[spirochete]

I feel like I should know this by now, but I just realized one quirk of R/S determination I'm not 100% sure of.

I know you can always count around the tetrahedron if your #1 priority is on a dash and your #4 priority is on a wedge.  Clockwise is R, counterclockwise is S.  If this is reversed and the #1 priority is on a dash and #4 is on a wedge then the opposite is true, eg clockwise now denotes "S" 

Also I know if neither of these priorities is on a wedge or dash you need to reorient the molecule in order to count properly.

But what if, for example, your #1 priority is on a wedge, and your #4 priority is in the plane of the paper?  Does clockwise always equal R in this case?  Or do you have to spin the tetrahedron until both #1/#4 are on dash/wedge?

[esar]

If priority #1 is up (wedge) and the clockwise order is (2, 3, 4) than the compound is 'R'.

I have always used a right-hand rule (similar to determining the sign of angular momentum in physics) to determine absolute configuration. If you can visualize the three dimensional tetrahedron you then never have to reorient the molecule on the page.  To do this, you put your right hand thumb in the direction of the #4 priority and your fingers straight with direction of the #1 priority. Then if you curl your fingers and encounter the #2 priority first, the compound is 'R'.  The only other possibility is that when you curl your fingers and encounter #3, the stereocenter is 'S'.

I am not sure if that is explained all that well, but that is what I do. I can never remember whether counterclockwise is S or R, but i know my right hand never changes its configuration

[spirochete]

That's a cool method but I'm looking for something to help people who want to minimize how much visualization they need to do.

Nobody else knows the answer to my question?  Should I phrase it differently?





BTW I remember clockwise=R and counterclockwise=S because clockwise is the "right" way to go and therefore counterclockwise is "wrong" aka "sinister" which is where S was derived from in latin.

Then I remember that the "correct" orientation is #1 wedge, #4 dash.  If it's in the "wrong" orientation I reverse the rules.

[azmanam]

If you want to minimize mental gymnastics, then condition people to reorient such that they are always looking at the molecule the same way (e.g. lowest priority pointing back, if that's easiest for them).  There are two operations I always find easiest for reorienting molecules that minimize mental gymnastics: the 120^o rotation and the 180^o rotation.  For each rotation, pick a bond that will remain stationary and that will be the bond around which you rotate - typically one of the in-the-plane bonds.

In the 120^o rotation, you don't change the position of any of the bonds, but the three other substituents (the ones not on the bond you're keeping stationary) rotation positions by one.  A second rotation rotates the three substituents to the next position, and the third rotation brings you back to your original orientation.

In the 180^o rotation, the other in-the-plane substituent stays in the plane, but moves to the other side of the paper.  The substituent in front moves to back, and the substituent in back moves to front.  A second 180 rotation will bring you back to the original orientation.

You can reorient any molecule to your desired orientation with a combination of only 120^o and 180^o, you just may need more than one rotation.  But you can eventually rotate any molecule to the orientation that's easiest to determine R/S.

Those are really hard to explain without pictures, so hopefully the following diagram will help.