I hat to ask such technical questions on this board, but I know there are some chemists out there who may have an answer. I have looked everywhere I can think of. All of my recources explain that there are two isomers of BINAP but none of them explain how R and S are determined.
Your answer can probably be found in this article. The guys that made it first must have determined which isomer is which somehow. [shudder]
Here is (basically) the way to do it (this is going to be tough in text, so bear with me a little if I ramble…)
What you do, is look for all the chiral centres (if you know what R and S are, I assume you know what a chiral centre is).
Imagine the compound in 3-D at each of these chiral centres. Assign each of the groups attached a priority preference (i.e. a,b,c,d or 1,2,3,4) according to atomic number -the group with the lowest atomic number is the least important (d, or 4).
If the atom is the same for two groups, then we proceed along the bonds until the first point of difference is reached, and then priority is assigned the same way - the lowest atomic number is least important (in the case of something like CH3 and CH2-C…, then the second one has priority since it has fewer low atomic number substituents).
Now, rotate the 3-D image so that the lowest priority group is pointing away from you, and trace a path from a to b to c (or 1,2,3). If this path circles clockwise, then the enantiomer is an R, and if its goes counterclockwise, then it’s an S group.
For large molecules, like BINAP, this is done for every chiral centre, so a given enantiomer of this compound would have several R or S designations, depending on the number of chiral centres (I see 2 in this case).
Odds are, though, that making this compound produces a racemic mixture anyways:D
Actually, now that I take a closer look at it - there are no chiral centres in BINAP! Each P is connected to two identical phenyls, and all the carbons are sp2 hybridized.
Is this a trick question?
Ok, pardon me for posting again - I keep coming back to this question, and thinking of something else (I’m still a student at this stuff, so thats my excuse!)
Here is my next crack at it:
Because of aromaticity, the P is actually “double bonded” to the aromatic rings, at any given time…really hard to explain, since this is the kind of thing I suck at in organic chem. BUT, naphthalene is more aromatic than phenyl, and so it resonates more with the P:…
Dammit, this is kind of beyond me at the moment…especially without diagrams…I think I’ll let someone who knows what they’re talking about WRT this compound take over from here.
But this page: http://www.uni-heidelberg.de/institute/fak12/OC/helmch/Vieweg/vieweg.htm
Has some diagrams that might help…
ARRRGGGHHH
mnemosyne wanders off, wondering if this is indeed the career path she wants to take…
mnemosyne,
I don’t have the structure of BINAP in front of me, so I don’t know if he was asking a trick question or not. I just wanted to commend you for trying to explain the designations for chiral centers in a text format.
I always thought it was hard enough to explain chiral nomenclature with molecular models.
If you have access to a library the answer can be found in Topics Stereochemistry, 1970, 5, 31. Otherwise go to http://people.ouc.bc.ca/woodcock/nomenclature/nom-940.htm
essentially you look down the chiral axis, cross your eyes and let your brain go mushy or something like that. You wil need a Chime plugin for the site (Netscape only)
slight correction - chime (http://www.mdlchime.com/chime/ )is available for windows for either IE or netscape, its only macusers that have to use netscape.
Thank you for your responses, I do have access to a library, and will be checking out Topics in Stereochemistry to get a picture of the procedure. (I am at a Macintosh which apparently does not have Netscape.)
You are right mnemosyne, there are no chiral centers on this molecule. That is what makes this problem so frustrating. I have asked several professors, and they too do not know, since it is not a common problem.
BINAP is chiral, because rotation about the single bond between the two napthalene groups are hindered. Make a model or something and you will see that the mirror image of each conformation is not superimposible.It is still worth visiting the website as you should be able to work out most of the details just from the text
Christopher - that’s the conclusion that I came to last night, though I don’t have a model for it (I contemplated going to ChemSketch and drawing it out in 3-D, but it’s on another OS and would have required a reboot). There ARE no chiral centres, but there are certainly steric issues and whatnot causing one shape or another. But why call them R or S? I’m going to see if I can look it up in that journal too, but I still think it should be called something else: although E/Z or Cis/trans don’t apply either…
For those of you still wondering, BINAP is found at www.chemsketch.com, if you search for it.
And thank you, Shark, I tried 
It is really difficult without models, since Organic chem is REALLY a visual thing, but with the help of my textbooks, I was able to at least lay out the basic rules (Solomons and Fryhle give a really good text explanation).
That site calls thins Ra/Sa, not R/S - its similar, but still not the same thing, as far as I can tell. This is actually not a nomenclature thing I was ever taught, though.
IANAC, but can’t you just measure the rotation of polarization of light passing through a sample? I seem to recall that that was the basis for the definitions.
Sometimes. Whether that works or not depends on what you know about the chirality and optical rotation of the precurser molecules. If you create a new chiral center in your syntehesis there’s no certain correlation between r and s and the experimentally determined d and l. Typically you end up doing NMR or examining the formation and properties of derivative complexes of the two isomers to determine which is which.