1|CHMSC CRIM AT SCI 11 -GENERAL CHEMISTRY
Organic Chemistry Chapter 2
DRAWING ORGANIC MOLECULES
This page explains the various ways that organic molecules can be
represented on paper or on screen - including molecular formulae, and
various forms of structural formulae.
The chlorines could be opposite each other or at right angles to each
other. But these two structures are actually exactly the same. Look at
how they appear as models.
Molecular formulae
A molecular formula simply counts the numbers of each sort of atom
present in the molecule, but tells you nothing about the way they are
joined together.
For example, the molecular formula of butane is C4H10, and the
molecular formula of ethanol is C2H6O.
Molecular formulae are very rarely used in organic chemistry, because
they don't give any useful information about the bonding in the
molecule. About the only place where you might come across them is
in equations for the combustion of simple hydrocarbons, for example:
In cases like this, the bonding in the organic molecule isn't important.
Structural formulae
One structure is in reality a simple rotation of the other one.
Note: This is all much easier to understand if you have actually
got some models to play with. If your school or college hasn't given
you the opportunity to play around with molecular models in the
early stages of your organic chemistry course, you might consider
getting hold of a cheap set. The models made by Molymod are
both cheap and easy to use. An introductory organic set is more
than adequate. Google molymod to find a supplier and more
about them, or click on the picture or text link below to see a
typical example from Amazon. (Don't click on the "Buy" button
unless you really want to buy it!)
Share the cost with some friends, keep it in good condition and
don't lose any bits, and resell it via eBay or Amazon at the end of
your course.
Alternatively, get hold of some coloured Plasticene (or other
children's modelling clay) and some used matches and make your
own. It's cheaper, but more difficult to get the bond angles right.
A structural formula shows how the various atoms are bonded. There
are various ways of drawing this and you will need to be familiar with
all of them.
Displayed formulae
Consider a slightly more complicated molecule, C2H5Cl. The displayed
formula could be written as either of these:
A displayed formula shows all the bonds in the molecule as individual
lines. You need to remember that each line represents a pair of shared
electrons.
For example, this is a model of methane together with its displayed
formula:
Notice that the way the methane is drawn bears no resemblance to the
actual shape of the molecule. Methane isn't flat with 90° bond angles.
This mismatch between what you draw and what the molecule actually
looks like can lead to problems if you aren't careful.
For example, consider the simple molecule with the molecular formula
CH2Cl2. You might think that there were two different ways of arranging
these atoms if you drew a displayed formula.
But, again these are exactly the same. Look at the models.
The commonest way to draw structural formulae
For anything other than the most simple molecules, drawing a fully
displayed formula is a bit of a bother - especially all the carbonhydrogen bonds. You can simplify the formula by writing, for example,
CH3 or CH2 instead of showing all these bonds.
So for example, ethanoic acid would be shown in a fully displayed form
2|CHMSC CRIM AT SCI 11 -GENERAL CHEMISTRY
and a simplified form as:
Organic Chemistry Chapter 2
horizontally. Anything else is simply hung off that chain.
It doesn't matter in the least whether you draw any side groups
pointing up or down. All of the following represent exactly the same
molecule.
You could even condense it further to CH3COOH, and would probably
do this if you had to write a simple chemical equation involving
ethanoic acid. You do, however, lose something by condensing the
acid group in this way, because you can't immediately see how the
bonding works.
You still have to be careful in drawing structures in this way.
Remember from above that these two structures both represent the
same molecule:
If you made a model of one of them, you could turn it into any other
one simply by rotating one or more of the carbon-carbon bonds.
How to draw structural formulae in 3-dimensions
The next three structures all represent butane.
There are occasions when it is important to be able to show the
precise 3-D arrangement in parts of some molecules. To do this, the
bonds are shown using conventional symbols:
All of these are just versions of four carbon atoms joined up in a line.
The only difference is that there has been some rotation about some of
the carbon-carbon bonds. You can see this in a couple of models.
For example, you might want to show the 3-D arrangement of the
groups around the carbon which has the -OH group in butan-2-ol.
Butan-2-ol has the structural formula:
Not one of the structural formulae accurately represents the shape of
butane. The convention is that we draw it with all the carbon atoms in a
straight line - as in the first of the structures above.
Using conventional bond notation, you could draw it as, for example:
This is even more important when you start to have branched chains of
carbon atoms. The following structures again all represent the same
molecule - 2-methylbutane.
