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Topic: Determining Stereochemical Relationships among compounds  (Read 3352 times)

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Offline vivekfan

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Determining Stereochemical Relationships among compounds
« on: October 17, 2009, 05:21:59 PM »
I have repeatedly read my stereochemistry chapter, and I understand how to draw a particular relationship if I'm asked, but if I'm given two compounds, I am having trouble figuring out what relationship they share: enantiomers, diastereomers, identical, or conformational.

Since I can't scan this document, I'm going to try and describe the compounds. I tried assigning R/S to each of the chiral centers in the compound and comparing them, but I think that only works when you know which relationship you want.

a) There is one compound with two carbons, and to the left carbon there is an H in the plane of the page,  methyl going into the page, and bromine coming out. On the right carbon, there is Bromine in the plane of the page, H going into the page and methyl coming out of the page.

The other compound listed next to it has two carbons. The left carbon has a methyl in the plane of the page, H going into the page, and Bromine coming out. The other carbon of this compound has methyl in the plane of the page, H going into the page, Bromine coming out.

What is the relationship between the two? I tried assigning R/S to the compounds and since one compound had R/S and S/R, I thought they were enantiomers, but the answer key said it was conformational. How do I approach these types of problems.

b) Then there was one compound that has two carbons. The left one has methyl going into the page, Bromine in the plane, and H out. The right one has bromine in the plane, H going in and methyl coming out.

The other compound has the left carbon with H in the plane, Bromine in, methyl out of the page. The right carbon has methyl in the plane, bromine in, H out.

what is the relationship? I am confused on how to approach these. Any tips or suggestions would be greatly appreciated!  Thanks in advance.

Offline Rabn

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Re: Determining Stereochemical Relationships among compounds
« Reply #1 on: October 17, 2009, 07:44:00 PM »
I would suggest you build the molecules with your modeling kit.  There is no substitute for building and handling the molecular models in order to understand stereochemical relationships.

Offline Dan

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Re: Determining Stereochemical Relationships among compounds
« Reply #2 on: October 18, 2009, 07:31:09 AM »
a).
What is the relationship between the two? I tried assigning R/S to the compounds and since one compound had R/S and S/R, I thought they were enantiomers, but the answer key said it was conformational. How do I approach these types of problems.

Ah, in this case it sounds like you missed the symmetry in these compounds - which are in fact the same. (2R,3S)-2,3-dibromobutane is a meso compound, it is exactly the same as (2S,3R)-2,3-dibromobutane, and I presume diagrams given depicted different conformers. You have too keep an eye out for meso compounds in these questions.

Quote
b) Then there was one compound that has two carbons. The left one has methyl going into the page, Bromine in the plane, and H out. The right one has bromine in the plane, H going in and methyl coming out.

The other compound has the left carbon with H in the plane, Bromine in, methyl out of the page. The right carbon has methyl in the plane, bromine in, H out.

what is the relationship? I am confused on how to approach these. Any tips or suggestions would be greatly appreciated!  Thanks in advance.

You could start by assigning R/S. Alternatively you can redraw them in similar conformations to make the differences easier to see.

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