alkanes and isomers

Alkanes and chain isomers

The table below contains the first 6 alkanes. The alkanes are a family of saturated hydrocarbons that you met in gcse science. Most of the molecules you will meet in your A-level organic chemistry course are are in some way based on this family of compounds. Most of the compounds you will meet are made by replacing a hydrogen atom 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 this group the functional group.

A functional group is the reactive part of a molecule which gives the molecule its distinctive chemical and physical properties. At first it can become a bind to learn the structure and reactions of these different functional groups, but as your knowledge and confidence with organic chemistry grows these functional groups and their distinctive reactions will become almost second nature to you.

The alkanes

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 Structure
methane 1 CH4
ethane 2 C2H6
propane 3 C3H8
butane 4 C4H10
pentane 5 C5H12
hexane 6 C6H14

Naming compounds

Hexane, shown opposite, is an alkane with the 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 molecules as well as hexane using all the carbon and hydrogen atoms. In fact you could build 6 different molecules using all the carbon and hydrogen atoms. These molecules would all have the same molecular formula but will look different from each each, that is they will have different structures. Molecules with the same molecular formula but different structures are called structural isomers.

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

There are 3 types of structural isomers:

On this page we will focus on chain isomers only. A chain isomer is where there are two or more ways of arranging the carbon skeleton in a molecule.

For example the diagram below 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 isomers and in this case a chain isomer. A structural isomer is simply a compound with the same molecular formula but a different shape.

How would we go about naming the 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 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

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. In the second example, which is also shown opposite, the longest chain is only 5 carbon 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:

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 ocyl nonyl decyl
formula CH3- C2H5- C3H7- C4H9- C5H11- C6H13- C7H15- C8H17- C9H19- C10H21-

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

So to name our isomer we have:

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:

This time 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?

The image oppsite 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:

Key Points

Practice questions

Check your understanding- questions on chain isomers