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STEREO CHEMISTRY

STEREO CHEMISTRY. Enantiomeric excess (optical purity). Example…[ a ] of (+)-alanine from fossil sample = + 4.25 o [ a ] of pure (+)-alanine = + 8.5 o EE = optical purity (%) = {[ a ] observed / [ a ] pure }x100 So…EE = op = {4.25 / 8.5} x 100 = 50%

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STEREO CHEMISTRY

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  1. STEREO CHEMISTRY

  2. Enantiomeric excess (optical purity) Example…[a] of (+)-alanine from fossil sample = + 4.25o [a] of pure (+)-alanine = + 8.5o EE = optical purity (%) = {[a]observed / [a]pure}x100 So…EE = op = {4.25 / 8.5} x 100 = 50% Interpretation…50% of sample is pure (+), other 50% is racemic mixture Each square represents 25% of the sample (+) (+) 50% is (+) (-) (+) 50% is racemic = 25% (+) + 25% (-) total sample = 75% (+) and 25% (-)

  3. Separation of Enantiomers • How can enantiomers be separated? • Fractional recrystallization… • Reaction to convert enantiomers to diastereomersfor separation • (disastereomers have different physical properties). • (2) Separate diastereomers  convert back to enantiomers • Chromatography - passage of a solution of enantiomers through a column packed with a chiral material - one enantiomer will have a greater affinity (like a right hand prefers a right glove) and travel slower through the column. • Chiral probes - materials capable of distinguishing between enantiomers

  4. Separation of a racemic aminoalkyne by fractional crystallization… ………..different solubilities………..

  5. Reactions involving chiral molecules (no change in chiral center, but R/S can change)

  6. Electrophilic Addition Reactions Reactions that form products with asymmetric centers… achiral

  7. Reactant has a chiral center & reaction makes a new one… R R

  8. Product with two chiral centers… * * * * * * * *

  9. Stereochemistry of H2 addition is syn….

  10. Peroxyacid reaction syn-addition

  11. Biological Molecules Enzymes are chiral reagents because their binding site is chiral

  12. Receptors - protein that binds a particular molecule • Also are chiral - allow selective binding of enantiomers • Example - receptors in the nose are able to distinguish between 10,000 different smells - each enantiomer fits into a different receptor The nose knows!

  13. Many drug molecules are synthesized as racemates..either both molecules have comparable activity or one is inactive. R = bronchodilator (albuterol) S = antagonist 19 of 20 essential amino acids are chiral….

  14. Other stereogenic centers…

  15. Broccoli contains sulforaphane… Increases the activity of certain enzymes capable of degrading toxic/carcinogenic compounds. Only (R)-stereoisomer is active and found in plants.

  16. Thanks

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