Step-by-Step guide to help with stereochemistry
Stereochemistry can be a difficult concept to grasp, especially if you are not good at visualizing 3D structures. I would recommend using a model kit to become more familiar with how to ‘see’ the structures that you will come in contact with on your homework and exams.
To start, find a model kit…
…and either a piece of paper or a whiteboard.
Now, draw your structure and assign the priorities to all your groups (1-4).
Next, build the structure with your model kit.
You can line it up with you drawing to make sure.
Locate your lowest priority group (in this case, it is hydrogen) and move it to the back. In other words, turn it away from so you can only see the groups that have priority 1 (highest) to 3.
Take you model and line it up with one structure…
…and then the next. Do this a couple of times to get a feel for how to ‘see’ this in your mind (how to translate it from paper to your mind).
Now that you have your lowest priority in the back, all you need to do is see if the numbers 1-3 go in a clockwise (R) or counterclockwise (S) direction. In this case, they go in a clockwise direction; therefore, the assignment is R.
Ok…now it is your turn:
What do you think….R or S?
I know in nomenclature that alcohol is before the Br, but in the chiral center would the priority be Br when determining R or S because its atomic weight is larger than O? Is the answer S?