7.6 The Cahn-ingold-Prelog R-S Notational System TABLE 7.1 Absolute Configuration According to the Cahn-Ingold-Prelog Notational System Step number Example CHaCH Given that the absolute configuration of (+)-2-butanol is H3C 1. Identify the substituents at the stereogenic center, In order of decreasing precedence, the four substitu and rank them in order of decreasing precedence ents attached to the stereogenic center of 2-butanol according to the system described in Section 5.4 Precedence is determined by atomic number, work- outward from the point of attachment at th stereogenIc center. 2. Orient the molecule so that the lowest ranked sub- As represented in the wedge-and-dash drawing at stituent points away from you the top of this table the molecule is already appro- stituent attached to the stereogenic center and 46. priately oriented. Hydrogen is the lowest ranked points away from us. 3. Draw the three highest ranked substituents as they CHaC the lowest ranked group points away from you. that ppear to you when the molecule is oriented so 4. If the order of decreasing precedence of the three The order of decreasing precedence is anticlockwise highest ranked substituents appears in a clockwise The configuration at the stereogenic center is s ense, the absolute configuration is R(Latin rectus, right, ""correct"). If the order of decreasing prece- CH3 k OH(highest dence is anticlockwise, the absolute configuration is S Latin sinister, "left") Often, the R or S configuration and the sign of rotation are incorporated into the name of the compound, as in(R)-(-)-2-butanol and (S)-(+)-2-butanol PROBLEM 7.7 Assign absolute configurations as r or S to each of the following (a) H3C -CH2OH CHC CH3CH2 (+)-2-Methyl-1-butanol (+)-1-Bromo-2-methylbutane H3 (d) H3c CHF CHaCI (+)-1-Fluoro-2-methylbutane (+)-3-Buten-2-oI Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteOften, the R or S configuration and the sign of rotation are incorporated into the name of the compound, as in (R)-()-2-butanol and (S)-()-2-butanol. PROBLEM 7.7 Assign absolute configurations as R or S to each of the following compounds: (a) (c) (b) (d) ()-3-Buten-2-ol C H HO H3C CH CH2 ()-1-Fluoro-2-methylbutane C H CH3CH2 H3C CH2F ()-1-Bromo-2-methylbutane C CH3 CH3CH2 H CH2Br ()-2-Methyl-1-butanol C H CH3CH2 H3C CH2OH 7.6 The Cahn–Ingold–Prelog R–S Notational System 269 TABLE 7.1 Absolute Configuration According to the Cahn–Ingold–Prelog Notational System Step number 1. Identify the substituents at the stereogenic center, and rank them in order of decreasing precedence according to the system described in Section 5.4. Precedence is determined by atomic number, work￾ing outward from the point of attachment at the stereogenic center. 2. Orient the molecule so that the lowest ranked sub￾stituent points away from you. 4. If the order of decreasing precedence of the three highest ranked substituents appears in a clockwise sense, the absolute configuration is R (Latin rectus, “right,” “correct”). If the order of decreasing prece￾dence is anticlockwise, the absolute configuration is S (Latin sinister, “left”). Example In order of decreasing precedence, the four substitu￾ents attached to the stereogenic center of 2-butanol are As represented in the wedge-and-dash drawing at the top of this table, the molecule is already appro￾priately oriented. Hydrogen is the lowest ranked sub￾stituent attached to the stereogenic center and points away from us. The order of decreasing precedence is anticlockwise. The configuration at the stereogenic center is S. 3. Draw the three highest ranked substituents as they appear to you when the molecule is oriented so that the lowest ranked group points away from you. HO± CH3CH2± CH3± (highest) H± (lowest)   CH3CH2 OH CH3 CH3CH2 OH CH3 (highest) (second highest) (third highest) ()-2-Butanol C H H3C CH3CH2 Given that the absolute configuration of ()-2-butanol is OH Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website