Consider
a slightly more complicated molecule, C2H5Cl. The
displayed formula could be written as either of these:
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 carbon-hydrogen 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 and a
simplified form as:
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:
The
next three structures all represent butane.
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.
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.
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 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 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.
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
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:
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:
Using
conventional bond notation, you could draw it as, for example:
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.
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 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 3-dimensions 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!
Skeletal
formulae
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:
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.
And
this is cyclohexene, which is similar but contains a double bond:
But the
commonest of all is the benzene ring, C6H6, which
has a special symbol of its own.