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Unformatted text preview: Stereochemistry I R and S in Alkanes A chiral carbon has four different groups attached to it, the differences may be several bonds away, but so long as they are different in any way the groups are considered different. o Stereocenters are chiral carbons and vice versa Chiral molecules are asymmetric molecules o They do not have an internal mirror plane o They nearly always have at least one stereocenter Chiral molecules may lack a mirror plane and have no chiral centers, they exist but are beyond the scope of this course (i.e. helicene) o They are optically active and rotate plane-polarized light Left (counterclockwise) rotation is (-) and levorotory Right (clockwise) rotation is (+) and dextrorotory Individual chiral centers are classified as either R or S. o R/S designation is completely unrelated to the direction of rotation of plane polarized light. Assigning R vs. S stereocenters The Absolute Configuration: o Working outward from the chiral carbon assign priority to each attached group. (Cahn-Ingold-Prelog Priority Rules) 1 st priority is assigned by atomic number, heavier atoms get priority In order 1 st Br then Cl then C and lastly H 4 th if isotopes are encountered the heavier isotope gets priority 14 C then 13 C then 12 C If two of the same atom are encountered priority is assigned by the first difference in bonded atoms Multiple bonds count as multiple single bonds to the same atom o Once priority is assigned rotate the lowest priority to the back If 1 2 3 proceeds clockwise the center is R (right turning) If 1 2 3 proceeds counterclockwise the center is S (left/sinister turning) 2 3 1 1 3 2 4 4 Counterclockwise (Leftward turning) S Clockwise (Rightward turning) R Examples: CH 2 CH 2 Br C OH H 3 C H F C H COOH CH 2 OH Br NC CHO Compound A: CH 2 CH 2 Br C OH H 3 C H 1 2 3 4 CH 2 CH 2 Br C OH H 3 C H 1 2 3 4 clockwise = R (2R)-4-bromo-2-butanol...
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