Structural Isomerism

STRUCTURAL ISOMERISM

There are three main structural isomers, which are chain isomers, position isomers, and functional group isomers. Structural isomerism can lead to a diverse number of these types of isomers so that, if there are 40 carbon atoms to a molecule, there would be about 62 billion possible structural isomers.

In chain isomers, the main carbon skeleton is not the same between the isomers. This can involve branched chains or continuous chains of carbon atoms. Figure 68 shows what structural isomers look like:

Figure 68.

Position isomers are based on the position of a functional group. The example in figure 68 is an example of position isomers. The name is going to be slightly different because of the position of a methyl group, an ethyl group, etcetera.

Functional isomers differ according to the type of functional group added to the chain. The chain can go from an alkane, for example, to a cycloalkane with the same number of atoms. There can be different functional groups that make up the different structures. With functional isomers, the structure differs only in the type of functional group added to the molecule.

Stereoisomerism

There are two types of stereoisomerism: geometric isomerism and optical isomerism. These aren’t rearrangements of molecules in 2 dimensions; rather, it is threedimensional changes in molecules in space. Because they look the same in 2D, they are more difficult to describe. Figure 69 shows what a stereoisomer of an organic substance looks like:

Figure 69.

Geometric isomerism is actually a term that isn’t used in the IUPAC system. Instead, it is referred to as cis-trans isomerism. It most commonly involves carbon double bonding. These bonds are different from single carbon bonds because they cannot rotate as freely as single bonds. This allows two different molecules to be made, depending on the side chains and functional groups added to them. A Cis-isomer or Z-isomer has the functional groups on the same side of the double bond, while the trans-isomer or Eisomer has the functional group on the opposite side of the double bond. This is better pictured in figure 70:

Figure 70.

Optical isomers are what are described in figure 71. These come in pairs and look like mirror images in a sense. They aren’t completely superimposable but are more like the left hand and the right hand, which aren’t directly mirror images of one another. These are lettered as S-isomers (meaning “left” in Latin or “sinister”) or R-isomers (meaning “right” in Latin or “rectus”). The letter is put in front of the name in order to describe which one it is. They are also referred to as enantiomers. Figure 71 shows the Senantiomer and R-enantiomer of a simple carbon molecule:

Figure 71.

Isomers of all kinds are important to chemistry because they have different chemical and physical properties. This applies even to enantiomers, which have very similar