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 in some way based on this family of compounds. Most of the organic 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 particular 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 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 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; that is they will have different structures. Molecules with 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:

• chain isomers
• position isomers
• functional group isomers

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 present in the longest chain will tell us the root name for the compound. This is outlined below:

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:

• 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.

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

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

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 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 use commas between numbers when naming compounds.

Key Points

• Structural isomers are molecules which have the same molecular formula but different structures.
• 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.

Check your understanding- questions on chain isomers

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