From four carbons on up, there are different ways that the atoms in these compounds can
be arranged. These are the isomers that were mentioned earlier.
Let's focus on the nature of these structural isomers before looking at how they are
named. Just knowing the number of carbon atoms in a particular compound does not
necessarily tell you what the structure is.
|The carbon atoms in a compound can sometimes form a single continuous
chain of carbon atoms, and sometimes they form a branched chain. That can also be
represented using these skeletal structures.
|C - C - C - C
|C - C - C
|If we add the ten hydrogens needed to fill out the bonding,
we get the molecular formula C4H10 that goes with the name butane.
However, each of these compounds has some properties that are different from the other.
These compounds have different properties because they have different structures. That is
why they are called structural isomers.
Isomers, you should remember, are different compounds with the same
molecular formula. Because these are different compounds, they have to have
Isomers should not be confused with the rotational shapes of molecules. The rotation of
single bonds is very important in the providing alkane molecules with a wide variety of
|Here we have space-filling models of n-hexane. By rotating the
bonds you can see that it can have a variety of shapes.
|The same can be seen using these ball and stick models.
shapes do not constitute different compounds. To get a different compound the bonding
pattern of the atoms would have to change.
One way to create different names for different compounds is to use prefixes.
|In order to distinguish between these two different kinds of butane, the
one which is strung out as a straight chain of carbons can be called normal-butane.
Normal- can be abbreviated by using an n-. This compound
can be called normal-butane or n-butane. The other one
which has a branched chain can be called isobutane.
|C - C - C - C
|C - C - C
The use of prefixes to identify isomers has serious limitations. Let me show you why.
|Let's go back to this list and add the number of isomers possible for each
of these. We just looked at the two structural isomers of butane. There
are three structural isomers for pentane. There are five
possible structural isomers for hexane. There are nine structural isomers
for heptane. As you increase the number of carbons which are being used in a compound, you
continue to increase the number of structural isomers that can exist.
I think you can see that the idea of introducing a new prefix to go in front of the
name of the alkane to identify each different isomer could get to be quite a chore.
Consequently, this method was dropped except for simple cases (such as the butanes and
pentanes), and another way of naming these isomers was developed.
The IUPAC approach is described in the next section.
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