ORGANIC CHEMISTRY 3540

STEREOCHEMISTRY

DIASTEREOMERS

MESO COMPOUNDS

RACEMIC MIXTURES

RESOLUTION

REACTION STEREOCHEMISTRY

DIASTEREOMERS

WHAT IF YOU HAVE 2 STEREOCENTERS?

THEN YOU HAVE FOUR STEREOISOMERS!
DETERMINE IF EACH STEREOCENTER IS R OR S IN THE FOLLOWING FOUR STEREOISOMERS OF 3-BROMOBUTAN-2-OL.

ENANTIOMERS ARE MIRROR IMAGES AND HAVE IDENTICAL PROPERTIES (MP, BP, DENSITY, SOLUBILITY, ETC.), EXCEPT DIRECTION OF OPTICAL ROTATION AND REACTIVITY WITH A CHIRAL ENVIRONMENT.

DIASTEREOMERS ARE NOT MIRROR IMAGES AND HAVE DIFFERENT MP, BP, SOLUBILITY, ETC., ETC.

WHICH STEREOISOMER IS (2R,3S)-2-BROMO-3-CHLOROBUTANE?

MESO COMPOUNDS

CONSIDER 2,3-DIBROMOBUTANE.

2,3-DIBROMBUTANE EXISTS AS THREE STEREOISOMERS, TWO OPTICALLY ACTIVE ENANTIOMERS AND ONE OPTICALLY INACTIVE MESO.

WHY IS THE MESO OPTICALLY INACTIVE?

OTHER MESO COMPOUNDS.

CAN YOU FIND THE PLANE OF SYMMETRY IN THIS MESO COMPOUND?

MORE THAN 2 STEREOCENTERS.

MAXIMUM NUMBER OF STEREOISOMERS IS 2ⁿ.

HOW MANY STEREOISOMERS FOR EXAMPLE 1, EXAMPLE 2, EXAMPLE 3.

RACEMIC MIXTURES OR RACEMATES

50-50 MIXTURES OF THE R AND S STEREOISOMERS ARE OPTICALLY INACTIVE.

50-50 MIXTURES OF TWO ENANTIOMERS ARE CALLED RACEMIC MIXTURES.

OPTICALLY INACTIVE BECAUSE THEY CANCEL EACH OTHER OUT.

MOST LAB COMPOUNDS ARE RACEMIC AND MOST NATURAL COMPOUNDS ARE JUST ONE ENANTIOMER.

HOW DO YOU SEPARATE ENANTIOMERS?

A. PASTEUR LUCKY IN THAT ENANTIOMERIC CRYSTALS SEPARATED OUT.

B. FORMATION OF DIASTEREOMERS.

C. BIOLOGICAL. ADD A BIOLOGICAL SYSTEM WHICH REMOVES ONE ENANTIOMER.

STEREOCHEMISTRY OF REACTIONS

CH₃CH₂CH=CH₂ + HBr ---> CH₃CH₂CHBrCH₃

WHAT IS THE STEREOCHEMISTRY FOR THE ADDITION OF HBr TO ALKENES?

CONCLUSION: ACHIRAL STARTING MATERIAL ALWAYS LEADS TO OPTICALLY INACTIVE PRODUCTS (RACEMIC OR MESO)!

CH₃CH₂CH(CH₃)CH₂CH=CH₂ + HBr ---> CH₃CH₂CH(CH₃)CH₂CHBrCH₃

WHAT IS THE STEREOCHEMISTRY FOR THE ADDITION OF HBr TO A CHIRAL ALKENE?

CONCLUSION: A REACTION BETWEEN AN ACHIRAL STARTING MATERIAL (SUCH AS HBr) AND A CHIRAL STARTING MATERIAL LEADS TO UNEQUAL AMOUNTS OF DIATEREOISMERIC PRODUCTS! IF THE REACTANT WAS OPTICALLY ACTIVE, THE PRODUCT WILL BE OPTICALLY ACTIVE.

IS THERE CHIRALITY IN NATURE?