What is an Epimer? - Definition in Chemistry & Examples

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  • 0:04 Almost the Same
  • 0:40 Definition of an Epimer
  • 1:50 Examples of Epimers
  • 3:30 Lesson Summary
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Lesson Transcript
Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The focal point of this lesson is the concept of epimers in chemistry. We'll take a look at the definition of an epimer followed by a close look at some specific examples.

Almost the Same

Have you ever known a set of identical twins? Identical twins can be hard to tell apart since they share virtually all of their physical characteristics. A lot of times however, the twins can be separately identified by a subtle difference that can be found once you get to know them and spend some time around them.

We can extend this concept to organic compounds in some respects, and the idea of identical twins that aren't quite the same is a good lead in to our current lesson. Today we are going to be learning about something called epimers by studying their definition, and then taking a detailed look at some specific examples as a reinforcement. Let's get started!

Definition of an Epimer

First, we need to know that stereoisomers are pairs of compounds that have the same chemical formula and the same atom connectivity, but they differ in their three-dimensional orientation. Imagine if you had two shapes, a triangle and a square, that both had the same mass and were both made out of the same kind of wood. They're the same in the sense that they both weigh the same and they're made from the same materials, but they're two different three-dimensional shapes.

Epimers are a specific type of stereoisomer that have multiple stereocenters, but only differ from one another by the configuration at one of the stereogenic centers. A stereocenter or stereogenic center is a carbon atom that has four different atoms or groups of atoms bonded to it. Consider as an example the molecule called ephedrine, which is used to combat low blood pressure and treat asthma.


Notice how the carbon atom labeled number one is bonded to a hydrogen, an alcohol group, a benzene ring, and another carbon atom. Similarly, the carbon atom labeled number two is bonded to a hydrogen, a nitrogen, a methyl group, and another carbon atom. Both of the atoms labeled one and two are classified as stereocenters since they contain four different groups bonded to them.

Examples of Epimers

Let's take a look at some specific examples of epimers to get a better understanding of how to recognize them. Glucose is a common sugar that circulates through our bloodstreams and is sometimes referred to as blood sugar. There are two forms of glucose, and the two forms differ from one another only in the relative configuration of a single -OH group.


In the alpha version of glucose, the -OH group, highlighted in red, is pointing down, and in the beta version the same -OH group is pointing up and to the right. Notice how all of the other -OH groups in glucose are exactly the same regardless of if it's the alpha or beta form. They only differ from one another at that single stereocenter - that's what makes them epimers!

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