylhexane.

Notice the Penta put in front of the methyl. When more than one of the same attachments is present we will add a prefix right after the numbers such as di, tri, tetra, and so on. This tells us there are 5 methyls total present and they are located twice at carbon 2, once at carbon 4, and twice at carbon 5. Since there is only one ethyl, we do not need to add a prefix.

Thanks for reading, and be sure to check out more of our tutorials, experiments and element articles.

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