The topics introduced in the next few days are arguably the key theme of Org 1 – stereochemistry.
Why is it the key theme? Because stereochemistry is a key element of many of the reactions you will soon encounter, and furthermore, a very testable element.
There are two important skills to master as soon as you possibly can.
0. Learn how to find the “hidden” hydrogens in line diagrams (you need to know this already).
1. Learn how to recognize chiral centers.
2. Learn how to determine R/S configurations.
A carbon attached to 4 different substitutents is a chiral center. Determining whether something is a chiral center might be slow at first. Give yourself freedom and time to work slowly and methodically: it will be rewarded.
Here’s a quick outline:
- Start at the carbon in question
- Examine the 4 bonds it is attached to.
- Ask: are any of these different? If all four are different, you can stop, knowing that this is a stereocenter. But if any are the same, you must keep branching out until either one of two things happen – 1) you find a point of difference, or 2) you come to the end and find they are they same.
- If two or more groups are the same then it is not a stereocenter.
If they’re all different (even by one atom) then you’ve found a stereocenter.
Here is an extreme example. Yes, it’s a stereocenter!
As far as determining R/S of a stereocenter, again, it’s best done one atom at a time. I really admire Steven’s approach. It goes one atom at a time. It’s slow and methodical, and it works. With time, you’ll get much faster at it.
Let me repeat: mastering these skills is essential if you wish to do well in Org 1. Stereochemistry really is the battleground of this course: everything you learn from here on out will funnel into it.
Tomorrow – a purr-fect way to tell how molecules are related.
Thanks for reading! James
PS – practice problems for finding chiral centers and determining R/S (Arizona State)