[qw098]

Hi guys,

I was wondering if it possible to distinguish enantiomers or diastereomers by either IR or NMR?

Thanks!!

My guess is that IR wouldn't help distinguish enantiomers nor diastereomers but I just want to make sure I tried googling it, but with no avail!

[Honclbrif]

What's the difference between the members of a diastereomeric pair? Might these differences conceivably be reflected in a spectrum?

What's the difference between the members of an enantiomeric pair? Might these differences conceivably be reflected in a spectrum?

[qw098]

In an enantiomer, all the chiral centers are opposite for the two compounds, while in a diastereomer, not all of the chiral centers are different between the two compounds.

[Honclbrif]

In an enantiomer, all the chiral centers are opposite for the two compounds, while in a diastereomer, not all of the chiral centers are different between the two compounds.

So what does this mean for the chemical and physical properties of a diastereomeric pair vs. the properties of an enantiomeric pair?

[qw098]

IR can't distinguish between enantiomers nor diastereomers while NMR can only distinguish between enantiomers and not diastereomers

[Honclbrif]

Not quite.

Diastereomers have different chemical and physical properties, while enantiomers have the same properties*. Do you think this would make it easier to distinguish between an enantiomeric pair or a diastereomeric pair?

*As long as they are in an achiral enviroment such the average NMR experiment

[qw098]

Ah ok.

For IR, enantiomers would have the same spectra, but diastereomers would have different spectra.

For NMR, the same holds true, with the exception if we do our NMR experiment in an achiral experiment

[Honclbrif]

Ah ok.

For IR, enantiomers would have the same spectra, but diastereomers would have different spectra.

For NMR, the same holds true, with the exception if we do our NMR experiment in an achiral experiment

Thats pretty much it. Depending on the exact structures, the spectra of the hypothetical diastereomers may be quite similar, but there should be some difference. The best way to get an achiral environment in an NMR experiment to distinguish enantiomers is to use a chiral shift reagent.

[qw098]

Thanks Honclbrif

[Dan]

For NMR, the same holds true, with the exception if we do our NMR experiment in an achiral experiment

Thats pretty much it. Depending on the exact structures, the spectra of the hypothetical diastereomers may be quite similar, but there should be some difference. The best way to get an achiral environment in an NMR experiment to distinguish enantiomers is to use a chiral shift reagent.

We need to switch "achiral" for "chiral" here.

Enantiomers have identical properties in an achiral environment.

They have different properties in a chiral environment, so they can be distinguished in a chiral environment such as a chiral solvent, or complexed with a chiral shift reagent.

[Honclbrif]

Argh! Brain fart! Thanks for catching that one, Dan.

[qw098]

Thanks Dan!

Funny, I had a test today... and I mentioned what you said Honclbrifl as an aside that you couldn't distinguish enantiomers by NMR.

The test said... "suggest possible structures" : I then drew a structure and wrote as a small aside, "the other enantiomer might exist, but NMR couldn't distinguish it.. then I mentioned that you could possibly use Mosher's acid in an NMR experiment to distinguish between the enantiomers..."

I later realized (after the test, about 2 hours after) that the compound didn't even have a chiral center... so obviously there isn't an enantiomer... brain fart on my part too

I tried to be "clever" and it bit me on the tongue I won't lose marks though, but I did sound like a nice idiot

[agugliotta]

Hello - a bit delayed to this forum, but in regard to IR with enantiomers: so if an unknown illicit drug were to come back as amphetamine hydrochloride with the IR absorbance spectrum, I would not be able to tell if it is l or d? 

A sample test question I have notes:  If the spectrum for a previously unidentified drug matches that of an l-amphetamine HCL, which is a justifiable conclusion?  1- the unknown IS l-amphetamine HCL or 2 - the unknown could be d-amphetamine HCL.  Wondering which would make more sense if IR cannot distinguish... thank you!!!