Organic Chemistry : Help with Enantiomers
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Example Question #65 : Stereochemistry
What is the absolute configuration of the molecule shown?
2S, 4S
2R, 4S
2R, 4R
2S, 4R
2R, 4S
This molecule has two stereocenters. Numbering from left to right they are carbon numbers 
For the chiral carbon on the left (carbon number 
For the chiral carbon on the left (carbon number 
Example Question #41 : Isomers
Assign absolute configuration to the tetrahedral asymmetric stereocenter (TAS) circled in red.
Trans
S
Cis
R
S
Aside from the hydrogen that extends "back into the page", priority 1 goes to the top right group (from the perspective of the TAS) because of the large chlorine atom. Priority 2 goes to the top left group because of the carbon atom attached to another carbon atom. Priority 3 goes to the methyl group because it is a carbon atom attached to hydrogen atoms, which have the lowest priority. When the hydrogen atom attached to the TAS is viewed as going back into the page, the circle created by going from priority 1 to priority 3 is counterclockwise, so we assign this TAS to be S. Cis and trans are irrelevant to TAS.
Example Question #42 : Isomers
Assign absolute configuration to the tetrahedral asymmetric center (TAS) circled in blue.
R
Cis
Trans
S
R
Aside from the hydrogen that extends "forward from the page", priority 1 goes to the top left group (from the perspective of the TAS) because of the chlorine atom. Priority 2 goes to the bottom left group because of the carbon atom attached to another two carbon atoms. Priority 3 goes to the bottom right group, which has a carbon attached to only one other carbon at the point of difference with the group that has priority 2. When the hydrogen atom attached to the TAS is viewed as going back into the page, the circle created by going from priority 1 to priority 3 is clockwise, so we assign this TAS to be R. Cis and trans are irrelevant to TAS.
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