6. Chirality - 2010 Department of Chemistry, The University...

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© 2010 Department of Chemistry, The University of Western Ontario 6.1 6. Chirality (text 6.1 – 6.10) A. Review of Chirality and R/S Configuration Chiral objects do not have a plane of symmetry, so they are not superimposable on their mirror images. Achiral (non-chiral) objects have a plane of symmetry and are superimposable on their mirror images. 2-Chlorobutane is chiral and not superimposable on its mirror image. The two structures are enantiomers . Carbon #2 is bonded to four different substituents (H, Cl, CH 3 , and CH 2 CH 3 ) 2-Chloropropane is achiral and superimposable on its mirror image. i.e. its mirror image is identical. Carbon #2 is bonded to four substituents, two of which are identical (CH 3 ).
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6.2 The most common source of chirality in a molecule is a stereocentre (chiral centre), usually an atom bonded to four different groups (substituents). Enantiomers are stereoisomers that do not interconvert at room temperature. They are different compounds. They have the same bonding sequence, the same formula, and the same number and type of functional groups, so their chemistry is exactly the same , EXCEPT: o The two enantiomers have different effects on the rotation of plane-polarized light. o They react differently with other chiral compounds (this is the very important in biochemistry). We use the R and S convention to specify the absolute 3D configuration of stereocentres, which are commonly marked in structures as . The convention assigns a priority order to the substituents on the stereocentre according the same priority rules we examined when looking that the E/Z convention in alkenes. Example on assigning priorities: H C CH 3 OCH 3 Cl
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6.3 Using the priority assignments, we can assign an R or S configuration label to the molecule as follows: 1. Draw or place the molecule with the lowest-priority group (#4) behind the plane. 2. Draw an arrow going from the highest priority group (#1) to the second-highest (#2) and then the third-highest (#3). o Clockwise = absolute configuration is R Counterclockwise = absolute configuration is S 1 2 3 4 1 3 2 4 RS What are the R and S configurations of the following? O CH 2 CH 3 H ( )-2-ethylcyclohexanone N H ( )- N -methylindole C C CH 2 CH 3 OH H H O Fischer projection = C C CH 2 CH 3 OH H H O C CH 2 CH 3 HO H = H O
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6.4 Past test question: Which one is an E,R stereoisomer? H CH 3 N H Cl H CH 3 N Cl H H CH 3 N Cl H H CH 3 N Cl H H H 3 C N Cl H
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6.5 B. Molecules with Two or More Stereocentres A molecule with 2 stereocentres will have up to 2 2 = 4 potential stereoisomers, e.g. 2,3,4- trihydroxybutanal.
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6. Chirality - 2010 Department of Chemistry, The University...

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