Organic Chemistry MCAT Practice Exam

What are enantiomers characterized by?

1. Nonsuperimposable mirror images

2. Identical properties, different optical rotation

3. Superimposable mirror images

4. Identical optical rotation

The correct answer is: Nonsuperimposable mirror images

Enantiomers are specifically defined as a pair of molecules that are mirror images of each other and cannot be superimposed on one another, much like how a person’s left and right hands relate to each other. This characteristic of being nonsuperimposable mirror images is fundamental to the concept of chirality in organic chemistry. Each enantiomer will have distinct spatial arrangements of atoms, which leads to differing interactions with polarized light, typically resulting in different optical rotation. While it is true that enantiomers have identical physical properties in terms of boiling point, melting point, and solubility when in a chiral environment, a significant distinction is their different optical activities. Therefore, the correct characterization of enantiomers emphasizes their nonsuperimposable nature, which is what sets them apart from other stereoisomers that may or may not possess chirality.