The table below contains the first 6 alkanes. The
alkanes are a family of saturated
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 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|
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:
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|
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:
|Number of carbon atoms||1||2||3||4||5||6||7||8||9||10|
So to name our isomer we 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.
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: