Question
The fact that enantiomers have identical physical and chemical properties prompts a question about racemic mixtures: how can one separate the mixture into its two constituent isomers?
- ____
Answer
The fact that enantiomers have identical physical and chemical properties prompts a question about racemic mixtures: how can one separate the mixture into its two constituent isomers?
- The answer lies in the relationship between enantiomers and diastereomers. Reacting two enantiomers with a single enantiomer of another compound will, by definition, lead to two diastereomers. Imagine, for example, two enantiomers that contain only one chiral carbon; these compounds could be labeled (+) and (–). If each is reacted with only the (+) enantiomer of another compound, two products would result: (+,+) and (–,+). Because these two products differ at some—but not all—chiral centers, they are necessarily diastereomers.
Note: In these solutions, the rotations cancel each other out, and no optical activity is observed. If enantiomerism is analogous to handedness, racemic mixtures are the equivalent of ambidexterity. These solutions possess no handedness overall and will not rotate plane-polarized light.
Note: Rotations measured at different concentrations and tube lengths can be converted to a standardized specific rotation using the following equation:
- where [α] is specific rotation in degrees, αobs is the observed rotation in degrees, c is the concentration in (g/mL), and l is the path length in dm.
Note: A polarizer allows light waves oscillating only in a particular direction to pass through, producing plane-polarized light, as shown in Figure 2.12.
Note: A simple 180° rotation around a vertical axis, as shown in Figure 2.11, allows the compound to be superimposed upon its mirror image.
Note: For example, the C-1 carbon atom in 1-bromo-1-chloroethane has four different substituents. As shown in Figure 2.10, this molecule is chiral because it is not superimposable on its mirror image.
Note: Chirality can also be thought of as handedness. In fact, one of the easiest visualizations of chirality is to think of your own hands, as shown in Figure 2.9. Although essentially identical, your left hand will not be able to fit into a right-handed glove.
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