1.
Please help me find the absolute stereochemistry (R or S) of the two stereocenters. I got S for the
leftmost one and R for the rightmost one but that is incorrect. Please explain
NH2-
Solution
The configurations given by you are correct but since the main group that is - NH2 and - CH3
are on dashed wedge bond, while assigning R ans S cofigurations dased wedge bond should be
assigned to lowest ranked atom/group based on their atomic numbers. In the present case at both
stereocenters the lowest ranked atom is H. So, H must be on dashed wedge bond.
Inorder to have that, we need to change the configurations, since we got S for left stereo center
we need to take it as R ( after rearranging the structure sothat H get dashed bond ) and for right
stereocnetr we got R but we need to take it as S ( after reaarnaging the structure sothat H is on
dashed wedge bond ) .
SO, The configurations are,
Left sterocenter = R
Right stereocenter = S
NOTE : Dahsed wedge bond always assigned to the lowest ranked atom/group to find R and S
configurations for stereocenters.