Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: agugliotta on February 02, 2016, 10:02:00 PM

Title: Enantiomers and IR
Post by: agugliotta on February 02, 2016, 10:02:00 PM
In regard to infrared and isomers (enantiomers in particular), can FTIR distinguish these molecules?  For example, if an IR absorbance spectrum for an unidentified drugs hits on l-amphetamine hydrochloride, does this mean that is a definitive id of the unknown drug? or could the unknown be d-amptheramine hydrochloride also?  .. what about cocaine enantiomers? Thank you!

I know IR is designed to give structural information, but I wasn't sure about isomers. Thanks.
Title: Re: Enantiomers and IR
Post by: Doc Oc on February 03, 2016, 09:46:53 AM
Enantiomers have the same physical traits so they are not distinguishable by IR.
Title: Re: Enantiomers and IR
Post by: Babcock_Hall on February 03, 2016, 11:09:31 AM
I agree with Doc Oc, but I will point out one possible exception.  Enantiomers are not distinguishable, unless they are in a chiral environment.  I would predict that if you dissolved each enantiomer in in S-2-butanol (for example) they might be distinguishable, but I don't know how practical such an experiment would be.
Title: Re: Enantiomers and IR
Post by: Corribus on February 03, 2016, 12:44:59 PM
You can also use FTIR/VCD to distinguish between enantiomers.

See Ito et al, JACS 1979 101, 2, 496-498. http://pubs.acs.org/doi/abs/10.1021/ja00496a045

A general description of VCD can be found here.

https://en.wikipedia.org/wiki/Vibrational_circular_dichroism
Title: Re: Enantiomers and IR
Post by: agugliotta on February 03, 2016, 09:55:19 PM
Thank you all!  So ultimately, with using FTIR, you wouldn't know which stereoisomer is present (i.e. l or d-amphetamine) as it could really be either one.

Thank you again.