,7.6 The Cahn Ingold Prelog R-S Notational System,1. need rules for ranking substituents at stereogenic center in order of decreasing precedence 2. need convention for orienting molecule so that order of appearance of substituents can be compared with rank The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog.,Two requirements for a system for specifying absolute configuration,1. Rank the substituents at the stereogenic center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked substituent points away from you.,The Cahn-Ingold-Prelog Rules (Table 7.1),Example,Order of decreasing rank: 4 3 2 1,1. Rank the substituents at the stereogenic center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked substituent points away from you. 3. If the order of decreasing precedence traces a clockwise path, the absolute configuration is R. If the path is anticlockwise, the configuration is S.,The Cahn-Ingold-Prelog Rules (Table 7.1),Example,Order of decreasing rank: 4 3 2,clockwise,R,anticlockwise,S,(S)-2-Butanol,Enantiomers of 2-butanol,(R)-2-Butanol,Very important! Two different compounds with the same sign of rotation need not have the same configuration.,Verify this statement by doing Problem 7.7 on page 269. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?,Stereogenic center in a ring,CH2C=C CH2CH2 CH3 H,7.7 Fischer Projections,Purpose of Fischer projections is to show configuration at stereogenic center without necessity of drawing wedges and dashes or using models.,Rules for Fischer projections,Arrange the molecule so that horizontal bonds at stereogenic center point toward you and vertical bonds point away from you.,Br,Cl,F,H,Rules for Fischer projections,Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.,Br,Cl,F,H,Rules for Fischer projections,Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.,Br,Cl,F,H,7.8 Physical Properties of Enantiomers,Same: melting point, boiling point, density, etc Different: properties that depend on shape of molecule (biological-physiological properties) can be different,Physical properties of enantiomers,O,O,CH3,CH3,H3C,H3C,CH2,CH2,Odor,()-Carvone spearmint oil,(+)-Carvone caraway seed oil,Ibuprofen is chiral, but normally sold as a racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.,Chiral drugs,