Question

Select the correct r or s designation for the stereocenter.
1) r
2) s

Answer 1

The correct R or S designation for the stereocenter is determined by assigning priorities based on atomic numbers, analyzing the arrangement of the groups around the chiral center, and considering the orientation of the lowest priority group.

Step-by-step explanation:

The correct R or S designation for the stereocenter is determined by assigning priorities to the groups attached to the chiral center and then analyzing their spatial arrangement. The priorities are assigned based on the atomic numbers of the atoms directly attached to the chiral center, with the highest atomic number receiving the highest priority (1) and the lowest priority (4) is often given to the atom with the lowest atomic number, typically hydrogen.

To assign the R or S descriptor:

1. Assign priority based on atomic number: -Br > -NH₂.
2. If the lowest priority group is already pointing away from the viewer (hashed wedge), no action is needed.
3. Draw an arrow from priority #1 to #2 to #3.
4. If the arrow is clockwise, the descriptor is R; if counterclockwise, the descriptor is S.

However, if the lowest priority group is not pointing away from you, you must first mentally or actually swap it to the back and then reverse the assignment you would otherwise make: if the sequence appears counterclockwise after this, the chiral center is actually R, and vice versa.