10 - The atom with the lowest atomic number is ranked...

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CH 310M/318M – Lecture 18 – W 3/3/2010 Reading Assignment : 3.6, 3.8 Textbook Problems : 3.2, 3.3, 3.20, 3.22-3.25 Graded Homework : HW#08 deadline = 5:00 p.m. on Thu 3/4 HW#09 deadline = 11:00 p.m. on Sun 3/7 Last Time: Bicycloalkanes – cis - and trans -decalin Physical and chemical properties of alkanes (end Chapter 2) Stereochemistry: chirality, enantiomers, chiral centers On Thursday, 3/4, Nate Bill will hold his office hours from 12:00 until 1:00 p.m. (instead of from 3:00 to 4:00 p.m.) Today : Chiral centers vs. stereocenters ( R )/( S ) configurations of chiral centers
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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) .
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Unformatted text preview: The atom with the lowest atomic number is ranked fourth (4) . 2) If a decision about priority cant 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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This note was uploaded on 10/25/2010 for the course CH 310M taught by Professor Iverson during the Spring '05 term at University of Texas at Austin.

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10 - The atom with the lowest atomic number is ranked...

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