Organic Chemistry MCAT Practice Exam

When comparing diastereomers, what is true about their optical activity?

1. They have identical optical rotation

2. They possess related optical activities

3. They can have unrelated optical activity

4. They are always optically inactive

The correct answer is: They can have unrelated optical activity

Diastereomers are stereoisomers that are not related as mirror images, which distinguishes them from enantiomers. Due to their distinct configurations at one or more stereogenic centers, diastereomers can exhibit different physical properties, including varying levels of optical activity. The correct assertion is that diastereomers can have unrelated optical activity. This means that the specific optical rotation values for diastereomers can differ significantly from each other. One diastereomer may rotate plane-polarized light to the right (positive optical rotation) while the other may rotate it to the left (negative optical rotation), or they could both have different magnitudes of rotation. This variability arises because the presence of multiple stereogenic centers can affect the spatial arrangement of the molecule and, consequently, its interaction with light. In contrast, enantiomers, which are stereoisomers that are non-superimposable mirror images, will always have equal but opposite optical rotation values. Therefore, when looking at pairs of diastereomers, the diversity in their configurations leads to the possibility of their optical activities being completely different or unrelated.