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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: sundberg on October 19, 2005, 08:51:15 AM

Title: Cotton-effect and octant rule
Post by: sundberg on October 19, 2005, 08:51:15 AM
I'm working on some problems, and one of them is to solve the absolute configuration of a cyclic ketone with the octant rule. ORD gave a positive cotton effect. Could someone please try and explain how to use this method? I really don't understand the octant rule and how to apply it.  ??? (Google didn't turn up much either)

Title: Re:Cotton-effect and octant rule
Post by: movies on October 21, 2005, 12:58:22 AM
There is a great explanation in "Stereochemistry of Organic Compounds" by Eliel, Wilen, and Mander.  I'll try to summarize:

First, draw a three dimensional representation of your molecule.  Next, superimpose three dimensional coordinate axes onto your molecule with the origin at the carbonyl carbon and one of axes coinciding with the C-O bond in the carbonyl group.  These are your octants.  If you were to look down the axis that contains the carbonyl group (with the O in front and the C in back), you would see what appear to be quadrants.  In general, the 'front' octants in this projection aren't needed, so they are ignored.  The four rear octants are labeled with the top left and bottom right octants + and the top right and bottom left octants -.  If most of the steric bulk (or more accurately, the most polarizable group) resides in a + octant, then a + CD is expected.

This seems to work pretty well if you are sure of the most favorable conformation of the molecule and the distribution of the bulk is unambiguous.  In conflicting cases where there are relatively similar polarizable groups in both a + and a - octant, then the predictions are not as good, although you generally still predict the correct sign of CD, the magnitude is often small and therefore kind of unreliable.

Here is a slide that I made for a recent presentation to our group.  I hope this helps.  The reference to the 1986 Tetrahedron review is also very useful and has a lot of examples.
Title: Re:Cotton-effect and octant rule
Post by: sundberg on October 23, 2005, 09:48:10 AM
Thanks, that was great.
Been sitting all weekend with my molecular kit and trying to figure out how it works and I finally got it. :D
(I have a really hard time visualising 3D structures)