# Diastereomers

Stereochemistry: Meso Compounds, Diastereomers

Tip: This isn't the place to ask a question because the teacher can't reply.

## Key Questions

• Diastereomers are a type of a stereoisomer. Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.

Also when two diastereoisomers differ from each other at only one stereocenter they are epimers.

n/aDiastereisomers are optically active because their compounds are also optically active.

#### Explanation:

Well, I don't quite like the question itself. It's like asking why people are short.

Well, not all people are short. Some individuals are short, whereas others are tall, and some are of intermideate height. You can ask why short people are short but you can't ask why all people are short.

So, returning to your question, you can ask why some compounds are optically active whereas others are not.

Then you can go with this question further:

If the compound is optically active its diastereisomers are also optically active (short people are short).

Ernest Z. wrote really great answer to your question but meso compounds are not considered as diastereisomers. (Well, It's about how different textbooks interprete it but as far as I remember the compounds are not diastereisomers). Anyways, I hope you've got the idea.

http://www.chem.ucla.edu/harding/ec_tutorials/tutorial72.pdf

Ask me if you need to know how to determine optical activity.

Epimers are always diastereomers.

#### Explanation:

Diastereomers are compounds that contain two or more chiral centres and are not mirror images of each other.

For example, the aldopentoses each contain three chiral centres.

Thus, D-ribose is a diastereomer of D-arabinose, D-xylose, and D-lyxose.

Epimers are diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center.

Thus, D-ribose and D-arabinose are epimers (and diastereomers), because they differ in configuration only at $\text{C-2}$.

D-ribose and D-xylose are epimers (and diastereomers), because they differ in configuration only at $\text{C-3}$.

D-ribose and D-lyxose diastereomers, but they are not epimers, because they differ in configuration at both $\text{C-2}$ and $\text{C-3}$.

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