R and S Chirality

[Jay2910]

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Hello,

I am having quite of bit of trouble understanding example 3.2. The link is below:
I get that I have to number from highest priority to lowest priority. I just don't get why my approach( that is going from group 1 to 2 bypassing group 3). Under this assumption, I was thinking that the molecule is actually S . .but its not.

So why is my approach wrong? and why is Berkley's right? They went from priority #1 to #4.


http://i1072.photobucket.com/albums/w371/Goldeolde/IMAG0206.jpg

Help would be awesome!
Thanks much!

 

[rjosh33]

Hello,

I am having quite of bit of trouble understanding example 3.2. The link is below:
I get that I have to number from highest priority to lowest priority. I just don't get why my approach( that is going from group 1 to 2 bypassing group 3). Under this assumption, I was thinking that the molecule is actually S . .but its not.

So why is my approach wrong? and why is Berkley's right? They went from priority #1 to #4.


http://i1072.photobucket.com/albums/w371/Goldeolde/IMAG0206.jpg

Help would be awesome!
Thanks much!

There's two potentially tricky parts to R and S naming. 1 is figuring out the priorities, and 2 is then deciding whether the priorities are clockwise or counterclockwise. You've done well on the first part, so I'll focus on what I do for the second.

Once you have the priorities labeled, you check to see where the #4 priority is pointing. If it's anywhere other than the back of the molecule (i.e., if it's in the plane of the page or on a wedge), you need to make a single-swap. A single-swap is when you swap the position of the #4 priority with whichever atoms(s) is on a dash, or pointing to the back of the molecule. Then, starting at priority 1 and going to 3 of the new configuration, you figure out the direction (clockwise or counterclockwise) you would have to go. CW is R, CCW is S. However, because we made the single-swap, the true configuration is the opposite of what we just figured out.

For example, in the attached molecule, priority 4 (H) is on a wedge, pointing out of the page. Therefore, it needs to be switched with whatever is on the dash, pointing into the page. We can see that OH is on the dash. Therefore, we swap these two. This means that priority 4 is now on the dash, and priority 1 is on the wedge. Now, starting at priority 1 and going to 2 and then to 3, we see we have to go counterclockwise. This means our converted molecule is S. Write down S. Remember, though, to get this configuration we had to make the single-swap, meaning it's the opposite of what we're trying to find. So, we cross out the S, and write down R. R is therefore the configuration of the original molecule.

This might seem confusing at first, but it makes more sense after a little practice.

 

[Jay2910]

Life makes sense again! Thanks!