How to Name Alkanes (Naming Alkanes)

Understanding how to name alkanes is a fundamental concept throughout the entire year of organic chemistry. The International Union of Pure and Applied Chemistry (IUPAC) has laid out a protocol for naming, which is used all over the world, and gives a standardized way of naming alkanes. Using this internationally standardized method, two people in completely different parts of the word can speak the same chemical language.

Naming molecules in organic chemistry is often referred to as nomenclature. These two terms can be used virtually interchangeable. As an example, “naming alkanes” and “alkane nomenclature” as the exact same thing.

 

The process of how to name alkanes can be simplified into 3 steps:

  1. Finding the longest carbon chain
  2. Identifying and correctly naming substituents
  3. Putting the name together in IUPAC format

 

      1. Finding the Longest Carbon Chain

The longest chain of uninterrupted carbons in the molecule is called the parent chain or the carbon backbone. Finding this is the first step in how to name alkanes. After finding the parent chain, count the number of carbons in that chain, and find the corresponding prefix in the table below. The prefix of the name gives the number of carbons in the parent chain.

The suffix gives information about the main functional group in the molecule. Since we are only talking about carbon-carbon single bonds (alkanes) in this chapter, the suffix is always “-ane.” Note how the prefix and the suffix combine in the following table.

 

Number of Carbons Prefix Carbon chain name
1 Meth- Methane
2 Eth- Ethane
3 Prop- Propane
4 But- Butane
5 Pent Pentane
6 Hex- Hexane
7 Hept- Heptane
8 Oct- Octane
9 Non- Nonane

 

Let’s look at some examples of naming alkanes.

 

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Here, the parent chain is the entire molecule. This chain has five carbons. If we look at the chart, we see this molecule is named pentane.

 

Here’s another example of naming alkanes:

 

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The longest chain in this molecule is NOT the chain that goes horizontally across the page. The carbon backbone or parent chain is indicated in red.

 

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This parent chain has 7 carbons, so the carbon backbone is called heptane (we’ll learn how to name the other part of the molecule that’s in black in the next section).

 

Keep it Simple

Remember, the parent chain is not always the most obvious choice, like the one that goes horizontally across the page, for example. A common trick by professors is to have a chain going across the page that, at first glance, looks like the carbon backbone but in reality, is not the correct choice.

 

What if you had to pick the parent chain when you have two chains of the exact same length? Our tie-breaker is to take a look off the number of substituents (portions of the molecule not in the parent chain) off each parent, and pick the parent chain that maximizes the number of substituents. Take a look at the example below.

 

Example

Identify the parent chain for the following molecule.

 

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Answer

After taking a close look at this molecule, it becomes clear that there are two, 8-carbon chains in the molecule, which we must choose between for the parent chain. Therefore, we must turn to our tie-breaker, which is the number of substituents off the parent chain. We want to choose the parent chain that maximized the number of substituents coming off the parent.

 

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Therefore, we choose the chain that leads to 2 substituents:

 

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2. Identifying and correctly naming substituents

A substituent is any part of the molecule that is not included in the parent chain. Identifying substituents is the second step of how to name alkanes.

In the example above reproduced here, the black portion (that’s circled in blue) would be a substituent as it is not in the carbon backbone.

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Alkyl substituents (substituents that contain only carbon and hydrogen) are named in almost the exact same way as the parent chain. The prefix is still decided by the number of carbons in the substituent but the suffix for an alkyl (the term for an alkane substituent) group is now –yl. So the substituent above has 1 carbon and is an alkyl substituent, so it would be named as methyl.

 

We now must identify the location of the substituent by specifying the parent chain the substituent is attached to. Each carbon in the parent chain is given a number label, ranging from 1 to the number of carbons in the parent chain. The purpose of the labeling is to allow us to specify the exact location of substituents. The carbon chain should always be numbered in a way that minimizes the number given to the substituent. For example, we have two ways we could number the parent chain in the molecule that was introduced above:

 

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We will therefore be sure to number the parent chain from left to right to minimize the numbering of the methyl substituent.

The fact that the methyl is on carbon 3 will be important later in forming the name.

 

3. Putting the name together in IUPAC format

 

A few other rules about naming must be covered about how to name alkanes before the systematic IUPAC name can be created.

  • Dashes (-) always go between numbers and letters (i.e. 3-methylhexane). Commas go between numbers that need to be separated.
  • If you have more than one of the same substituent (e.g. multiple methyl groups), you add a prefix in front of the substituent.
Number of same substituents Prefix
2 di-
3 tri-
4 tetra-

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  • If you have multiple substituents, then you put them in alphabetical order in the official IUPAC name. For example, if you have a propyl on C3, ethyl on C2, and methyl on C1, you would name it as 2-ethyl-1-methyl-3-propyl(insertparentchain).

 

Substituents come before the parent chain name in the full IUPAC name. The general formula for a simple IUPAC name is:

#-SubstituentParentChain

Where “#” is the location of the substituent, “Substituent” is the name of the substituent, and “ParentChain” is the name of the parent chain.

 

 

In the naming alkanes example previously introduced (shown below), the substituent is a methyl on the 3rd carbon of the parent chain, and the parent chain is hexane. Putting all this information together, the name of the molecule is 3-methylhexane.

 

This image illustrates the step-by-step process of how to name alkanes

 

 

 

 

 

 

 

 

 

 

Let’s do another example about how to name alkanes. Take a moment, and try to name the molecule below.

 

Example

Name the following alkane.


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Answer

6-ethyl-3,5-dimethylnonane

 

First, we must identify the parent chain. Don’t be fooled into choosing the 8-carbon chain that goes horizontally across the page as there is a 9-carbon chain also in the molecule that’s our parent chain. Therefore, our parent chain is nonane.

 

Looking at the substituents in the molecule, an ethyl and 2 methyl groups are present. To minimize the numbering of the substituents, we number from right to left. This gives us the substituents 3,5-dimethyl and 6-ethyl. Note that we have 2 methyl groups, so we do NOT name these as 3-methyl-5-methyl. Instead, the substituents are combined in naming as 3,5-dimethyl.

 

Now to place the name together, we must alphabetically compare the substituents to see which will be listed first in the name. We compare the “m” in methyl to the “e” in ethyl. Since the “e” in ethyl comes before the “m” in methyl, we list ethyl first. Note that the prefix “di-” has no impact on the alphabetical ordering of the substituents. Placing the entire name together, we get 6-ethyl-3,5-dimethylnonane.

 

This provides a step-by-step example of naming alkanes.

 

 

 

 

 

 

 

 

 

Example

Name the following molecule.

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Answer

6-ethyl-2,3,3-trimethyloctane.

 

The parent chain in this molecule is the chain going horizontally across the page. Note there is one other chain that could be chosen as the parent. The same answer is found either way.

14Once the parent is identified, it is clear there are 4 substituents in the molecule. We number the parent chain from left to right to minimize the numbering of the substituents. We can now get more specific about the location of our substituents: 3 methyl groups are coming off carbons 2, 3, and 3, and an ethyl group is on carbon 6.

 

We put the methyl groups together to make 2,3,3-trimethyl. To decide which substituent comes first in the name, they are arranged in alphabetical order (not including the prefix tri-). Because “e” comes before “m,” the ethyl comes first. The final name is therefore:

6-ethyl-2,3,3-trimethyloctane.

This provides a step-by-step illustration of how to name alkanes for a molecule in organic chemistry.