Stereochemistry Stereochemistry refers to the 3-dimensional properties and reactions of molecules. It has its own language and terms that need to be learned in order to fully communicate and understand the concepts. Definitions Stereoisomers compounds with the same connectivity, different arrangement in space Enantiomers stereoisomers that are nonsuperimposible mirror images; only properties that differ are direction (+ or -) of optical rotation Diastereomers stereoisomers that are not mirror images; different compounds with different physical properties More Definitions Asymmetric center sp3 carbon with 4 different groups attached Optical activity the ability to rotate the plane of plane polarized light
Chiral compound a compound that is optically active (achiral compound will not rotate light) Polarimeter device that measures the optical rotation of the chiral compound Plane-Polarized Light Plane-Polarized Light through an Achiral Compound Plane-Polarized Light through a Chiral Compound Polarimeter Measures Optical Rotation Specific Rotation, ]] ]] = ] / cl = observed rotation c = concentration in g/mL l = length of tube in dm Dextrorotary designated as d or (+), clockwise rotation Levorotary designated as l or (-), counterclockwise rotation
Specific Rotations of some Common Organic Compounds Compound ] # * centers Penicillin V +233.0 3 Sucrose +66.5 10 Camphor +44.3 2 MSG +25.5 1 Cholesterol -31.3 8 Morphine -132.0 5 Chirality Center Carbon has four different groups attached Enantiomers nonsuperimposible mirror images mirror
plane OH H HO2C OH CH3 (S)(+) lactic acid from muscle tissue o  = +13.5 CH3 H CO 2H (R)(-) lactic acid from milk o  = -13.5 Enantiomeric Excess (Optical Purity)
observed rotation rotation of pure enantiomerx 100 = enantiomeric excess (e.e.) o observed rotation = +109 109.0 e.e. = 123.0 x 100 = 88.6% e.e. H H (S)-(-) Limonene o [= from lemons (R)(+) Limonene o  = +123.0 from oranges
88.6% (+) 11.4% racemic actually 94.3% (+) Biological Activity (R)(+) Thalidomide O O N H (S)(-) Thalidomide O H N O a sedative and hypnotic O O H
N O a teratogen H N O SSRI Efficacy depends on Stereochemistry NC O * N(CH3)2 (+/-) Celexa (-) Lexapro F Absolute Configuration Use Cahn, Ingold, Prelog priorities Place the lowest priority group back
(focus down C - 4 bond) draw arrow from 1-2-3 1 1 clockwise counterclockwise 4 4 2 2 3 3 (R) (S) Assign Priority to each Group on Asymmetric Center I Cl H I rotate F
4 H F3 1 Cl 2 focus down C-4 bond Lactic Acid 4 H HO2C 2 1 OH 1
OH CH3 3 (S) 4 H CO2H 2 CH3 3 (R) C.I.P. Priorities Low High CH2CH2CH3 CH(CH 3)2 O
CH2CH2OH CH 2CH CH2CH2CH3 CH=CH 2 CO2H CH 2Cl CH2CH2Br CH(CH3)2 Fischer Projections H HO 2C CH3 OH
OH OH H CO 2H CH3 H CO 2H CH3 Horizontal bonds approach you (wedge bonds) Vertical bonds move away (dashed bonds) Assigning Absolute Configuration to Fischer Projections H HO2C H
CH3 (S) OH OH OH rotate CO2H H CO2H CH3 CH3 (S) (S)
Rotation of the Projection 90o Reverses Absolute Configuration OH H 90 CO2H CH3 (S) H o CH3 OH CO2H (R) 90
CH3 o HO2C H OH (S) 90 o CO2H HO CH3 H (R) Diastereomers Stereoisomers That Are Not Mirror Images H OH
3 2 CO2H H OH 3 2 CO2H Br H H Br (2S,3S) (2S,3R) same stereochemistry at C2 (S) opposite stereochemistry at C3 Fischer Projections with 2 Chiral Centers CO2H H
2 Br 3 OH H CO2H H 2 OH H 3 Br CH3 CH3
(2S,3S) (2S,3R) 2 Chiral Centers 4 Stereoisomers Identical, Enantiomers or Diastereomers? CH2CH3 H a) Br CH3 & H CH2CH3 b)
Br HO H C CH3 CH3 C H CH3 NH2 CH3 & H NH2 H OH CH3
Tartaric Acids R,R CO2H H HO OH H S,S CO2H HO H H OH CO2H CO2H R,S CO2H
S,R CO2H H OH HO H H OH HO H CO2H CO2H Racemic Mixture R,R
S,S CO2H CO2H H OH HO H CO2H HO H H OH CO2H Racemic Mixture (Racemate): 50/50 mixture of enantiomers o
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