You need to assign priorities for the groups atached to the stereocenter by taking a look at the atomic number of the atom that is directly attached to the carbon atom, i.e. to the stereocenter.
So, for the groups you have, the atoms attached to the stereocenter are
#W ->#the #"N"#in #"NH"_2#;
#X ->#the #"C"#in #"CH"_3#;
#Y ->#the chlorine atom, #"Cl"#;
#Z ->#the #"C"#in #"CH"_2"NH"_2#.
The idea here is that you assign priorities in order of decreasing atomic number. The atom will the biggest atomic number will get priority
In this case, you have nitrogen,
So chlorine will get priority
But what do you do for the two carbon atoms? Which one gets priority
To decide that, look at the atoms that are attached to these two carbon atoms in their respective groups.
#"H"_2"C" - "NH"_2#
So, for group
#"For group X: " "H " "H " "H" #
#"For group Z: " "N " "H " "H"#
The first point of difference between these two lists will constitute the tiebreaker. In your case, the tiebreaker is the nitrogen atom, atomic number of
This means that group
So, this is what you have
Now, in order to assign an
So basically, you need
Now, you could rotate the molecule in your mind to see how the configuration would change if group
Since this answer is already pretty long, I'll show you the quick way.
So, you know that can determine the configuration of the stereocenter by ignoring the group with the lowest priority and checking to see the direction in which the remaining priorities increase
- if they increase counterclockwise, then you have
- if they increase clockwise, then you have
Since the group with the lowest priority is not on the dash, you can switch it with the group that is on the dash so that it lands on the dash.
Keep in mind, however, that switching the position of any two groups will change the stereocenter configuration!.
In your case, if you switch
The priorities increase from priority
However, since you switched two groups to get this configuration, you have to reverse it to get the actual one.
Therefore, the stereocent is not
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 (
The rule is very simple, you switch places between the lowest priority substituent (
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).
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.