9.2 Chiral and Achiral Molecules

Figure 9.12: Chiral and Achiral Molecules

A chiral molecule is a molecule that contains one or more carbons with four nonidentical substituents. The carbon atom(s) that fit the latter description is / are called (a) chiral / asymmetric center(s).

When a chiral molecule is a mirror image of itself, it is called an enatiomer. Otherwise, it is a diasteriomer.

Note that enantiomers are also called optical isomers. The term “optical isomer” arises from the fact that enantiomers rotate planes of polarzied light in opposite directions.

9.2.1 Chiral centers of glucose

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Figure 9.13: Fischer Projections of Glucose

A Fischer projection is a simple way of indicating chiral molecules by showing their three-dimensional structure in two dimensions without showing all of the wedges and dashes on the chiral centers.

In a typical Fischer projection, horizontal lines on a chiral center represent the wedges and vertical lines represent the dashes.

Note that glucose also has a dexter (i.e., “D”) and a laevus (i.e., “L”) conformation (as seen in figure 9.13) depending on rotation of polarized light.

9.2.2 Aldose and ketose stereoisomers

Figure 9.14: Aldose Stereoisomers

A molecule with \(n\) chiral centers can have \(2^n\) possible stereoisomers. Figure 9.14 displays some stereoisomers of aldoses.

Figure 9.15: Ketose Stereoisomers

Figure 9.15 displays some stereoisomers of ketoses.

9.2.3 Epimers

Figure 9.16: Epimers

An epimer is a sugar that only differs in the configuration around one carbon atom.

For instance, D-glucose and D-mannose in figure 9.16 only differ in their C2 carbon stereochemistries. D-glucose and D-galactose, on the contrary, only differ in their C4 carbon stereochemistries.

9.2.4 In summary…

Figure 9.17: Summary of Stereochemistry

Figure 9.17 summarizes the content that has been mentioned in the previous subsections.