[ostudent]

Hi everyone,

I've been having a bit (a lot) of difficulty with determining the configuration of a stereocenter. I know how to do the 'easy way', but I want to learn to think about molecules spatially.

Sometimes, I can do it and other times I try the same method and get the wrong answer, which probably means my thinking isn't correct.
For example, when I look at the figure 1 below...

First, I decide that the structure needs to be rotated 90 degrees so that the 4 can take the dashed position (away).
Then, I draw the figure the way I think it would look if rotated 90 degrees (see figure 2).
Finally, I see that the 1-2-3 are oriented counter-clockwise which means that the correct configuration is S.

The answer key says R and I know it's right because I can get there the other, easier way.
I watched a lot of videos and it's frustrating not being able to learn it myself...

I'm grateful for any feedback. Thank you.

[phth]

You are right, but the convention is to flip the molecule over because group #4 is pointing towards us so if we flip it over then it goes 1 top-2 bottom right-3 bottom left. Can you practice with a model kit? Ask the professor to borrow one I'm sure they would loan one to you for a little bit

[crawlingmcedge]

Sorry, but what do you mean think about them spatially? They are already contain 3 dimensional information don't they?

Try doing what I do. Put priority 4 into the plane of the page so you end up with triangle of 1,2,3 - one bond vertical and 2 bonds 'horizontal'.

 Then assign R/S and imagine the molecule rotate towards you on an axis of the 'horizontal' bonds.

[kriggy]

Your thinking seems correct but your execution is not. The best way to do it is to keep one substituent in place and rotate the other three. In your case, you keep either 1 or 2 on the same spot and rotate the other three. Im not 100% sure how you did the rotation but I think I figured it out where you made a mistake:

[ostudent]

Thank you all so much. @kriggy your drawings are very clear. I worked out your method and I think I understand.