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Organic Chem Stereochemistry question

Discussion in 'MCAT Discussions' started by NineSixteen, Aug 12, 2002.

  1. NineSixteen

    NineSixteen Senior Member
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    So someone asked this question in my MCAT review class and I don't have an answer...wonder if anyone has any thoughts. It's tough to explain without drawing, but I'll try.

    Say you have a center carbon with 4 groups bonded and you are trying to figure out if that carbon is chiral, and if it is, whether the absolute configuration is R or S. Here are the four groups. One is a Hydrogen. One is an -OH. The other two are alkene groups, geometric isomers of one another, as in they are the same molecule but one is E and the other is Z. Is that center carbon chiral? If so, which alkene gets the higher priority - E or Z?

    Another variation on that same thought. What if the two groups (besides the H and OH) were chiral carbons themselves...idential except one group is R and the other is S? Now is the center carbon chiral? Is it R or S?

    I think these questions are beyond the scope of anything one would see on the MCAT, but thought provoking just the same.
     
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  3. Mudd

    Mudd Charlatan & Trouble Maker
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    Intriguing questions:

    Consider the second one first:

    If you recall from sugar chemistry, you can oxidize a sugar to an aldaric acid (diacid with two COOH groups at the ends of the chain) using nitric acid. Typically, the test is used to determine if the molecule forms a meso compound.

    If we start with ribose and the c-2 epimer of ribose (arabinose), then treat them with nitric acid, we get the exact scenario you describe. The ribose product is described as being meso, and having no optical rotation. The arabinose product is optically active with an specific rotation of +42 degrees.

    However, neither compound is said to have a stereogenic center at carbon three (according to Caserio-- the only orgo book I can find at the moment with something like that). In other words, they choose not to identify the central carbon, which I assume means they do not consider it to exhibit chirality.

    To answer your first question, I looked at Lowry and Richardson, and they only briefly describe prioritization. From what I can extract, and they don't address an exact case like your alkene scenario, it would come down to alphabetical prioritization. How disappointing, not that I can think of a better way. As long as the atomic numbers of the connected atoms match (no isotopic enrichment is involved), then last on the procedure is alphabetical order (which I assume would include R-center over S-center and E-geometry over Z-geometry, although that rule negates the sugar example).

    So basically, I have been absolutely no help other than to add a bit more confusion. Great question-- although maybe we should finish this after 8/17!!!

    Good luck!
     
  4. conure

    conure Master Distiller
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    This is kind of an interesting question.

    ultimately I think neither of these would be chiral. Because CIP only indicates what is bonded, not how it is bonded. Thus as you move across the double bonds of the alkenes labling each atom bonded you end up with the same things bonded to both thus not chiral.

    As for the second example the two substituents on the carbon in question must be alkanes and again as long as the same things are bonded to each then R/S makes no difference.

    There may be something out there that CIP does include stereochem but I'm not aware of it.
     
  5. limit

    limit Molesting my inner-child
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    In terms of organic chemistry principles tested on the MCAT, I highly doubt they would request you to determine R/S configurations or chirality based on priorities other than oxidation states and atomic weights -- neither can differentiate between geometric isomers. Still, it would be rather interesting to isolate and dump such a molecule into a polarimeter.
     
  6. NineSixteen

    NineSixteen Senior Member
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    Interesting thoughts, thanks y'all.

    I'll stick with my first answer to my student, which was, "That won't be tested on the MCAT...too complex, don't worry about it." Y'all seem to concur with that, eh?
     
  7. limit

    limit Molesting my inner-child
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    I believe R/S configurations were tested on a recent MCAT, so this question (if modified to have a solution that fits neatly with the general consensus of the the textbooks) would be fair game. Just get the basics of R/S, E/Z and priorities and your students should be okay.
     
  8. Mudd

    Mudd Charlatan & Trouble Maker
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    That would have been my answer to them. It is well beyond the scope of the MCAT. Of course, once I got home, being the absolute geek that I am, I would have gone fishing through through every textbook I have to get an answer. And you can bet I'd never admit to anyone that I had been such a geek as to look it up.

    Overall, limit is absolutely right... the MCAT surveys the basics of orgo in terms of stereochemistry.
     

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