Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rutherford on July 30, 2013, 10:43:08 AM

Title: Axial chirality
Post by: Rutherford on July 30, 2013, 10:43:08 AM
As I couldn't find any good text about this topic, I have to ask a question:

Why are axial enantiomers represented as this:
(http://upload.wikimedia.org/wikipedia/commons/7/7b/Atropisomer.svg)
but not as in the attached picture?
Title: Re: Axial chirality
Post by: Dan on July 30, 2013, 02:23:03 PM
Because they are not planar. If they were planar, as you have drawn, they would not be chiral (mirror plane in the plane of the page). Your representations do not indicate which enantiomer you have.
Title: Re: Axial chirality
Post by: Rutherford on July 31, 2013, 06:17:19 AM
I think also that these conformations I drew would be very unstable, because of steric hindrances. Then that's why the true enantiomers can't interconvert easily. How do I know which one is Sa, and which one is Ra?
Title: Re: Axial chirality
Post by: Dan on July 31, 2013, 08:06:03 AM
R/S assignment example here (http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/stereochemie/weitere_chiralitaetselem.vlu/Page/vsc/en/ch/12/oc/stereochemie/benennung_axial/benennung_axial.vscml.html).
Title: Re: Axial chirality
Post by: Rutherford on July 31, 2013, 08:47:53 AM
Ok, thanks.
Title: Re: Axial chirality
Post by: Rutherford on July 31, 2013, 08:53:18 AM
Why aren't allenes planar as alkenes?
Title: Re: Axial chirality
Post by: Dan on July 31, 2013, 09:07:30 AM
It's to do with the molecular obitals - you can easily google stuff like this yourself.

e.g. http://bouman.chem.georgetown.edu/S02/lect12/lect12.htm
Title: Re: Axial chirality
Post by: Rutherford on July 31, 2013, 09:18:23 AM
Sorry, I tried, but couldn't so easy. Thank you for the help, it is clear now.