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QUESTION

# R/S Absolute configuration: how to decide if it is R or S configuration ?if it given that (1) NH2 will be in the position of W (2) CH3 will be X (3) Cl will be Y (4) CH2NH2 will be in the position of Z I will attach a pic to show the place of compounds

The configuration is (R) .

Let us write the structure of your molecule first and assign priorities:

Priorities are assigned based on the atomic number. The rule is the following: Assign priorities (1, 2, 3, or 4) to the atoms directly bonded to the stereogenic center in order of decreasing atomic number.

Next we look at the lowest priority substituent (CH_3), it should be facing backward; which is not in this case!

The rule is very simple, you switch places between the lowest priority substituent (CH_3) and the actual substituent facing backward (CH_2NH_2), here is what you get:

Label the new configuration after rotation; it is going to be (S) . Flip the configuration: Which means since it is (S) it will become (R).

You're DONE.

Note: some textbooks mention that you have to rotate the molecule, which means you have to work your imagination, since it is a 3D molecule and you are working on a 2D surface (the paper or computer monitor) sometimes it becomes confusing, this method is way simplier.

Here is a video on labeling stereogenic centres R and S. There is part 1 and part 2.