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03042009 - (4 2 If a decision about priority can’t be...

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CH 310M/318M – MWF 9:00 – Lecture 19 – W 3/4/2009 Reading Assignment : Sections 4.1 & 4.2 Last Time : Stereocenters vs. chiral centers Properties of enantiomers Optical activity Assigning ( R )/( S ) designations to chiral centers Textbook Problems : 3.4, 3.5, 3.7, 3.8, 3.10(a), 3.25-3.28, 3.30-3.32, 3.34, 3.37 Graded Homework : HW#08 deadline = 3:00 p.m. on Thu 3/5 HW#09 deadline = 3:00 p.m. on Mon 3/9 Today : Assigning ( R )/( S ) designations to chiral centers (continued…)
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Assigning R , S Stereochemical Designations This system is based on the Cahn–Ingold–Prelog priority rules. These rules are used to assign relative priorities to the four different groups attached to a chiral center. 1) Looking at the atoms directly attached to the chiral center, assign priorities in order of decreasing atomic number . The atom with the highest atomic number is ranked first (1) . The atom with the lowest atomic number is ranked fourth
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Unformatted text preview: (4) . 2) If a decision about priority can’t be reached by applying rule 1, continue moving away from the chiral center, and compare atomic numbers of the second grouping of atoms. Continue on as necessary, always moving away from the chiral center, until the first point of difference is reached. 3) Multiply-bonded atoms are considered as if they were an equivalent number of singly-bonded atoms. 4) Once priorities have been assigned, the molecule is oriented so that the lowest priority group (4) points directly away from you. Then, observe how the other 3 groups are arranged around a circle. In effect, draw a Newman projection looking down the bond between the stereocenter and the group of priority (4) If moving from 1 2 3 is a → → clockwise movement, the configuration is R . If moving from 1 2 3 is a → → counterclockwise movement, the configuration is S ....
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