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Topic: Isomer Hierarchy  (Read 12298 times)

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Offline Yggdrasil

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Re: Isomer Hierarchy
« Reply #15 on: May 12, 2006, 01:11:40 PM »
For most intents and purposes, an chemist would not consider conformational isomers to have different chemical or physical properties.  Since rotation about most sigma bonds has a very low energy barrier, there will be a rapid conversion between the conformational isomers at room temperature.  So, in theory most conformational isomers will have different properties (for example, different dipole moments), but in practice, you can't observe these different properties most of the time because of the rapid interconversion of conformers.  The molecule will appear as a time-averaged combination of its conformers.

However, in biochemistry, conformational isomerism is extremely important because different conformers of a protein have vastly different properties and there are various methods to distinguish between conformers (mainly between the random coils of denatured proteins and the secondary structures of folded proteins; one such technique is circular dichroism spectroscopy).

There is some overlap in the chart.  For example, some simple conformational isomers can be mirror images, but these may be limited to achiral conformational isomers (I'm not completely sure that this statement is correct).  However, there are conformational isomers which are not enantiomers (e.g. folded v. unfolded proteins). 

I think for a first-year college audience the diagram is fine.  It's not 100% correct, but the main differences are minor details which would be hard to incorporate without making the diagram overly complicated.

Offline movies

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Re: Isomer Hierarchy
« Reply #16 on: May 12, 2006, 07:35:05 PM »
You can separate the enantiomers of trans-cyclooctene at low temp.  That is a case where conformational isomers are truly optically active.

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