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Topic: question: sterochemistry  (Read 9226 times)

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Offline refid

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question: sterochemistry
« on: May 15, 2007, 06:04:13 PM »


Wondering if the first two images are correct & im unsure of the relationship of the last one.. my guess would be diastereomers as well
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Offline movies

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Re: question: sterochemistry
« Reply #1 on: May 15, 2007, 08:42:28 PM »
Yep!
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Offline Dolphinsiu

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Re: question: sterochemistry
« Reply #2 on: May 16, 2007, 03:21:34 AM »
In my memory,

R,S <-> S,R = identical. meso compund
S,S <-> R,R = enantiomer
« Last Edit: May 16, 2007, 03:43:54 AM by Dolphinsiu »
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Offline sjb

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Re: question: sterochemistry
« Reply #3 on: May 16, 2007, 03:33:51 AM »
Quote from: Dolphinsiu on May 16, 2007, 03:21:34 AM
In my memory,
...
S,S <-> R,R = identical. meso compund

No!

Consider 2-bromo-3-chloropentane. This does not have an internal mirror plane of symmetry and so the (2R,3R) compound does not exist in a meso form.

Additionally, you may want to recall that a meso form suggests a mirror plane, so the 2 configurations will be different (one R, the other S).

See eg http://www.cem.msu.edu/~reusch/VirtTxtJml/sterism3.htm for a fairly decent run through of things.

S
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Offline Dolphinsiu

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Re: question: sterochemistry
« Reply #4 on: May 16, 2007, 03:43:08 AM »
Sorry, I have reversed the situation. Actually I know. Very sorry!
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Offline english

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Re: question: sterochemistry
« Reply #5 on: May 17, 2007, 05:28:11 PM »
Last one is the same as the second one.
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Offline Borek

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Re: question: sterochemistry
« Reply #6 on: May 17, 2007, 06:06:54 PM »
Quote from: g_english on May 17, 2007, 05:28:11 PM
Last one is the same as the second one.

"The first in line is tied to a tree and the last is being eaten by ants." ;)
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Offline sjb

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Re: question: sterochemistry
« Reply #7 on: May 18, 2007, 06:36:40 AM »
Quote from: Dolphinsiu on May 16, 2007, 03:43:08 AM
Sorry, I have reversed the situation. Actually I know. Very sorry!

It's still not 100% correct, your concept would only be true if the substituents on the asymmetric centres were the same.

Going back to 2-bromo-3-chloropentane. This does not have an internal mirror plane of symmetry and so the (2R,3S) compound does not correspond to a meso form.

S
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Offline english

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Re: question: sterochemistry
« Reply #8 on: May 18, 2007, 07:55:39 AM »
What sjb is trying to point out is that you cannot say a compound is meso by merely looking at its name from Cahn-Ingold rules (R-S nomenclature).

You have to look at the actual molecule.



If I have R,R and a separate molecule that is S,S, these could be enantiomers and chiral or identical and achiral.  To know, I would need the structure.
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Offline movies

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Re: question: sterochemistry
« Reply #9 on: May 18, 2007, 06:52:13 PM »
I don't think you could have an R,R (or S,S) molecule that is achiral.  To get a meso stereoisomer, you would need R,S and either an inversion center or a plane of symmetry.
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Offline english

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Re: question: sterochemistry
« Reply #10 on: May 18, 2007, 06:54:48 PM »
Quote from: movies on May 18, 2007, 06:52:13 PM
I don't think you could have an R,R (or S,S) molecule that is achiral.  To get a meso stereoisomer, you would need R,S and either an inversion center or a plane of symmetry.

Ah, right!  Didn't think of that. :P
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