The only difference between these is a slight rotation of the bond
between the centre two carbon atoms. This is shown in the two models
below. Look carefully at them - particularly at what has happened to
the lone hydrogen atom. In the left-hand model, it is tucked behind the
carbon atom. In the right-hand model, it is in the same plane. The
change is very slight.
The two structures on the left are fairly obviously the same - all we've
done is flip the molecule over. The other one isn't so obvious until you
look at the structure in detail. There are four carbons joined up in a
row, with a CH3 group attached to the next-to-end one. That's exactly
the same as the other two structures. If you had a model, the only
difference between these three diagrams is that you have rotated
some of the bonds and turned the model around a bit.
To overcome this possible confusion, the convention is that you always
look for the longest possible chain of carbon atoms, and then draw it
It doesn't matter in the least which of the two arrangements you draw.
You could easily invent other ones as well. Choose one of them and
get into the habit of drawing 3-dimensional structures that way. My own
3|CHMSC CRIM AT SCI 11 -GENERAL CHEMISTRY
habit (used elsewhere on this site) is to draw two bonds going back
into the paper and one coming out - as in the left-hand diagram above.
Notice that no attempt was made to show the whole molecule in 3dimensions in the structural formula diagrams. The CH2CH3 group was
left in a simple form. Keep diagrams simple - trying to show too much
detail makes the whole thing amazingly difficult to understand!
Organic Chemistry Chapter 2
Note: Explaining exactly what this structure means needs more
space than is available here. It is explained in full in two pages
on the structure of benzene elsewhere in this site. It would
probably be better not to follow this link unless you are actively
interested in benzene chemistry at the moment - it will lead you off
into quite deep water!
Skeletal formulae
Deciding which sort of formula to use
In a skeletal formula, all the hydrogen atoms are removed from carbon
chains, leaving just a carbon skeleton with functional groups attached
to it.
For example, we've just been talking about butan-2-ol. The normal
structural formula and the skeletal formula look like this:
There's no easy, all-embracing answer to this problem. It depends
more than anything else on experience - a feeling that a particular way
of writing a formula is best for the situation you are dealing with.
Don't worry about this - as you do more and more organic chemistry,
you will probably find it will come naturally. You'll get so used to writing
formulae in reaction mechanisms, or for the structures for isomers, or
in simple chemical equations, that you won't even think about it.
There are, however, a few guidelines that you should follow.
In a skeletal diagram of this sort


there is a carbon atom at each junction between bonds in a
chain and at the end of each bond (unless there is
something else there already - like the -OH group in the
example);
there are enough hydrogen atoms attached to each carbon
to make the total number of bonds on that carbon up to 4.
Beware! Diagrams of this sort take practice to interpret correctly - and
may well not be acceptable to your examiners (see below).
There are, however, some very common cases where they are
frequently used. These cases involve rings of carbon atoms which are
surprisingly awkward to draw tidily in a normal structural formula.
Cyclohexane, C6H12, is a ring of carbon atoms each with two
hydrogens attached. This is what it looks like in both a structural
formula and a skeletal formula.
What does your syllabus say?
Different examiners will have different preferences. Check first with
your syllabus. If you've down-loaded a copy of your syllabus from your
examiners' web site, it is easy to check what they say they want. Use
the "find" function on your Adobe Acrobat Reader to search the organic
section(s) of the syllabus for the word "formula".
You should also check recent exam papers and (particulary) mark
schemes to find out what sort of formula the examiners reallyprefer in
given situations. You could also look at any support material published
by your examiners.
Note: If you are working to a UK-based syllabus and haven't got a
copy of that syllabus and recent exam papers, follow this link to
find out how to get them.
What if you still aren't sure?
Draw the most detailed formula that you can fit into the space
available. If in doubt, draw a fully displayed formula. You would never
lose marks for giving too much detail.
And this is cyclohexene, which is similar but contains a double bond:
Apart from the most trivial cases (for example, burning hydrocarbons),
never use a molecular formula. Always show the detail around the
important part(s) of a molecule. For example, the important part of an
ethene molecule is the carbon-carbon double bond - so write (at the
very least) CH2=CH2 and not C2H4.
Where a particular way of drawing a structure is important, this will
always be pointed out where it arises elsewhere on this site.
But the commonest of all is the benzene ring, C6H6, which has a
special symbol of its own.