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Exploring Stereochemistry: Enantiomers, Optical Purity, and Separation Methods

Learn about enantiomeric excess, optical purity, and methods to separate enantiomers in stereochemistry. Discover how fractional recrystallization, chiral chromatography, and chiral probes aid in separation. Explore reactions forming products with asymmetric centers, the stereochemistry of H2 addition, and the role of chiral molecules in biological systems.

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Exploring Stereochemistry: Enantiomers, Optical Purity, and Separation Methods

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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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