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STEREO CHEMISTRY. 2 Enantiomeric excess (optical purity) Example…[  ] of (+)-alanine from fossil sample = + 4.25 o [  ] of pure (+)-alanine = + 8.5.

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Presentation on theme: "STEREO CHEMISTRY. 2 Enantiomeric excess (optical purity) Example…[  ] of (+)-alanine from fossil sample = + 4.25 o [  ] of pure (+)-alanine = + 8.5."— Presentation transcript:

1 STEREO CHEMISTRY

2 2 Enantiomeric excess (optical purity) Example…[  ] of (+)-alanine from fossil sample = o [  ] of pure (+)-alanine = o EE = optical purity (%) = {[  ] observed / [  ] 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 3 Separation of Enantiomers How can enantiomers be separated? Fractional recrystallization… (1)Reaction to convert enantiomers to diastereomers for 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 4 ………..different solubilities……….. Separation of a racemic aminoalkyne by fractional crystallization…

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7 7 Reactions involving chiral molecules (no change in chiral center, but R/S can change)

8 8 Electrophilic Addition Reactions Reactions that form products with asymmetric centers… achiral

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10 10 Reactant has a chiral center & reaction makes a new one… R R

11 11 Product with two chiral centers… * ** * ** * *

12 12 Stereochemistry of H 2 addition is syn….

13 13 Peroxyacid reaction  syn-addition

14 14 Enzymes are chiral reagents because their binding site is chiral Biological Molecules

15 15 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!

16 16 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….

17 17 Other stereogenic centers…

18 18 Broccoli contains sulforaphane… Increases the activity of certain enzymes capable of degrading toxic/carcinogenic compounds. Only (R)-stereoisomer is active and found in plants.

19 19 Thanks


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