I in assigning R/S stereochemistyr

[future_vet]

(There is no problem statement etc.)

I am having a very hard time figuring out how to assign R/S stereochemistry on certain compounds. I can't rotate molecules in my head without making mistakes, and so I rotate them on paper.

Let's take this example:
http://avogadro.chem.iastate.edu/CHEM331/331FinalKeyF03.pdf
Go to page 6 please, we have to label compounds.

When a molecule has 2 atoms in the plane, + 1 in the front and 1 in the back, I can rotate them. I keep one atom fixed, usually #2 (4 being the smaller, 1 the largest), and rotate the other 3.

However, and this is where I need help, I don't know what to do when I have molecules such as the ones on the left, where 2 are in the back and 2 in the front, and none in the plane of the paper/screen.

How can I rotate those on paper in order to assign R/S stereochemistry?

Thank you for any help you can give me!

~J.

 

[future_vet]

Actually I found this:
http://chem.pdx.edu/~wamserc/C334F99/4notes.htm

Thanks!

 

[Blueshift5]

I understand your struggle with assigning R/S stereochemistry. It can be a challenging task, especially when dealing with complex molecules. The key to successfully assigning R/S stereochemistry is to visualize the molecule in three-dimensional space. This can be done by using molecular modeling software or by building physical models.

In the example you provided, it appears that the molecules on the left have a tetrahedral geometry with no atoms in the plane of the paper. In this case, you can assign R/S stereochemistry by determining the priority of the substituents attached to the central carbon atom. This can be done by using the Cahn-Ingold-Prelog rules, which prioritize substituents based on atomic number.

Once you have determined the priorities of the substituents, you can then rotate the molecule in your mind or on paper to align the lowest priority substituent (represented by a dashed line) away from you. Then, you can visualize the remaining three substituents in a clockwise or counterclockwise direction and assign the configuration as R or S accordingly.

I hope this explanation helps you in your understanding of assigning R/S stereochemistry. It may take some practice, but with time and experience, it will become easier. If you need further assistance, I suggest consulting with a colleague or seeking guidance from a chemistry tutor. Best of luck!