Chapter 7 STEREOCHEMISTRY

CHIRALITY AND ENATIOMERS Chiral Object – an object that IS NOT superimposable with its mirror image. Achiral Object – an object that IS superimposable (i.e. identical) with its mirror image. Enantiomers – stereoisomers, which are non-superimposable mirror images. Stereochemistry

CHIRAL CARBON ATOMS Chiral Carbon Atoms – carbon atoms bonded to FOUR DIFFERENT groups. Chiral atoms are generally called chirality centers. * * mirror Some rules: If a molecule has NO chirality center, it is usually, but not necessarily achiral. If a molecule has just ONE chirality center, it is definitely chiral. If a molecule has MORE THAN ONE chirality center, it may or may not be chiral. Stereochemistry

THE MIRROR IMAGES OF CARBON CENTERS WITH THREE DIFFERENT SUBSTITUENTS ARE SUPERIMPOSABLE IDENTICAL WITH THE ORIGINAL Stereochemistry

THE MIRROR IMAGES OF CARBON CENTERS WITH FOUR DIFFERENT SUBSTITUENTS (CHIRAL CARBON ATOMS) ARE NOT SUPERIMPOSABLE 120o * * * mirror NOT IDENTICAL WITH THE ORIGINAL Stereochemistry

CARBON CENTERS WITH FOUR DIFFERENT SUBSTITTUENTS (CHIRAL CARBON ATOMS) DO NOT HAVE INTERNAL MIRROR PLANES OF SYMMETRY Stereochemistry

CHIRAL COMPOUNDS ALSO DO NOT HAVE INTERNAL MIRROR PLANES OF SYMMETRY MIRROR IMAGE IDENTICAL WITH ORIGINAL MIRROR IMAGE IS NOT IDENTICAL WITH ORIGINAL Stereochemistry

THE CAHN – INGOLD – PRELOG CONVENTION Assign a “priority” and a number to each group, bonded to the chiral carbon atom. Highest priority group gets number 1, second in priority – number 2, third in priority – number 3, lowest in priority – number 4. Orient the molecule in a fashion such that the lowest in priority group (number 4) is in back and view the molecule along the bond from the chiral center to group #4. Draw a curved arrow, starting from #1, through #2, to #3. If the arrow points in a clockwise direction – R-enantiomer, if the arrow points in a counter-clockwise direction – S-enantiomer. Stereochemistry

THALIDOMIDE: DISASTROUS BIOLOGICAL ACTIVITY OF THE “WRONG” ENANTIOMER Stereochemistry