Orgo: S/R determination

[Kussemek]

After having fallen behind, and now trying to pick up where I left off...I am having problems determining if a molecule rotates s or r. I get the point of assigning the priorities, but I cant seem to get the S/R determination consistently. Does anyone have any methods that they can share?

---

Members don't see this ad.

 

[Daichi Katase]

Ok, after assigning priorities and putting hydrogen in the back....if the priorities rotate counterclockwise, then you have "S" and if they rotate clockwise, then you have "R"

 

[jackets5]

also you dont have to have the hydrogen (or lowest priority group on the bottom) if it is on the side just take the opposite if it clockwise then you have an "S" compound and vice versa. I like it since it eliminates any chance of screwing up moving the groups around

Ok, after assigning priorities and putting hydrogen in the back....if the priorities rotate counterclockwise, then you have "S" and if they rotate clockwise, then you have "R"

 

[Kussemek]

it sounds easy enuf, but my mcat teacher was talking about something with your hand, and if its right handed its R and left S...am I missing something?

 

[jon0013]

it sounds easy enuf, but my mcat teacher was talking about something with your hand, and if its right handed its R and left S...am I missing something?

you always want your "H" group to be in the wedged compartment. if i remember correctly my orgo prof said something like switch the H group with the group in wedge. and then take the other 2 remaining groups and swap their positions. can anyone verify this?

 

[hb2998]

Regarding if the lowest priority groups isn't in one of the wedged positions (front/back) do this, it will make your life much easier than any formulated routine. All those other tricks stated above are much easier in right condition.. use them if you can! I'm gonna suggest two methods that work great for me, but it may be hard for some people who don't really like turning the molecule in their head. Hold the molecule still in space.. think of it in a 3D way, and I imagine moving your head to a location where the lowest priority group is farthest away from you. (If a sunflower, think of the lowest priority group as the stem.) Then assign priority.

If you don't like fixing the molecule in space, and moving around it.. You don't really need the lowest priority group to be in the back, you just need its bond to be perpendicular to the radius of the hand motion as you're turning your hand either clock or counter clockwise. You can just turn your hand coordinate your hand lets say perpendicular to the paper, making sure that you're achieving the desired perpendicular bond/radius of hand motion... then see if your turning your hand clock wise or counter clockwise..

 

[QofQuimica]

it sounds easy enuf, but my mcat teacher was talking about something with your hand, and if its right handed its R and left S...am I missing something?

This is how I learned to do it as an undergrad, although I don't normally teach my students this method because many of them find it confusing. If you want to use this technique, what you do is align your thumbs (of both hands) in the same direction of the fourth priority group (which isn't always H, incidentally) and then curl your fingers. If the priorities go in the direction of the fingers on your right hand, the configuration is R. If they go in the direction of the fingers on your left hand, the configuration is S. The benefit of this method is that you don't have to worry about swapping groups if your low-priority group is sticking out at you; you merely need to point your thumbs outward, and the technique still works just as easily as if it were pointing away.

 

[skp]

This is how I learned to do it as an undergrad, although I don't normally teach my students this method because many of them find it confusing. If you want to use this technique, what you do is align your thumbs (of both hands) in the same direction of the fourth priority group (which isn't always H, incidentally) and then curl your fingers. If the priorities go in the direction of the fingers on your right hand, the configuration is R. If they go in the direction of the fingers on your left hand, the configuration is S. The benefit of this method is that you don't have to worry about swapping groups if your low-priority group is sticking out at you; you merely need to point your thumbs outward, and the technique still works just as easily as if it were pointing away.

POST OF THE YEAR

THANKS Q