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07-16 - Stereoisomers, R-S Nomenclature.docx

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Jeff Landry

 Equilibrium in Center: K = 1, = 0  Equilibrium to the Right: K > 1, < 0  Equilibrium to the Left: K < 1, > 0 Stereoisomerism  If you can superimpose two drawings on each other, they’re the same thing  “Configuration of Stereocenter”: R or S (Absolute Configuration)  X-ray Crystal Structure: “photo”  Polarimeter: Optical rotation of plane polarized light: o Polarize the light (so that only one plane of light continues), then you can see how it rotates o Image → Dextrorotatory (clockwise) → (+)-enantiomer o Mirror Image → Levorotatory (counter-clockwise) → (-)-enantiomer  R,S-Nomenclature: Rules in order of decreasing priority: a, b, c, d o A, b, c clockwise: R o A, b, c counterclockwise: S  When you have a carbon tetrahedral with 3 actual connected things and one electron pair on top (amine?), it rapidly inverts by switching the electron pair group to the bottom (right through the Carbon), then pushing the other three up  Priority Rules: 1. Order by atomic number, i.e. H = 1 → lowest 2. If same priority at first atom, go to first point of difference and judge 3. Multiple bonds: Add double or triple representations of atoms at the respective other end of the multiple bond Multiple Stereocenters  One trick to draw enantiomer is to switch position in every bond (wedge to dash and vice versa)  Diastereomers are stereoisomers that are not image-mirror-image related  If two molecules are super-imposable, they are just two drawings of the same thing
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