Determining R/S without molecular model kit

[russellfx]

---

Members don't see this ad.

When I took organic I was allowed to use a molecular model kit which greatly helped me in determining R and S configurations for chiral compounds. Now that I'm studying for the MCAT I'm having difficulty with this topic. Of course its easy if the lowest priority is already placed behind the page or if its in front of the page coming at you (which you would just reverse) -- but what if its on the plane of the page - how can I determine R/S? Or any other possible combination if there is any?

Thanks!

 

[mauve]

what i find helpful is holding one of the four atoms stationary and rotating the other three until H ends up at the back. Then I figure out the R or S.

hope that helps

On a related note, I don't know how to figure out the R or S if something is drawn in Haworth projection 😕

 

[Revilla]

I learned a method involving placing the lowest weighted atom in the "back" of the structure, then looking at the other three head on, ranked in ascending number based on molecular weight. Then I could determine R or S based on if the numbers ran clockwise or counterclockwise. I'll try to look at my notes and explain this a bit better later on.

This is how I learned it as well. Don't assume you'll always have a H. On my MCAT, there was no hydrogen so you had to move the lowest priority to the back and use the other three to determine R or S.

R = clockwise
S = counterclockwise

 

[russellfx]

wait so can i just assume everything is the opposite of what I would attain if the lowest priority is not in the back?

 

[RPedigo]

wait so can i just assume everything is the opposite of what I would attain if the lowest priority is not in the back?

Here's what you do:

First situation: Lowest priority group is already in the back
1. Assign R/S configuration as normal

Second situation: Lowest priority group is not in the back
1. Switch the lowest priority group's position with the group that is in the back
2. Now the lowest priority group is in the back
3. Assign R/S configuration with your new switched-group molecule
4. Switch the configuration because you switched the groups around. That is, if you just found that the site on our switched-group molecule is S, change it to R. If you found that the site on our switched-group molecule was R, switch it to S. If you switch the places of two groups, you must also switch the answer you find.


Try practicing it a bit and get back to us. Good luck! 👍

I also moved this thread to the Q&A subforum.

 

[161927]

Keep all the bonds and shading the same.

If you move a pair of atoms around TWICE you end up with the starting configuration.

Switch the one in the back with H. Then switch any other two atoms. Then assign priorities.

 

[requeim]

the way i learned this is:

If the lowest priority group is already in the back,
1. Assign configuration (R/S)

If the lowest priority group is NOT already in the back,
1. Assign configuration (R/S)
2. Since lowest priority group is not in the back, switch whatever configuration you got from part 1.

 

[futuredoctor10]

^Yep that's how I learned it too, like requiem

 

[tco]

Well, I'll just tell you the method that I use. I dunno if anyone else has mentioned it yet or not:

Figure out what the priority order is. Typically the molecule will be situated so that the lowest priority is either out of the page or into the page.

If the lowest is out of the page or into the page, treat it as if it is into the page. (Meaning just forget about the lowest priority atom and continue with these steps)

Determine whether the molecule is (R) (clockwise counting of 1,2,3) or (S) (counterclockwise counting of 1,2,3).

If the lowest priority atom is into the page, that's the answer.

If the lowest priority atom is out of the page, flip configurations.

If this wasn't explained well enough, let me know, I'll give an example.

 

[unsung]

When I took organic I was allowed to use a molecular model kit which greatly helped me in determining R and S configurations for chiral compounds. Now that I'm studying for the MCAT I'm having difficulty with this topic. Of course its easy if the lowest priority is already placed behind the page or if its in front of the page coming at you (which you would just reverse) -- but what if its on the plane of the page - how can I determine R/S? Or any other possible combination if there is any?

Thanks!

I have a way to do this that might not be easy to explain, but it always works for me. If you're staring at a Fischer projection, imagine the side (horizontal) arms coming out at you, like it's giving you a hug. The vertical segments go back into the page away from you.

Then just assign the priorities. Say it's like this:

......1
4__|__2
......|
......3

(Ack, I can't get this to indent correctly. Ignore the dots! It's supposed to look like a Fischer diagram, so just bear with me.)

Now picture drawing a circle starting at 1 and going toward 3. What plane would the circle be in? It would be a plane that's like cutting into the surface of the picture, right? Imagine that circle as a steering wheel. Imagine sitting in front of that steering wheel and turning the wheel in the direction from 1 to 3.

Which way does the wheel turn? Right? Then we know it's "rectus" or (R). Left? Then we know it's "sinister" or (S). So for the Fischer diagram above, the wheel would turn left, and we know it's (S).

No need to fiddle around with putting the lowest priority in "back" or "reversing" the result from a Fischer etc. etc. Just always picture the Fischer the way it actually shows the molecule-- with horizontal segments poking out at you and vertical segments falling backwards.

Here's another one:


.....1
4__|__2
.....|
.....3

Draw a circle, imagine a steering wheel, imagine sitting behind the wheel. Which way does it turn? This one turns right, so it's (R).

I used to hate Fischers until I got comfortable with them. Sugars are easy to see with Fischers.

With perspective drawings, the steering wheel thing is even easier. Basically, the trick I use is to always picture things from the perspective of the person behind the wheel. I mean, just imagine someone in a car driving towards you, and they turn the wheel left (their left)... that's pretty easy for anyone to see, right? Nobody gets confused about how their left is actually our right, etc. How about watching a person drive from a side perspective? Say you're sitting shotgun, it's pretty easy to see, right?

So why do people have trouble seeing it in o-chem? It's a matter of perspective. Just realize where the circle (assign priorities first), then trace from 1 to 3 (turn your wheel). And voila.

If that's not so clear, let me know. Maybe I can explain it better.

 

[Steveoph]

wait so can i just assume everything is the opposite of what I would attain if the lowest priority is not in the back?

Thankfully yes! If the molecule shown has a H towards and another group away, figure it out as if the H was behind and the other group was towards, then just switch the result. So if you got R, it's actually S. Saves you from doing mental spinning of the molecule.