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A Few Organic Chem Questions (Stereochemistry)

by royblaze
Tags: chem, organic, stereochemistry
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royblaze
#1
Sep13-11, 03:38 PM
P: 74
I am currently learning about stereochemistry in my O-chem class. I just have a few questions that I were running through my head. I know there are a lot of questions here, but they're not HW-related at all; my prof. just taught the lecture and I have had these questions on my mind.

1. How are stereogenic centers determined? Are they just locations where enantiomers can exist via "group swapping?"

2. How can one determine a "chiral diastereomer" of a organic molecule? More specifically, what if the molecule is a meso-compound (where a plane of symmetry can exist); how can you find a "chiral diastereomer" if chirality means that it can't super-impose, while the diastereomer means that they are not enantiomers? Is the diastereomer in reference to the original molecule (i.e. diastereomers are just stereoisomers that are not enantiomers with each other)?

3. Is a meso compound really just as simple as locating a plane of symmetry? Can you rotate about single bonds to "force" a meso compounds exist?

Thank you.
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royblaze
#2
Sep14-11, 01:59 PM
P: 74
After much reading, consultation, and model building, I have answered these questions in a way that, for the most part, makes sense for myself.
Borek
#3
Sep14-11, 02:08 PM
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Can you try to summarize you PoV?

royblaze
#4
Sep14-11, 09:24 PM
P: 74
A Few Organic Chem Questions (Stereochemistry)

Okay, here goes, from what I understand:

1) How are stereogenic centers determined? Are they just locations where enantiomers can exist via "group swapping?"

Stereogenic centers are (for the context of o-chem I) carbons where there are four different substituent groups. Stereogenic centers are also "centers" where the substituents could potentially attach in a different orientation (i.e. enantiomers, where the mirror image has different "viewed" orientation of substituent groups). "Group swapping" isn't really a thing; rather, it's just that stereochemistry is observable if two groups could potentially attach in a different orientation (i.e. methyl and ethyl on a single carbon bonded to R and R').

Stereogenic centers cannot exist when two substituents are the same, since the --Prelog thing has ties in priority at those locations.



2) How can one determine a "chiral diastereomer" of a organic molecule? More specifically, what if the molecule is a meso-compound (where a plane of symmetry can exist); how can you find a "chiral diastereomer" if chirality means that it can't super-impose, while the diastereomer means that they are not enantiomers? Is the diastereomer in reference to the original molecule (i.e. diastereomers are just stereoisomers that are not enantiomers with each other)?

A chiral diastereomer is merely just a chiral molecule that is NOT superimposable with OTHER stereoisomers. In accordance to meso-compounds, a chiral diasteromer could just be an enantiomer with a different stereochemistry about a single carbon having different "into the plane, out of the plane" substituents. One thing I am not sure of, though, is that a chiral diastereomer is no longer meso once it becomes chiral (?).



3) Is a meso compound really just as simple as locating a plane of symmetry? Can you rotate about single bonds to "force" a meso compounds exist?

A meso compound requires at least 2 identical substituents on symmetrically located stereogenic centers, as far as I can tell. You can rotate about single bonds to "see" if a meso exists or not.


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