How To Draw And Identify Enantiomers And Diastereomers?

Enantiomers are molecules that are nonsuperimposable mirror images of one another. This the official definition but we will use some tips and tricks to make the concept more understandable! . Please note that in order to have an enantiomer, a molecule has to be chiral in the first place. If you are not familiar with chirality, make sure to review it.


ENANTIOMERS

How to draw enantiomers?

There are two ways to draw enantiomers:

  1. Literally draw the mirror image of a molecule. This is probably the most popular way.

2. Redraw the molecule as it is and switch every chiral center (switch wedges to dashes and dashes to wedges).



How To Identify Enantiomers

Before we begin this discussion, please note that there are chiral molecules that do not have any chiral carbons (carbons attached to four different groups). Therefore, my tricks will apply to most but not all molecules.

  1. Identify chiral carbons in both molecules and assign R and S absolute configuration to them.

  2. Compare the chiral centers in the two molecules. If all of the chiral centers have opposite configuration going from one molecule to another, then those molecules are enantiomers. For example RR on one molecule and SS in another molecule. Make sure to compare the same chiral carbons to one another. For example, let's say there are two chiral carbons in molecule A, one with a bromine and another with a chlorine. Compare the stereochemistry of the carbon with bromine in molecule A to the stereochemistry of carbon with bromine in molecule B.

  3. Watch out for MESO molecules. MESO molecule are achiral and therefore, their mirror images will be the SAME molecule! Forgot how to identify a meso compound? Go HERE


IF a molecule is MESO, it's mirror image will be the same molecule, not enantiomer.

DIASTEREOMERS

Diastereomers are molecules that are nonsuperimposable non mirror images of one another. Again there are always exceptions, but these tips and tricks should help you deal with most of the cases.


How To Identify Diastereomers?


  1. Identify chiral carbons in both molecules and assign R and S absolute configuration to them.

  2. Compare the chiral centers in the two molecules. If at least one chiral carbon switches it's configuration (R/S) and at least one stays the same, then the two molecules are diastereomers.



Let's take a look at these two molecules. Both of them have two chiral center. The top chiral center has the same configuration (R) for both of them while the bottom chiral center is opposite. That means the two molecules are diastereomers.


How to draw diastereomers:

The more chiral centers a molecule has, the more diastereomers it can have. If there are two chiral centers, keep one the same and switch stereochemistry of the other. Then, do the opposite.


Let's say that we want to draw diastereomers of molecule A. Molecule A has two chiral centers. Both molecule B and molecule C are diastereomers of molecule A. In molecule B, the left chiral center stays the same while the right changes and it is the opposite in molecule C.


How To Identify Same Molecules?

Same molecules will have the same configuration (R or S) of the chiral centers. If all the chiral centers match, the two molecules must be same. Also, if we have a meso molecule, it's mirror image will be the same molecule as well.


Now, let's bring it all together.

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