For a molecule to have optical activity, it must ____

For a molecule to have optical activity, it must not only have chiral centers within it, but must also lack a plane of symmetry.

Note: A molecule with chiral centers that has an internal plane of symmetry is called a meso compound, an example of which is shown in Figure 2.14.

- As shown in this image, D- and L-tartaric acid are both optically active, but meso-tartaric acid has a plane of symmetry and is not optically active. This means that even though meso-tartaric acid has two chiral carbon atoms, the molecule as a whole does not display optical activity. Meso compounds are essentially the molecular equivalent of a racemic mixture.

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.

Note: In this image, one can see that I and II are mirror images of each other and are therefore enantiomers of each other. Similarly, III and IV are enantiomers. However, I and III are not. These are stereoisomers that are not mirror images and are thus diastereomers. Notice that other combinations of non-mirror-image stereoisomers are also diastereomers: I and IV, II and III, and II and IV.

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.

More cards in Chapter 2: Isomers