IN-CLASS PROBLEM. ChemistryOnline, STEREOCHEMISTRY OF TETRAHEDRAL CENTERS. ChemistryOnline, No Plane of Symmetry

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1 hapter 5 Draw the structure of bromocyclopentane. Stereochemistry Reproduction or distribution of any of the content, or any of the images in this presentation is strictly prohibited without the expressed written consent of the copyright holder. Draw the structure of cis-1-bromo-3-chlorocyclopentane. STEREEMISTRY F TETRAEDRAL ENTERS The spatial arrangement of groups around a tetrahedral carbon (the stereochemistry) can be shown using molecular models, or represented using dashed lines and wedges. 2 N 3 STEREEMISTRY F TETRAEDRAL ENTERS It is important to be able to visualize this stereochemistry in order to test molecules for internal planes of symmetry. STEREEMISTRY F TETRAEDRAL ENTERS The net effect of this asymmetry is to generate a molecule which is not superimposible on it s mirror image. Plane of Symmetry No Plane of Symmetry 2 N 3

2 STEREEMISTRY F TETRAEDRAL ENTERS Bottom Line: ne consequence of tetrahedral geometry is an internal asymmetry which occurs whenever there are four different substituents arranged around a tetrahedral center. STEREEMISTRY F TETRAEDRAL ENTERS A carbon which is attached to four different substituents is called a chiral carbon, and a pair of non-superimposible mirror images are called enantiomers. A pair of enantiomers. STEREEMISTRY F TETRAEDRAL ENTERS There must be four different substituents attached to a carbon in order for it to be chiral. STEREEMISTRY F TETRAEDRAL ENTERS There must be four different substituents attached to a carbon in order for it to be chiral. 3 3 STEREEMISTRY F TETRAEDRAL ENTERS For each of the molecules shown below, indicate each of the chiral centers with an asterisk ( * ) STEREEMISTRY F TETRAEDRAL ENTERS For each of the molecules shown below, indicate each of the chiral centers with an asterisk ( * ) b d.

3 STEREEMISTRY F TETRAEDRAL ENTERS For the molecule shown below, indicate each of the chiral centers with an asterisk ( * ) STEREEMISTRY F TETRAEDRAL ENTERS Enantiomers are identical in every physical and chemical property (except in their interactions with other chiral molecules) except for the fact that they rotate the plane of plane polarized light in opposite directions, and hence chiral compounds are often termed optically active. Eight hiral enters 2 8 Stereoisomers Magnetic or Electric omponent Absorbed Differently; Resultant Vector is Displaced Magnetic or Electric omponent Absorbed Differently; Resultant Vector is Displaced A solution containing one enantiomer will therefore rotate polarized light one direction......and the other enantiomer will rotate polarized light in the opposite direction. Plane Polarized Light an Be Described as Identical Magnetic & Electric Vectors; Resultant is Vertical Plane A solution containing equal concentrations of both will therefore have no effect on plane polarized light. A mixture of equal concentrations of two enantiomers is called a Racemic Mixture. SPEIFI RTATIN [α] D The Specific Rotation is equal to the observed rotation (α) divided by the the pathlength of the cell (l) in dm, multiplied by the concentration () in g/ml [ α] D = bserved Rotation (degrees) Path length, l (dm) oncentration, (g / ml ) = α l ASSIGNING ABSLUTE NFIGURATIN The direction in which an optically active molecule rotates light is specific for a given molecule, but is not related to the absolute orientation of groups in that molecule around the chiral center. [α] could be either + or

4 ASSIGNING ABSLUTE NFIGURATIN In order to signify the absolute configuration, a system of nomenclature has been established in which groups around the chiral center are assigned priorities. The lowest priority group is placed towards the back, and the direction (clockwise or counterclockwise) of a line connecting the remaining groups is determined. ASSIGNING ABSLUTE NFIGURATIN In order to signify the absolute configuration, a system of nomenclature has been established in which groups around the chiral center are assigned priorities. The lowest priority group is placed towards the back, and the direction (clockwise or counterclockwise) of a line connecting the remaining groups is determined. ounterclockwise is denoted S II ockwise is denoted R II III IV I I IV III S (sinister; left) R (rectus; right) ASSIGNING ABSLUTE NFIGURATIN III II IV I I II IV III S (sinister; left) R (rectus; right) WELL, ALL F TIS IS FINE BUT, PRIRITIES? The ahn-ingold-prelog Rules 1. Rank atoms directly attached to the chiral center according to atomic number. 2. If there is a tie at any substituent, look at the second, third, etc., until a difference is found. Multiple bonds bonds as multiples of that same atom. 3. Rotate the lowest priority group to the back; if a line connecting the three highest priority groups in ascending order goes clockwise, the molecule is R; if the connecting line proceeds counterclockwise, the molecule is S. 1. The substituent below with the highest ranking according to the R,S rules is: 2. The substituent below with the lowest ranking according to the R,S rules is:

5 2 N 3

6 2 N 3 2 N ( 3 ) ( 3 ) 2 ( 3 ) s N N N 2 N 2 3 N

7 Draw the structure of the following molecules: (R)-2-bromobutane (S)-2-bromo-2-chlorobutane (S)-1-bromo-1-chloroethane Draw the structure of the following molecules: (R)-3-chloro-3-methylhexane Use an asterisk (*) to show the chiral centers in the molecules (1R,3S)-1,3-dimethylcyclohexane 3 3 (1R,3S)-1-chloro-3-methylcyclopentane 3 3 IN-LASS Stereoisomers PRBLEMthat are not mirror images of each other are called diastereomers. Unlike enantiomers, Determine diastereomers the absolute have completely configuration different of chemical the molecule and physical properties from each other. Use an asterisk (*) to show the chiral centers in the molecules Mirror Images 3 3 * * * 3 3 * Identical A pair of diastereomers.

8 IN-LASS MES PRBLEM MPUNDS are compounds which contain chiral centers, but the molecule as a whole is achiral. Use an asterisk These molecules (*) to show contain the an chiral internal centers plane of in the symmetry. molecules IN-LASS MES PRBLEM MPUNDS are compounds which contain chiral centers, but the molecule as a whole is achiral. Use an asterisk These molecules (*) to show contain the an chiral internal centers plane of in the symmetry. molecules h my Goodness they are identical!! Meso ompound. ow many chiral centers are there in these molecules? Are the molecules chiral? In-ass Quiz! Use an asterisk ( *) to show the chiral centers in the molecules 2,4-dimethylpentane 5-ethyl-3,3-dimethylheptane 4,5-dimethyloctane

9 Determine the absolute configuration of both of the chiral centers in the molecule

10 Determine the absolute configuration of the chiral centers in the molecule shown below: Determine the absolute configuration of the chiral centers in the molecule shown below: Determine the absolute configuration of the chiral centers in the molecule shown below: