Alkanes and chain isomers

Functional groups

The table below contains the first six alkanes; the alkanes are a family of saturated hydrocarbons that you met in gcse science. Now many of the molecules you will meet in your A-level organic chemistry course are in some way based on this family of molecules, simply because many of the organic compounds you will meet in chemistry are made by replacing one or more of the hydrogen atoms in an alkane molecule with another atom or a group of atoms. These new atoms or group of atoms maybe responsible for the reactions of the molecule, we call these reactive parts of a molecule the functional group and you will very quickly realise that these functional groups are the key to understanding many of the reactions that molecules undergo.

The alkanes

The alkanes are a family of saturated hydrocarbons that contain the C-C and C-H groups only, that is they contain a single covalent bond between all the atoms in the molecule. The general formula for the alkanes is C_nH_2n+2, now the table below is included as a simple reminder of the names, structure and formula for the first six alkanes. I included the 3d-structures to remind you of the fact that there is free rotation around the C-C and C-H bonds and that the geometry around all the carbon atoms in the alkane is tetrahedral.

Name	Number of carbon atoms	Molecular Formula
methane	1	CH₄
ethane	2	C₂H₆
propane	3	C₃H₈
butane	4	C₄H₁₀
pentane	5	C₅H₁₂
hexane	6	C₆H₁₄

Structural isomers

A 3d model of showing the structure of hexane.

Hexane (shown opposite) is a saturated alkane with the molecular formula C₆H₁₄. If you had a model kit with 6 black atoms of carbon and 14 white atoms of hydrogen I am sure you could build lots of different shaped molecules other than the one shown. In fact you could build five different shaped molecules using all the carbon and hydrogen atoms. These molecules would all have the same molecular formula but they will all look different from each; for example the image below shows two molecules, both of which have the same molecular formula (C₆H₁₄ but they have different structures, they obviously look different from each other. 3D models showing two structural isomers of hexane.

Molecules such as this which have the same molecular formula but different structures are called structural isomers.

A structural isomer is a molecule with same molecular formula but a different structural formula

There are 3 types of structural isomers that you need to know about, these are:

chain isomers
position isomers
functional group isomers

On this page we will focus on chain isomers only, for more information on position and functional group isomers click the links at the top or bottom of the page. A chain isomer is where the molecules have the same molecular formula but differ in the arrangement of the carbon atoms in the main chain or backbone of the molecule. In other words, chain isomers have different carbon skeletons or branching patterns. For example the image above shows two molecules; both of which have the same molecular formula (C₆H₁₄) however it is clear from the image that these two molecules despite having the same molecular formula have very different shapes or structures. These two molecules are an example of a structural isomer and in this case a chain isomer.

Naming isomers

How would we go about naming the two chain isomer of hexane shown above? To name the compounds we first have to identify the longest chain of carbon atoms present in each molecule. The number of carbon atoms present in the longest chain will tell us the root name for the compound. This is outlined below:

Number of carbon atoms present in the longest carbon chain	1	2	3	4	5	6	7	8	9	10
root name	meth	eth	prop	but	pent	hex	hept	oct	non	dec

3d model showing chain isomer of hexane.

In the example above hexane clearly has 6 carbon atoms; so the root name will be hex-. The second part of the name -ane; tells us that it belongs to the alkane family. Recall from gcse science that the alkanes are all saturated hydrocarbons, that is they contain only single covalent bonds. In the second example; which is also shown opposite the longest chain of carbon atoms is only 5 atoms long and there is a side chain or branch on the third carbon atom. This branch contains the -CH₃ group. These branches are very common in molecules and are named simply from the number of carbon atoms present in them as shown in the table below.

So to name our isomer of hexane we have:

The longest chain is 5 carbon atoms- so the root name will be pent-
There is a methyl (CH₃-) branch or side chain on the third carbon atom in the molecule.

