Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: laotree on August 26, 2005, 11:46:08 PM
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R or S
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There are no stereogenic centres in these two molecules ( stereogenic centre = tetrahedral carbon atom to which four different substituents are attached). Therefore, it is pointless to talk of R and S configuration.
However, the two molecules are chiral , i.e. not superimposable to their mirror images.
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Incorrect, the second structure is R.
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Incorrect, the second structure is R.
Yup. Although some people prefer to use different notation for things with an axis of chirality but not a chiral center. R/S is still probably the most common.
And the first one does have a chiral center. It's R. The Z-double bond is higher priority by the CIP system.
Also, stereogenic centers aren't only chiral carbon atoms, the carbons of alkenes can be stereogenic too since the alkene can be E or Z.
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These two are someting easy to ignore or misunderstand for newcomers in organic chemistry.
The first one movies is right. The second one is aixal chiral, could be named by R/S or M/P. There are a lot aixal chirality exist in organic and natural product chemistry, not limited in biphenyl ring!
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How to determine R/S configuration to the 2nd structure?
Z has a higher priority, then, how about R & S?
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How to determine R/S configuration to the 2nd structure?
Z has a higher priority, then, how about R & S?
Same way as usual. The Z alkene has 1st priority, the E alkene is second, the methyl is third.
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For the second structure look down the longest side of the molecule. Look at the attachments sticking out on the sides and see if they're attachments are attached spirally clockwise or counter-clockwise. Clockwise will give you R and counterclockwise will give you S.
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For the second structure look down the longest side of the molecule. Look at the attachments sticking out on the sides and see if they're attachments are attached spirally clockwise or counter-clockwise. Clockwise will give you R and counterclockwise will give you S.
Did I illustrate it correctly?
1. Look at the molecule from the axis shown by the figure or from opposite side.
2. Assign the highest priority to the substituent (isopropyl group) which is closest to us.
3. If the substituents are attached spirally clockwise according to priority, then it is (R) configuration.
Am I right?
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perfect
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Same way as usual. The Z alkene has 1st priority, the E alkene is second, the methyl is third.
About the 2nd question, I mean when two of the substituents which are R/S relationship to each other, which one has a higher priority, R or S?
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From Winga's illustration....
Why is 1st substituent a higher priority group than 2nd substituent???
They look the same to me, they are both propal groups???
Or does it have to due with the orientation of those groups when looking down the line of sight in Winga's drawing???
This is the part I dont like about Organic...it is hard to picture, But a nice illustration by Winga..
Winga------> What program did you draw that with???
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From Winga's illustration....
Why is 1st substituent a higher priority group than 2nd substituent???
They look the same to me, they are both propal groups???
Or does it have to due with the orientation of those groups when looking down the line of sight in Winga's drawing???
This is the part I dont like about Organic...it is hard to picture, But a nice illustration by Winga..
Winga------> What program did you draw that with???
About the priority of substituents, I illustrated that the one closer to our eyes will have a higher priority.
I used ChemDraw to draw the organic molecules and edited it by Macromedia Flash.
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Could someone describe about M/P configuration?
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Winga, do you have an example of where you would have to pick between an R and an S sidechain? I'm trying to think of an example where the compound wouldn't be meso because of a plane of symmetry.
Could someone describe about M/P configuration?
M is like R, P is like S. For axial chirality you also sometimes see aR and aS instead of just R and S.
With respect to Blueshawk's question, Winga is correct. The substituents on the "front" ring are automatically given higher priority than those on the "back" ring. So, the isopropyl groups are priority 1 and 3, respectively. The H atoms on the other side of the rings are 2 and 4, respectively.
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Winga, do you have an example of where you would have to pick between an R and an S sidechain? I'm trying to think of an example where the compound wouldn't be meso because of a plane of symmetry.M is like R, P is like S. For axial chirality you also sometimes see aR and aS instead of just R and S.
With respect to Blueshawk's question, Winga is correct. The substituents on the "front" ring are automatically given higher priority than those on the "back" ring. So, the isopropyl groups are priority 1 and 3, respectively. The H atoms on the other side of the rings are 2 and 4, respectively.
Xylitol has R and S side chains but it's also a meso compound due to a plane of symmetry.
If both are either R or S, then it's chiral.
I'm not quite sure what example you are thinking of. You mean a compound containing R and S side chains without a plane of symmetry?
About assigning the priority of the 2nd structure, what H atoms we need to count?
Is it only the H atoms opposite to isopropyl groups?
As you said isopropyl groups are priority 1 and 3 while H atoms are 2 and 4, should we follow the path 1 --> 2 --> 3 --> 4? Or 1 --> 3 then, 2 --> 4?
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I was trying to understand what you meant by your earlier question:
About the 2nd question, I mean when two of the substituents which are R/S relationship to each other, which one has a higher priority, R or S?
For your other question, the H atoms in question are at the other ortho position to the biphenyl linkage (meta to the isopropyl groups). For determining the R/S configuration, you go 1 --> 3 --> 2 --> 4, clockwise is R, counterclockwise is S.
Or, if you prefer: isopropyl in front --> isopropyl in back --> H in front --> H in back
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I was trying to understand what you meant by your earlier question:For your other question, the H atoms in question are at the other ortho position to the biphenyl linkage (meta to the isopropyl groups). For determining the R/S configuration, you go 1 --> 3 --> 2 --> 4, clockwise is R, counterclockwise is S.
Or, if you prefer: isopropyl in front --> isopropyl in back --> H in front --> H in back
Aren't isopropyl groups also at the ortho postion to the biphenyl linkage? ???
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Yes. I'm talking about the other ortho position where there are hydrogens.
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Big discuss! Just what we need!
Thanks, Guys!