Organic Chemistry MCAT Practice Exam

What is the relationship between two enantiomers?

1. They differ at one chiral center

2. They are non-superimposable mirror images

3. They are identical in all properties

4. They have different molecular masses

The correct answer is: They are non-superimposable mirror images

Enantiomers are a specific type of stereoisomers that are characterized by having the same chemical formula and connectivity of atoms but differ in the spatial arrangement of those atoms. The defining feature of enantiomers is that they are non-superimposable mirror images of each other. This means that if you were to place one enantiomer over the other, you wouldn't be able to entirely overlap all parts of the molecules due to their distinct three-dimensional shapes. This non-superimposable relationship arises because enantiomers contain at least one chiral center, leading to two different orientations for the substituents around that center. Therefore, while enantiomers possess the same physical and chemical properties in a symmetrical environment (such as melting and boiling points), they can exhibit significantly different behaviors in the presence of other optically active substances, including biological systems. Thus, the correct choice accurately captures the essence of the enantiomer relationship, emphasizing that they cannot be superimposed upon each other, making them distinct despite their similarities. The other options describe characteristics that do not accurately represent the definition of enantiomers.