Sciencemadness Discussion Board

Cis/Trans isomerism of alkane 1,2 diol

qeezur - 3-12-2016 at 19:53

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 ?

Harristotle - 3-12-2016 at 23:37

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.

Tsjerk - 4-12-2016 at 03:14

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.

qeezur - 4-12-2016 at 04:19

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.


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

qeezur - 4-12-2016 at 04:22

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 ?

CuReUS - 4-12-2016 at 05:13

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

Metacelsus - 4-12-2016 at 07:54

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.

Dr.Bob - 4-12-2016 at 16:50

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.

qeezur - 5-12-2016 at 12:04

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.


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 ?


unionised - 5-12-2016 at 12:19

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.