In this video you will learn how to use different types of formula to represent organic molecules.
Organic molecules can be shown in a variety of ways using different types of formula.
The formula used most frequently in our videos has been the “displayed formula”. This
is because displayed formula provides to clearest visual structure of an organic molecule. Here
is the molecule butene shown in displayed formula. It shows all the atoms in the correct
order with lines to represent all single and double covalent bonds.
Drawing all the bonds for very common molecules can be quite repetitive. For example, here
is butanoic acid in its displayed formula. Since this is a common molecule and as chemists
we are clear with the bonding arrangement of the carboxylic acid functional group, we
also use structural formula to represent organic molecules. Here is the structural formula.
As you can see the covalent bond between the OH is not shown as we know this is the arrangement.
In fact, we would simplify this further and remove all the displayed bonds like this…
Structural formula is often used when writing equations but the formula does mean that it
isn`t always clear what arrangement bonds are in. Here is another example – first
the displayed formula of propane. If we then take out the lines representing the covalent
bonds and group our atoms together in the order they are arranged, we make the structural
formula of propane. Pause the video now and try and draw both
the displayed formula and structural formula of pentane which has 5 carbon atoms.
Here is the displayed formula of pentane and here is its structural formula.
Molecular formula is another type used and shows us only the number of each type of atom
in the compound. To work out the molecular formula, you simply add up each type of atoms.
For example, here is the displayed formula for hexane. As you can see, it has 6 carbon
atoms and it has 14 hydrogen atoms. The molecular formula of hexane is therefore C6H14. This
alone is not very useful as it does not show us how the atoms are arranged and where the
bonds are. If you look at these molecules, they all have 6 carbon atoms and 14 hydrogens
– although you may not be able to name these different molecules, you can see they are
very different but all have the same molecular formula
This is the molecule glucose. Pause the video now and complete the molecular formula for
glucose. You should have counted 6 carbon atoms, 12
hydrogen and 6 oxygen atoms so the molecular formula is C6H12O6.
Sometimes the formula is reduced even further to make an Empirical formula. The empirical
formula is the simplest whole number ratio of atoms in a compound. We need to find the
highest common factor, a number that all the numbers, 6,12,6 will divide into. They will
all divide by 6 so if we do this, we get C1, H2 , O1, we don`t show the 1s and so the empirical
formula is CH2O. Although empirical formula doesn`t show us the molecule or how it is
arranged, it does tell us the ratio of atoms in the molecule!
Now at the end of this video, you should understand how to draw organic molecules in displayed,
structural, molecular and empirical formula.