Help with R and S Stereochemistry

[thomasfx10]

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I need help trying to figure out the stereochemistry on the 2 quaternary carbons in the image in this link.

It it states the it is S- stereochemistry. I am not seeing that. I normally look for "H" and see if it is up or back. However, this does not have a "H" so I am suppose to look for the next lowest group ... CH2?

What is the highest group then?


Thanks for any help!

//https://picasaweb.google.com/117776877065299429288/May232011?authkey=Gv1sRgCI3uvafg6LahCA#

 

[GreenRabbit]

When you're ordering the groups, if all of the directly attached atoms are the same, you look at the groups attached to those atoms.

In this compound, the highest group attached to the bottom left quaternary carbon is the one going to the top right. That group has a carbon attached to 2 carbons, one of which is attached to an O. That is higher than the carbon going down, which also is attached to 2 carbons, but neither of which have an O. The lowest group is the CH3 coming toward you (that carbon is only attached to 3 hydrogens). The 2nd lowest group is the carbon going to the top left, because it is attached to only 1 other carbon. Therefore the config is S.

For the top right quaternary carbon, the lowest group is again methyl. The highest group goes to the top right, the 2nd lowest group again goes to the top left, and the config. is S.

 

[thomasfx10]

So the CH3 is the lowest! Then follow the chain to the "O". Now I see it. That rotates in the "R" direction but since the lowest group is up and not back, then the R becomes and S. I hope that is correct


Thanks!

 

[gunj122]

edit

 

[GreenRabbit]

So the CH3 is the lowest! Then follow the chain to the "O". Now I see it. That rotates in the "R" direction but since the lowest group is up and not back, then the R becomes and S. I hope that is correct


Thanks!

Yep. But don't just look for the O. If I had 2 groups... one with -CH2CHO and the other with -CH(CH3)2, the CH(CH3)2 would be higher.

 

[Swagster]

I can't view the picture. It says there's not one loaded. What source is it from?