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UMass Amherst CHEM 261 - Enantiomers and Chiral Centers
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Lecture 11Chem 261 1st EditionOutline of Last Lecture:I. Chair Flip reviewII. Converting 2D to 3D images (or vice versa)III. DisubstitutedCyclohexanesIV. Polycyclic MoleculesV. Stereochemistry and Tetrahedral Centers introOutline of Current Lecture:I. EnantiomersII. Chirality CentersIII. Naming EnantiomersIV. DiastereomersEnantiomers:Chiral = no plane of symmetry (hands)Achiral = has a plane of symmetry Enantiomers: A carbon bound to 4 DIFFERENT substituents (key word, different substituents! Can be functional groups, not just one atom!)Ex. CH3X NOT enantiomerCH2XY NOT enantiomerCHXYZ IS an enantiomer! (because the carbon has 4 different substituents!)**may have to “twist” molecules to compare themChirality Centers:A chirality center is a tetrahedral carbon bound to 4 different substituents.**A chirality center is ALWAYS a tetrahedral carbon, but a tetrahedral carbon isn’t always a chirality center!!An asterisk (*) on the carbon denotes a chirality centerWhy are chirality centers important? Optical activity! (some chiral molecules are optically active.The direction of the light depends on the enantiomer, which can be mathematically calculated, and how much light gets rotated depends on its specific rotation, which can be calculated experimentally) -D = dextrorotary (+), clockwise direction-L = levorotary (-), anticlockwise directionSpecific Rotation: +/- enantiomers = pretty much the sameNaming Enantiomers: Use the Cahn-Ingold-Prelog systemR,S- naming of enantiomers. Configuration depends on positions of functional groups1. Priority assignment (based on the atomic number of the substituents. The highest atomic number gets the highest ranking, which is 1. The substituents are then numberedfrom 1 to 4)2. Tie-Breakers (If you have the same atoms, go to the next atom in the substituent)3. Double bonds (Example, if the carbon is double bound to oxygen, rewrite it so that the carbon is instead single bound to two oxygen atoms)4. Redraw the configuration so that the substituent with the lowest priority points to the back (dashed wedge)5. Draw the arrow of rotation going from highest substituent (1) to the lowest. Ignore the substituent numbered 4 because it is in the back6. If the direction goes anticlockwise, it is “S”. If the direction goes clockwise, it is “R”**Any time you change two substituents, you either get the original molecule or the enantiomer, depending on the direction**If you put the lowest substituent in the front, the direction must be reversed Diastereomers:Def: molecules with more than one chiral center. They are not super imposableEnantiomers: need an “R” for each “S” and vice versa**with multiple chiral centers in diastereomers, at least one is unchangedMesocompounds: overall achiral compound but has chiral centers. They have a plane of symmetry, and are optically


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