So the name of the isomer will be 3-methylpentane. Note we use hyphens between letters and numbers in the names of molecules. The side-chains listed in the table below are often called alkyl groups, hence the names methyl, ethyl, butyl etc:

Number of carbon atoms	1	2	3	4	5	6	7	8	9	10
branch name	methyl	ethyl	propyl	butyl	pentyl	hexyl	heptyl	octyl	nonyl	decyl
formula	CH₃-	C₂H₅-	C₃H₇-	C₄H₉-	C₅H₁₁-	C₆H₁₃-	C₇H₁₅-	C₈H₁₇-	C₉H₁₉-	C₁₀H₂₁-

Example 2 - what is the name of the hydrocarbon below?

3D model of 3-methylpentane, an isomer of hexane

So to name our isomer we have:

The longest chain is 6 carbon atoms- so the root name will be hex-
There is a -CH₃ branch on carbon atom number 3

So its name will be 3-methylhexane.

If we had started numbering the carbon atoms from the other end of the molecule then we would have:

The -CH₃ branch is on carbon atom number 4. The longest chain of carbon atoms is still 6 carbon atoms long, so the name of the molecule this time would be 4-methylhexane.
This example demonstrates an important rule when naming compounds; that is that any substituents or branch chains must be given the lowest number possible. So the molecule above would be named 3-methylhexane and NOT 4-methylhexane.

Example 3 - what is the name of the hydrocarbon below?

Space filled model showing chain isomer hexane.

The image opposite shows a space filled model of a branched chain hydrocarbon molecule. What is the systematic name of this molecule?

Use the same method as shown above:

The longest chain of carbon atoms in this molecule is 4. This means the root name will be but-
On carbon atom number 2 there are 2 methyl groups (-CH₃).

The name of the molecule will be 2,2-dimethylbutane
The 2,2 indicate which of the carbon atoms in the longest chain of carbon atoms the two methyl groups are attached to. The prefix di is used to indicate that there are 2 groups and we always use commas between numbers when naming compounds.

Formula, names and branches

Hexane as mentioned above has five in the structural isomers; specifically it has five chain isomers. Let's have a quick look at the formulae for these isomers.

n-hexane or simply hexane is a straight chain isomer with no branches. The prefix "n" before the name of the molecule is commonly used to indicate the fact that we are dealing with a molecule with a straight chain of carbon atoms. The condensed formula of n-hexane is: CH₃CH₂CH₂CH₂CH₂CH₃

2-methylpentane or isohexane is also a chain isomer of hexane. This isomer has five carbon atoms in the longest chain with a methyl group (-CH₃) attached to the second carbon atom as a branch. Its condensed formula is: CH₃CH(CH₃)CH₂CH₂CH₃
The brackets around the methyl group (CH₃) on carbon atom number 2 indicates that this group is a branch off the main carbon chain. You may also see the prefix "iso" in the name of many organic compounds, it simply indicated that there is a methyl branch on carbon atom number 2.

3-methylpentane is another chain isomer of hexane. It has a methyl branch (-CH₃) on carbon atom number 3, its condensed formula is: CH₃CH₂CH(CH₃)CH₂CH₃

2,2-dimethylbutane is an isomer of hexane where the longest carbon chain in the molecule consists of 4 carbon atoms, there are two methyl branches (-CH₃) attached to carbon atom number 2 in the molecule, its condensed formula is: CH₃C(CH₃)₂CH₂CH₃

The last chain isomer of hexane is 2,3-dimethylbutane, here the longest carbon chain consists of 4 atoms and there are two methyl branches (-CH₃) on carbon atoms number 2 and 3. Its condensed formula is: CH₃CH(CH₃)CH(CH₃)CH₃

The image below shows four of the five chain isomers of hexane:

3d models of the chain isomers of hexane

Key Points

Structural isomers are molecules which have the same molecular formula but different structural formula.
Many molecules can have the same molecular formula and exist as straight chains but may also have branches or side chains.
To name a molecule identify the longest chain and give all side chains the lowest possible branch number.
The names of any side chains are listed alphabetically in the final name of the molecule, so for example ethyl would come before methyl in the name of an organic molecule.