Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: CHEKAL on January 06, 2013, 04:13:07 PM
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how would i be able to distinguish between cis and trans diastereomers using H-NMR? i know its something to do with the coupling constant.
also, how would i be able to determine a rough ratio of cis:trans diasteroisomers using H-NMR?
thanks
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What are cis and trans diastereoisomers? What exactly are you referring to here?
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i have a compound with two chiral centres, cis being both centres have the same configuration relative to one another, trans being different configuration to one another
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i have a compound with two chiral centres, cis being both centres have the same configuration relative to one another, trans being different configuration to one another
You mean like this, for example?
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yes, ive run the nmr spectrum of the compound, i just need to distinguish between them and determine an approximate ratio of the two relative configurations
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Well you can look at the ratio of the peaks next to the chiral centers.
The coupling constants for the H-H coupling should be different, look up the Karplus equation.
This of course if you can differentiate the peaks.
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which configuration would have a higher coupling constant?
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Probably the anti configuration. As I suggested look up the Karplus equation.
This tells you the relationship between the J value and the angle between the two protons.
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how would i do it in the case of no peak of the adjacent carbons to the chiral centres? i have an aziridine and the predicted spectrum nor my actual spectrum does not show any singlets that havnt been accounted for
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You mean you have a cis and trans aziridine? Like this?
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yes, i will get a structure uploaded, watch this space
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structures
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I would expect that the trans coupling would be larger than the cis coupling.
You should easily see this in the NMR.
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how would i be able to see this?
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The chemical shifts of the protons should be quite different, each will be a doublet the distance between the lines is the coupling constant, J in Hz.
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most lines are doublets yes, does that mean a triplet would have 6 peaks too?
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A triplet has three peaks, in a 1:2:1 ratio.
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then which peaks would appear as doublets due to coupling between the different configurations?
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Each of the protons on the aziridine ring will appear as a doublet, integrating for 1H.
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oh i see! thanks :) does this mean the difference in hight between the two peaks would give an approximate ratio of the two configurations in the yield?
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Yes that is correct. If you measure the integrals you can estimate the proportions of each diastereoisomer.