Stereoisomers

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A few things that I wanted to confirm:

Enantiomers-identical physical properties (except the rotation of plane polarized light) and identical chemical properties (except when reacting with other chiral molecules).

Diastereomers: different chemical and physical properties.

Meso compounds: according to TPR, .meso compounds have an internal mirror plane, have an enantiometer that is identical to it, and do not rotate plane-polarized light. Also, a molecule with an odd number of chiral centers cannot be meso.

However, wikipedia says that diastereomers include meso compounds?

http://en.wikipedia.org/wiki/Stereoisomer#Diastereomers

(scroll down to Diastereomers-its in the first paragraph). How can meso compounds have an enantiomer, and yet also be classified as a diastereomer?
.
 
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I believe what's being said is that meso compounds are a subcategory to diastereomers.

Everything you said is right.
 
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I think you're misreading.

Take compound A. It has four chiral centers, S,S,S,S.

R,R,R,R would be the enantiomer of compound A.

A plethora of compounds would be the diastereomers of compound A.

One of these diastereomers is S,R,S,R. This compound happens to be a meso compound.

The enantiomer of this meso compound is R,S,R,S, which is equivalent to the meso compound itself.
 
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loveoforganic said:
I think you're misreading.

Take compound A. It has four chiral centers, S,S,S,S.

R,R,R,R would be the enantiomer of compound A.

A plethora of compounds would be the diastereomers of compound A.

One of these diastereomers is S,R,S,R. This compound happens to be a meso compound.

The enantiomer of this meso compound is R,S,R,S, which is equivalent to the meso compound itself.
Click to expand...

Ohh, okay. So, the R,S,R,S and the S,R,S,R-would they technically be enantiomers, and NOT the same compound, right?
 
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They would be the same compound. Draw out 1-S,3-R-dimethylcyclohexane. Now draw out 1-R,3-S-dimethylcyclohexane.

I was having a hard time getting that straight for a sec too, but that example solidified it for me.
 
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loveoforganic said:
They would be the same compound. Draw out 1-S,3-R-dimethylcyclohexane. Now draw out 1-R,3-S-dimethylcyclohexane.

I was having a hard time getting that straight for a sec too, but that example solidified it for me.
Click to expand...

Got it, thanks!
 
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I was always told that the term enantiomers meant "non-superposable mirror images." Meso compounds are always superposable on their mirror images and can not be considered enantiomers under this definition.

Oh and check out this meso compound with an odd number of chiral carbons....
 

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loveoforganic said:
The central carbon (the one attached to bromine) isn't chiral.
Click to expand...
Yes it is, check this link out:
http://en.wikipedia.org/wiki/Xylitol
Notice how there's 3 stereochemical indications (2R,3r,4S), kind of pointless to have the middle term if it's not a stereocenter.
The carbon is chiral but the molecule as a whole is not. If you were to switch the hydrogen and the bromine you would get a different stereoisomer.
Also notice how the two other substituents are not identical but are enantiomeric, if you were to switch the configuration of only one of the two larger substituents the middle carbon would no longer be chiral.
 
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wanderer100 said:
H, Br,CHFCH3 (R), and CHFCH3(S)
Click to expand...

Hey, the last two are the same. When considering "different substituents", we take the actual atoms/structure of the substituent into consideration, not its chirality.
 
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pookiez88 said:
Hey, the last two are the same. When considering "different substituents", we take the actual atoms/structure of the substituent into consideration, not its chirality.
Click to expand...
You sure about that 😉?
Try using a molecular model kit, and try superimposing the molecule that I posted with a molecule that is identical but the positions for the B and for the H are switched......
 
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wanderer100 said:
You sure about that 😉?
Try using a molecular model kit, and try superimposing the molecule that I posted with a molecule that is identical but the positions for the B and for the H are switched......
Click to expand...

I meant they are the same in terms of molecular structure. When assigning priorities, we consider the molecular weight of the substituents. CHFCH3 is the same as CHFCH3. We don't consider the chirality of the substituents when assigning priorities...
 
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pookiez88 said:
I meant they are the same in terms of molecular structure. When assigning priorities, we consider the molecular weight of the substituents. CHFCH3 is the same as CHFCH3. We don't consider the chirality of the substituents when assigning priorities...
Click to expand...
We usually don't because we rarely encounter it. However it can serve as a "tiebreaker." BTW according to the link I posted to wiki the stereochem indication for xylitol is (2R,3R,4S), why is it 3R if there's no stereocenter?


(It should actually be "3r" not "3R" but that is relatively unimportant)
 
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wanderer100 said:
We usually don't because we rarely encounter it. However it can serve as a "tiebreaker." BTW according to the link I posted to wiki the stereochem indication for xylitol is (2R,3R,4S), why is it 3R if there's no stereocenter?


(It should actually be "3r" not "3R" but that is relatively unimportant)
Click to expand...

Hey, where on the wiki page does it indicate the chiral carbons (2R,3R,4S)? I can't seem to find it 😕
 
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Interesting...I've never heard of that before. But how can that molecule be chiral if it doesn't have 4 different substituents?
 
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pookiez88 said:
Interesting...I've never heard of that before. But how can that molecule be chiral if it doesn't have 4 different substituents?
Click to expand...
The carbon can be chiral because there are 4 different substituents. It might be hard to understand without having a model kit in front of you, but if you can't superimpose one substituent on another, what looks like 2 identical substituents are in fact different.
The molecule is not chiral, it is meso, and there are 2 alternative meso forms, each of which is optically inactive.
 
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wanderer100 said:
The carbon can be chiral because there are 4 different substituents. It might be hard to understand without having a model kit in front of you, but if you can't superimpose one substituent on another, what looks like 2 identical substituents are in fact different.
The molecule is not chiral, it is meso, and there are 2 alternative meso forms, each of which is optically inactive.
Click to expand...

Okay, do we normally see this in fischer projections??? Should I consider that only in fischer projections, or with regular ball and stick/straight chain models as well? This is probably beyond the MCAT though, right, since it would be a bit hard to visualize that without a model kit.
 
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pookiez88 said:
Okay, do we normally see this in fischer projections??? Should I consider that only in fischer projections, or with regular ball and stick/straight chain models as well? This is probably beyond the MCAT though, right, since it would be a bit hard to visualize that without a model kit.
Click to expand...
Well I can see it in the Fischer, it's not that hard to see it.....
I'd say it's beyond the scope of the MCAT, and is definitely hard to visualize if you never seen this kind of thing before (i.e. 99% of test takers).

The top set of molecules can not be superimposed upon each other, because the two substituents are different (R and S)

The bottom set can be superimposed on each other, because the substituents are the same (just rotate one of the molecules 180 degrees).
 

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wanderer100 said:
Well I can see it in the Fischer, it's not that hard to see it.....
I'd say it's beyond the scope of the MCAT, and is definitely hard to visualize if you never seen this kind of thing before (i.e. 99% of test takers).

The top set of molecules can not be superimposed upon each other, because the two substituents are different (R and S)

The bottom set can be superimposed on each other, because the substituents are the same (just rotate one of the molecules 180 degrees).
Click to expand...

Okay, thanks!
 
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Xylitol...So it's meso, but isn't it optically active? Aren't mesos supposed to be optically inactive?
 
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ratatat said:
Xylitol...So it's meso, but isn't it optically active? Aren't mesos supposed to be optically inactive?
Click to expand...
Xylitol does not have a left hand and a right hand version (it has no enantiomers and is achiral) so it is optically inactive.


When I say that it has no enantiomers I mean that it has no non-superimposable mirror images, since it is its own mirror image.
 
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