qeezur
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Cis/Trans isomerism of alkane 1,2 diol
Dear friends,
I have a compound containing saturated 1,2 diol group (partially racemic). I have to prove whether they are cis or trans to each other. What are my
options ? This diol came from acidic hydrolysis (deprotection) of solketal (1,2 protected form of glycerol). Mechanism of the step that forms this
diol points towards enantiomeric Trans but I have to prove it.
1) I know NOE/NOESY are often used for this purpose. Which proton to irradiate in this case ?
2) Is there any other technique as well that can be used for this purpose ?
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Harristotle
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Hi, I don't understand the question. If the hydrocarbon backbone is saturated, surely the cis/trans issue doesn't arise as the -OH is free to rotate.
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Tsjerk
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In a 1,2-diol with a saturated backbone there is cis/trans isomerization because the alcohol groups make the molecule rigid. The energy to convert one
into the other is low though because no covalent bonds need to be broken.
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qeezur
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Quote: Originally posted by Harristotle  | | Hi, I don't understand the question. If the hydrocarbon backbone is saturated, surely the cis/trans issue doesn't arise as the -OH is free to rotate.
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Thanks for replying,
Lets say the two OH of the diol get esterified with long chain fatty acids (partial glyceride) Or the diol moiety is contained within a large molecule
which restricts the rotation of the diol. In these two cases how can we know whether they exist in cis or trans form ?
even if OH of a diol are free to rotate, I believe they would prefer to be in the trans form considering that free rotation would put the two OH at
gauche and then eclipsing conformations between the two OH which means alot of torsional strain. Trans form will be more populated at low temperature.
I dont have the exact strain value for two OH eclipsing or gauching each otherwise the population density ratio between cis and trans would have been
yielded out.
The core question remains, How do we spectroscopically or through some other means verify that which conformation the OH are preferring
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qeezur
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| Quote: Originally posted by Tsjerk | | In a 1,2-diol with a saturated backbone there is cis/trans isomerization because the alcohol groups make the molecule rigid. The energy to convert one
into the other is low though because no covalent bonds need to be broken. |
I am thankful that you commented,
and I agree with you. Cis trans stereomerism exists in saturated free Diol as well. At low temperature the J values of Glycerol always show trans
preference in simple 1H-NMR. What are your thoughts on how do we conduct a NOE/NOESY method to verify our argument. Which proton to
irradiate ?
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CuReUS
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See this paper, they use GC retention time and mass spectra to prove that the stereochemistry was trans-http://cbs.ewha.ac.kr/pub/data/2003_05.pdf
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Metacelsus
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You might be able to just use the Karplus equation to determine the dihedral angle between the C-H protons, and deduce the sterochemistry from that.
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Dr.Bob
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If the diol was on a cyclic system, then you can get cis or trans. In a non-cyclic system, a diol might have enantiomers or diasteromers, depending
on the substitutions on the carbon. Solketal can exist in two enantiomers, R or S at the central carbon. Once you deprotect the ketal, you have
glycerol, which is symmetric, thus achiral. So without more info, it is hard to say, but NMR will only work on diatereomers, not enantiomers. A
simple optical rotation can help to see if the molecule is enriched in any enantiomers, any rotation other than zero indicates an excess of one
enantiomer, but without a known standard to compare it with, that is hard to do much else with. For enantiomers, you would need chiral
chromatography on HPLC, chiral NMR shift agents, or to make a derivative with a known chiral compound to create diastereomers, then NMR may allow a
determination of chirality. This is typical graduate level chemistry, if so, in theory someone in your lab would know what tools you have available
for that work. If at home, you should go for the PhD, as I still have a hard time doing chiral evaluations after 30 years of chemistry.
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Texium
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Thread Moved
4-12-2016 at 17:34 |
qeezur
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CuReUS, Thanks for the reference, I have read it.
Metacelsus, Thanks for the advise, Karplus relationship usually works for rigid system though but can be mildly useful in my case.
| Quote: Originally posted by Dr.Bob | | If the diol was on a cyclic system, then you can get cis or trans. In a non-cyclic system, a diol might have enantiomers or diasteromers, depending
on the substitutions on the carbon. Solketal can exist in two enantiomers, R or S at the central carbon. Once you deprotect the ketal, you have
glycerol, which is symmetric, thus achiral. So without more info, it is hard to say, but NMR will only work on diatereomers, not enantiomers. A
simple optical rotation can help to see if the molecule is enriched in any enantiomers, any rotation other than zero indicates an excess of one
enantiomer, but without a known standard to compare it with, that is hard to do much else with. For enantiomers, you would need chiral
chromatography on HPLC, chiral NMR shift agents, or to make a derivative with a known chiral compound to create diastereomers, then NMR may allow a
determination of chirality. This is typical graduate level chemistry, if so, in theory someone in your lab would know what tools you have available
for that work. If at home, you should go for the PhD, as I still have a hard time doing chiral evaluations after 30 years of chemistry.
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Thanks Dr. Bob,
My issue is not with system being chiral or achiral (It is in fact chiral at C2 of the diol). But thats not my concern here. My issue is that whether
the diol is Cis or Trans, being saturated some may argue that diols can rotate and conformations will change with energy but again the anti/trans
conformation will be more highly populated at least theoretically because of lesser torsional strain. My only question is that how can one
spectroscopically determine that which conformation is more populated ? Cis or trans ?
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unionised
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Cis diols significantly increase the strength of boric acid by forming complexes.
So, if you make a solution of borax and measure the pH then and the diol and measure the pH you can tell if there's a cis diol present- if there is,
the pH changes.
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