Alkane IUPAC Naming Explanation | Examples | Nomenclature

Naming a organic compound is a multi step process and should be done very carefully. But naming an alkane compound is the easiest in organic compound IUPAC nomenclature. When naming alkane, that includes selecting longest carbon chain, identifying substituents, numbering carbon chain properly and some other steps. We will discuss those steps in several examples to write IUPAC names of alkanes properly.

Examples of Alkane IUPAC naming

• CH₄ - methane
• CH₃CH₃ - ethane
• CH₃CH₂CH₃ - propane

You can see every name in above example are finished with ane suffix. In the IUPAC nomenclature, names of alkane should be finished with ane suffix.

Also you see, another suffix (meth, eth, prop) is used in front of ane suffix. These meth, eth, prop suffixes say, how many carbon atoms are in the root chain (will explain latter in the tutorial) of the molecule. Also we call these meth, eth, prop, more are word roots.

Lets take one example (propane) to explain in detail.

Propane - C₃H₈

In C₃H₈ molecule, there are three carbon atoms. Also there are only single bonds between carbon atoms (C-C). So this is an alkane and name should be finished as ane. Due to exist of three carbon atoms, prop suffix is used.

So final name is propane.

What is the IUPAC name of this molecule? CH₃-CH₂-CH₂-CH₃

This is a simple thing. But we do this in several steps to understand well.

• Check bonds between carbon atoms. There are single bonds between carbon - carbon atoms. So this molecule is an alkane and name should be finished as 'ane'.
• There are no sub-branches of carbon atoms and substituents.
• Count number of carbon atoms. In this molecule, there are four carbon atoms. So 'but' suffix is used.
• So, final name is butane.

Carbon chain length and word root

Chain Length	Word Root
C1	Meth
C2	Eth
C3	Prop
C4	But
C5	Pent
C6	Hex
C7	Hept
C8	Oct
C9	Non
C10	Dec
C11	Undec
C12	Dodec
C20	Icosane

What is the IUPAC name of this alkane? CH₃CH₂CH₂CH₂CH₂CH₂CH₃

Seven carbon atoms are in this alkane molecule. For seven carbon atoms what is the word root? heptane is the word root and IUPAC name is heptane.

Steps of IUPAC naming of alkane

1. Select the longest continuous carbon chain (root chain / main chain / highest number of carbon atoms)
2. Identify the substituent groups
3. Numbering the carbon chain
4. First, name the substituent groups
5. Name the main chain

Each steps are explained in detail below.

Identify longest continuous carbon chain

Identify longest continuous carbon chain (which includes maximum number of carbon atoms). It is called root chain. Then other carbon atoms which not included in root chain are defined as substituents or branched chains.

There are five carbon atoms in the root chain and two substituents in the below organic molecule.

Naming substituents groups in IUPAC naming

Substituents are parts which are not included in main branch. Some substituents and their substituent names are noted below.

• -CH₃ - methyl
• -CH₂CH₃ - ethyl
• -CH₂CH₂CH₃ - propyl
• -Cl - chloro
• -Br - bromo
• -I - Iodo

When writing IUPAC name, we first write substituents chain name and then root chain. Don't think about much in this level. You will understand more when examples are discussed.

In the above example, you can see two -CH₃ groups are as substituent groups. According to the IUPAC names for substituents, methyl should be the name for -CH₃ groups.

Identify substituents in a molecule.

We do several examples to understand whether substituent groups are present or not and if they are present, what are the names we are going to applied.

CH₃CH₂CH₂CH₃

You cannot separate carbon atoms to two or more carbon chains in this molecule. So there is only one carbon chain. Instead of carbon and hydrogen, no other element is combined. So there is no more substituents.

(CH₃)₂CHCH₃

You can draw two carbon chains. You should select the main carbon chain to contain highest number of carbon atoms. Then other carbon part become a substitute chain (substitute group).

Position of substituents are attached is Important!

In the above molecule, methyl groups are attached to two different carbon atoms. You will understand now, attaching position is important in naming the molecule.

So how to give position?

Numbering the root chain or main chain

Give numbers to the carbon atoms as 1,2,3,....

In the numbering, we can do it two ways. But,

• After numbering, substituent groups should have less numbers.

This is the correct way to numbering.

This is wrong.

With substituent group names, their attached positions also should be given. in the example, two methyl groups are attached to 2^nd and 3^rd carbon atoms respectively. Due to two same substituent are attached, di suffix should be written after the positions-.

Naming substituent group

2,3-dimethyl

Naming Root Chain | Main Chain

According to the number of carbon atoms in the root chain, final name should be finished. In this example, there are five carbon atoms in the root chain. So pent prefix is used and name should be finished with ane suffix.

Alkane IUPAC naming - Example 1

1. Select longest carbon chain (root chain) which is circled and bold in Fig 2. There are six 6 carbon atoms in the root chain.
   
2. Determine substituent(s). Substituents are highlighted (black bold) in fig 2.
   CH₃ - methyl
   CH₂CH₃ - ethyl
   Substituent are named according to alphabetical order, Therefore first comes ethyl group and then methyl group and substituent part is named as below.
   
   
   
3. Start numbering carbon chain. ethyl group gets lower position (3) because it exists first in the name according to the alphabetical order of substituents.
   
   
   
4. Substituent Name: 3-ethyl-5-methyl
   
   
5. You can write the name now. Finish the name by adding ane suffix. Hex suffix is added due to six carbon atoms in the root chain, finish the name as hexane. Then complete the name as
   3-ethyl-5-methyl hexane