Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rutherford on February 20, 2014, 01:30:46 PM
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How many stereoisomers of cefalotin exist?
I'd say 4: 2*2 for stereocenters.
But do the two nitrogen count as stereocenters?
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4 unless I am missing something!
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I am not sure about the nitrogen atoms, they are tetrahedrons and have three different substitutes + one electron pair. Is the isomerization between R and S slow enough so they can be regarded as different compound, I don't know.
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Since we are discussing stereo chemistry.
How many isomers are there of this compound? I think there are six. 3 stereo centers. So six isomers. Am I wrong?
Am I missing something with the epoxide?
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I'd go with six.
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I see 6, I don't think you are missing anything.
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Ok cool. I was doubting myself for some reason.
Also, when it comes to nitrogen being considered a stereocenter, what are the conditions for it to be one? I know in most biological compounds only carbon is considered. When do you consider nitrogen?
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Usually the inversion of an amine is very fast. Put some really bulky groups on it and you can slow it down so that you can see the isomers in the NMR time scale. It's a bit like chairs interconverting, at low temp you see axial and equatorial protons at RT you just see one big signal in the NMR.
But yes you can have chiral nitrogen atoms.
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Think about phosphines, now they can be chiral!
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Here, the nitrogens' environment seems very rigid, so maybe nitrogen should be considered as a stereocenter.
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I see 6, I don't think you are missing anything.
8, surely?
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Yes, getting blind in my old age. Or my screen needs cleaning:)
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I know other elements can be chiral centers. This is a topic I am ill equipped to talk about though. Gonna put my nose into some information.
@sjb
You think 8 for my compound?? Please explain further.
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When I learned it (and what I accepted as fact?!?) was that Nitrogens rapidly interconvert between R and S, therefore giving them chirality. As was mentioned, tertiary nitrogens might have a bigger energy barrier to rotation, however I am somewhat certain that it is, in fact, always, chiral. Unless, of course, it is a Carbon DB Nitrogen. Then no free rotation….and no stereo center :).
-Zack
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You think 8 for my compound?? Please explain further.
2n, not 2n.
Try writing them all out:
RRR
RRS
etc. etc.
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Is only the tertiary nitrogen a stereocenter, or both are?
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Brain fart.
I did indeed write them out a bit ago and it is 8.
Dummy.
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Only quaternary nitrogen atom does not interconvert between R and S configurations. In the case of tertiary one only sterically hindered N atom involved in three-fold ring (oxaziridines or aziridines) shows sufficient inversion barrier. First reports ~1976.
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So there are 8 stereoisomers? The tertiary nitrogen can be regarded as a stereocenter.
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All this talk of lone pair tunnelling is largely irrelevant; both nitrogens are amides, not amines, so are sp2 not tetrahedral. The one in the ring is fixed, so definitely won't have any E/Z isomerism. The open chain amide can potentially adopt E or Z stereochemistry, so overall there are 8 stereoisomers when the 2 stereogenic centres are considered.
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I don't think that we can strictly say that they are sp2, as they are actually something between sp2 and sp3. Now I really don't know what to say about the number of the stereoisomers.
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Nope, amides are most definitely sp2. Have a look at any protein crystal structures and you'll see hundreds of them...
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Okay, I read a little about it. So, there are 2 carbon stereocenters and one nitrogen atom causing E/Z isomerism.
The other nitrogen (in the rings) is still problematic for me. It can't be planar as the angle tension would be very big (30°). It has to be near sp3, and that would imply that it is a stereocenter.
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Yeah I guess it's probably slightly distorted from being planar, but even so it won't add to the stereoisomer count as the ring junction could never be trans...
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How do you mean trans? Why wouldn't it be possible as I draw in the picture? The six membered ring could also be below the plane.
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I agree both cis stereoisomers are possible, but the two trans ones arent:
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Okay, so it is 8 as you said. Thank you very much.