What are Diastereomers? - Definition, Examples & Separation

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  • 0:04 Different By Space
  • 0:50 Diastereomers
  • 2:01 Examples
  • 3:17 Separation of Diastereomers
  • 3:46 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 will be on a specific type of stereoisomer called a diastereomer. Our primary points of discussion will include how they are defined, specific examples, and how they are separated from one another.

Different By Space

What if you found two rocks and you weighed them on a balance and they had the exact same mass? Then when you determined what they were made of, you found out they contained the exact same mineral compositions. But, let's suppose that even though they had the same mass and were made of the exact same things, they had two very different shapes. If your friend tried to convince you that your rocks were exactly the same would you agree?

Of course not, they're two completely different shapes right?! Did you know that organic molecules sometimes have the exact same problem? Sometimes we can have two molecules with the same mass, the same chemical make-up, but they have totally different shapes. Today we're going to be talking about diastereomers, a topic within the broad context of stereoisomerism, which deals with the three-dimensional orientation of molecules.

Diastereomers

Let's get started by talking about the definition of diastereomers. Diastereomers are stereoisomers with two or more organic compounds that have at least two stereocenters with different configurations at some of the stereocenters but the same configuration at others. These configurations are not mirror images of one another. Stereoisomers are compounds that have the same chemical formula, the same atom connectivity, but a different three-dimensional orientation or shape. A stereocenter is an atom (usually carbon) within a molecule that contains four different atoms or groups of atoms bonded to it.

Consider as an example a simple carbon atom that has a hydrogen, fluorine, bromine, and hydroxyl (-OH) groups bonded to it. This carbon would be classified as a stereocenter since it indeed has four different groups bonded to it.


An asymmetric carbon atom is bonded to four different substituents
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When considering whether a pair of molecules is related as being diastereomers of one another, it's important to examine each stereocenter within both molecules. If all of the stereocenters within two compounds are exactly opposite of one another they are simply mirror images of one another, making them not diastereomers. If some of the stereocenters within one of the compounds are the opposite in configuration but others are the same, these compounds are diastereomers.

Examples

Although understanding the definition of diastereomers is important, it's even more valuable to examine some example cases. What if a compound contained both a chlorine and a bromine atom but they differed in the way they occupied three-dimensional space.


These compounds are diastereomers of one another
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Notice that in this diagram one of the compounds has chlorine going away from us into the page (it has a dashed bond) and the bromine is also going into the page. On the other compound, the bromine is the same as the first in that it's going into the page but notice how the chlorine now is a solid wedged bond coming out at us. Since the compounds are different in the configurations of the chlorine atoms but the same with the bromine atoms, they are diastereomers. Notice also how they are not mirror images of one another.

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