Alkanes - Structure and Nomenclature

Alkanes are organic compounds which consists of single-bonded carbon and hydrogen atoms. Alkanes are also known as saturated hydrocarbons. They have general formula C_nH_2n+2. There are three types of alkane compound found in nature, that is straight chain alkane, branched alkane and cyclic alkane. In this article we will learn about IUPAC nomenclature of straight chain and branched alkane compounds. 

Key words: Alkanes, Saturated hydrocarbons, Common names of alkane, IUPAC nomenclature. 

Table of Contents

1. Nomenclature of single chain alkanes
2. Nomenclature of branched alkanes
3. Examples
4. Conclusion

Here we will learn how to assign names to the straight chain and branched alkane compounds. We will cover nomenclature of cyclic alkane compound in separate article. Please see the link below for nomenclature of cyclic alkane compounds. (Link : https://www.chemistrywithdrsantosh.com/2020/09/nomenclature-of-cycloalkanes.html)

1. Nomenclature of single chain alkanes

Nomenclature of straight chain alkane compounds is defined by the number of carbon atoms present in the chain. For example, two carbon bearing alkane in known as "ethane" and four carbon atoms bearing alkane is known as "butane". Here the these names consists of prefixes like "eth" for 2 carbons and "but" for 4 carbons followed by suffix "ane" which represents the parent name "alkane". Some of the common single chain alkanes are shown in Table 1.

Formula	Number of carbons	Name
CH4	1	Methane
CH3-CH3	2	Ethane
CH3-CH2-CH3	3	Propane
CH3-CH2-CH2-CH3	4	Butane
CH3-CH2-CH2-CH2-CH3	5	Pentane
CH3-CH2-CH2-CH2-CH2-CH3	6	Hexane
CH3-CH2-CH2-CH2-CH2-CH2-CH3	7	Heptane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3	8	Octane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3	9	Nonane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3	10	Decane

Table 1

2. Nomenclature of Branched alkanes

How to assign name of branched alkenes then?

For example

For this we need to follow few steps;

Step 1: Select the longest chain of carbons and give numbering so that the branched carbon gets least number.

Step 2: Name the longest carbon chain alkane. It is known as “Parent alkane”.

Here in the given example, longest carbon chain is of 6 carbons so the parent alkane is “Hexane”.

Step 3: Next step is to identify the branched group (also known as a substituent). Here it is 2 carbons alkane which is Ethane. Now remove the suffix “ane” and replace it with “yl” (This is for any alkane substituents ; Table 2).

Alkane	Number of carbons	As a substituent
Methane	1	methyl
Ethane	2	ethyl
Propane	3	propyl
Butane	4	butyl
Pentane	5	pentyl
Hexane	6	hexyl
Heptane	7	heptyl
Octane	8	octyl
Nonane	9	nonyl
Decane	10	decyl

Table 2

Coming to the given example, the substituent is “Ethyl” and it is located at carbon number 3

Step 4: Write the substituent along with the carbon number before the parent alkane. Therefore the name of the compound is “3-Ethyl-hexane”.

Now let’s take another example.

Step 1: Select the longest chain of carbons

Step 2: Name the longest carbon chain alkane which is “Hexane”

Step 3: Next step is identify the branched group. Here it is “Chlorine”. Now remove the suffix “ine” and replace it with “o”. (This is for any halogen substituents; Table 3)

Halogen	Formula	When it is substituent
Fluorine	F	Fluoro
Chlorine	Cl	Chloro
Bromine	Br	Bromo
Iodine	I	Iodo

Table 3

Coming to the given example, the substituent is “Chloro”

Step 4: Write the substituent along with the carbon number before the parent alkane.

Therefore the name of the compound is “3-chloro-hexane”

Let’s take another example with more than one substituent.

Here the parent alkane is “hexane”. But there are two methyl groups (derived from methane) present at carbon number 2 and 3. At this situation we need another kind of prefixes along with carbon numbers as shown in the Table 4.

Number of similar groups	Prefix
Two	di
Three	tri
Four	tetra
Five	penta
Six	hexa
Seven	hepta
Eight	octa
Nine	nona
Ten	deca

Table 4

So, name of the given alkane is “2,3-dimethyl-hexane”

Let’s take one more example,

Now here there are two substituents are present at C-2 and C-3. Here the “methyl” and “ethyl” groups should be written in alphabetical order.

Therefore the name is “3-ethyl-2-methyl-hexane”.

Let’s take one more example

Here the substituents are ethyl (at C-3), methyl (at C-2) and bromo (at C-4). Write the substituents in alphabetical order.

So name of the alkane is “4-bromo-3-ethyl-2-methyl-hexane”

3. Examples

Some other examples are given below for practice,

4. Conclusion

Finally to summarize the topic for the nomenclature of alkanes we need to follow five rules.     

1. Select the longest chain of the carbon and name it as Parent alkane.
2. Start the numbering from more branched end.
3. Assign the substituents in alphabetical order.
4. If similar substituents are present then use the prefixes such as “di”, “tri”, “tetra” etc.
5. Write the substituents before parent alkane along with the carbon number on which it is present.

That's all for this topic. If you have any questions please feel free to ask me. Also suggest me if any changes or additions are required. Thank you!