Stereochem

HowAboutDAT said:

Can someone put the meaning of diastereomers, enantiomers, meso compounds, constitutional isomers and anything else of that sort in layman's terms? Thanks!

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I'll do the easiest one.

Enatiomer - Non-superimposable mirror images. They are molecules that have the exact same formula but they're arranged differently. Meaning if they were placed side-by-side you couldn't twist them in anyway that would get them to perfectly overlap.

Example:

Meso Compounds: These are compounds that have internal plane of symmetry. Consider the following image. Notice how the bottom half is the mirror image of the top half. Meso compounds are not optically active. Why?
Just write the configuration of the two chiral centers (Designated with * in the following image). The top chiral center has an S-configuration, while the bottom chiral center has an R-configuration. In conclusion, these two chiral centers rotate light the same number of degrees but in opposite direction. The end result is a compound that has no optical activity.


Diastereomers: Are compounds that have the same configuration at one or more chiral centers, but opposite configuration at others. Consider the following image. Notice that there are two diastereomers for the given compound. Let's see if the above definition applies:
-Compare the lower left compound to the top compound--The bottom chiral center has the exact configuration as that of tthe top compound, but the top chiral center has an opposite configuration (Notice how the location of H and OH are switched). Since this compound has the same configuration at the bottom chiral center and the opposite configuration at the top chiral center it qualifies as an diastereomer.

-Compare the lower right compound to the top compound--The top chiral center has the exact configuration as that of tthe top compound, but the bottom chiral center has an opposite configuration (Notice how the location of H and OH are switched). Since this compound has the same configuration at the bottom chiral center and the opposite configuration at the top chiral center it qualifies as an diastereomer.

In the above post, notice that each of the two diastereomers are considered enantiomers of each other. So, what is the difference between diastereomers and enantiomers?
Remember we said that diastereomers have the same configuration at one or more chiral centers, but opposite configuration at others.
Enantiomers on the other hand have opposite configuration at ALL chiral centers.

nice nze! well done.

question tho... say you have a cis 1,2 dichlorocyclopentane. That's a meso compound and its optically inactive.

What about a trans 1,2 dicholorcyclopentane. There is also a plane of symmetry there, correct? Does that make it optically inactive as well?

pbure9 said:

nice nze! well done.

question tho... say you have a cis 1,2 dichlorocyclopentane. That's a meso compound and its optically inactive.

What about a trans 1,2 dicholorcyclopentane. There is also a plane of symmetry there, correct? Does that make it optically inactive as well?

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First part is tru. 2nd false. check out teh attachment. its hsould be optically active.

Sublimation said:

First part is tru. 2nd false. check out teh attachment. its hsould be optically active.

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Srry it didnt upload the image here it is.

The Trans Isomer does NOT have an internal plane of symmetry. Think about it; One Cl group is above the ring and the other is below the ring; whereas, if you put a mirror in between you should either see both groups above the ring or both groups below the ring, which is the case for Cis Isomers. Since the Trans Isomer does not have an internal plane of symmetry, it can NOT be a meso compound. So, it's optically active! You can double check this, by determining the configurations at each of the chiral centers. Both chiracl centers will either be R or both will be S. So, they're both rotating light the same number of degrees in the SAME direction. Thus, they won't cancel each other out, which means the compound will be optically active.

Hope this clarifies it!

pbure9 said:

nice nze! well done.

question tho... say you have a cis 1,2 dichlorocyclopentane. That's a meso compound and its optically inactive.

What about a trans 1,2 dicholorcyclopentane. There is also a plane of symmetry there, correct? Does that make it optically inactive as well?

Click to expand...

HowAboutDAT said:

Thanks for the explanation guys... Just to sum things up, Lets say you have a compound with two chiral centers - 1R, 2R

The enantiomer of this compound is 1S, 2S

The diastereomers of this compound are 1R, 1S and 1S, 2R?

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Correct! Except the first diastereomer should be 1R,2S (I'm pretty sure you knew it; you just typed it wrong)

sweet, i was wondering what it was. Thanks nze and sublimation!

HowAboutDAT said:

awesome, thanks. yeahh... I was just giving a random example actually. While we're on the stereochemistry topic, just as practice, in the two examples posted by razblo, the first one is S and the second is R, correct?

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Correct!