Structural Isomers
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Structural Isomers

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.
"straight"
chain
C - C - C - C
branched
chain
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 different names.

Rotational Shapes

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 molecular shapes.

Here we have space-filling models of n-hexane. By rotating the bonds you can see that it can have a variety of shapes. Space-filling models of hexane showing different shapes of the molecule. [61modhex1.jpg]
The same can be seen using these ball and stick models.

These different shapes do not constitute different compounds. To get a different compound the bonding pattern of the atoms would have to change.

Ball-and-stick models of hexane showing different shapes of the molecule. [61modhex2.jpg]

 

Common Names

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

normal-butane
n-butane

C - C - C
|
C

isobutane

 

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.
Name Molecular
Formula
Number of
Structural
Isomers
methane CH4 1
ethane C2H6 1
propane C3H8 1
butane C4H10 2
pentane C5H12 3
hexane C6H14 5
heptane C7H16 9
octane C8H18 18
nonane C9H20 35
decane C10H22 75

 

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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