What is the Difference Between Enantiomers & Diastereomers?

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  • 0:03 Enantiomers & Diastereomers
  • 1:14 Understanding Enantiomers
  • 2:46 Understanding Diastereomers
  • 3:20 Enantiomers vs. Diastereomers
  • 4:16 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Enantiomers and diastereomers are types of stereoisomers. In this lesson, we'll learn what the difference is between these types of stereoisomers and how to differentiate between them.

Enantiomers & Diastereomers

Take a look at your hands - they are non-superimposable mirror images of each other. This means that they are mirror images of each other, but you can't stack them on top of each other and have them line up exactly the same.

Your hands are enantiomers of each other. This is because while your hands have the same connections (or fingers), they're attached in a different order. But, they're attached in the exact opposite order of each other, making them mirror images.

Now, imagine that your fingers got mixed up on your left hand, and now its thumb comes between the middle finger and the ring finger. Both hands still have the same connections (or fingers) but they are in a different order, just as with enantiomers. But, the difference is that your hands are no longer mirror images of each other. This would make them diastereomers.

So, stereoisomers (chemicals with the same connections, but different orientations) are broken into two main categories: enantiomers and diastereomers.

  • An enantiomer is a stereoisomer that is a non-superimposable mirror image of each other
  • A diastereomer is a stereoisomer with two or more stereocenters, and the isomers are not mirror images of each other

Understanding Enantiomers

In order for two molecules to be enantiomers, every single stereocenter needs to be in the opposite orientation of each other. Stereocenters can be given an 'R' or an 'S' designation, so in a molecule that has three stereocenters, with all of them in the 'R' designation, then the enantiomer will need to have all of the stereocenters in the 'S' designation.

If the A molecule has two stereocenters, the first in the 'R' orientation and the second in the 'S' orientation, then the enantiomer will need to have the first stereocenter in the 'S' orientation and the second in the 'R' orientation. Every stereocenter needs to be in the opposite direction.

Sugar molecules are named based on the 'R' or 'S' orientation of the last stereocenter. If the last stereocenter has the OH on the right in the Fischer Projection, or in the 'R' orientation, then the entire molecule is labeled as a 'D' sugar. If it is on the left, then the entire molecule is labeled as a 'L' sugar.

For example, there are two forms of glucose (the most common sugar molecule), D-glucose and L-glucose. In nature you'll almost never find L-glucose. They're both glucose molecules though, because it isn't only that last stereocenter that has been switched around: every stereocenter is on the opposite side, making them enantiomers of each other:

D and L glucose

You can see that the orientation of D-glucose is: R, S, R, R, while the orientation of L-glucose is: S, R, S, S. These are exact opposites of each other, as seen here:

Orientation of stereocenters

Understanding Diastereomers

If a molecule has more than one stereocenter and every single stereocenter isn't in the opposite direction then they are not enantiomers, but diastereomers. In other words if two stereoisomers are not enantiomers, then they are diastereomers.

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