Organic Chemistry MCAT Practice Exam

What is an epimer?

1. A type of geometric isomer

2. A subset of diastereomers differing at one chiral center

3. A mirror image of a chiral molecule

4. A type of enantiomer

The correct answer is: A subset of diastereomers differing at one chiral center

An epimer is specifically a type of diastereomer that differs from another compound in configuration at just one of its multiple chiral centers. This definition is crucial because the distinguishing feature of epimers is this singular difference, which leads to distinct physical and chemical properties between the two molecules. For instance, in carbohydrate chemistry, glucose and galactose are epimers; they differ only in the configuration around one specific carbon atom (C4), while the configurations of the other chiral centers remain the same. This concept is significant in organic chemistry as it helps categorize stereoisomers based on their relationships and reactivity. This understanding is why the other answer choices do not fit the definition of an epimer. For example, geometric isomers relate to different orientations in space around a double bond, which is not relevant to epimers. Similarly, mirror images of chiral molecules represent enantiomers, while epimers do not necessarily exhibit this property, as they are not mirror images of each other, but rather variations at just one stereocenter.