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Two requirements for a system for specifying absolute configuration

Two requirements for a system for specifying absolute configuration. 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

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Two requirements for a system for specifying absolute configuration

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  1. Two requirements for a systemfor specifying absolute configuration 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.

  2. The Cahn-Ingold-Prelog Rules 1. Rank the substituents at the stereogenic center in order of atomic number (of the immediate atom) The atom with the highest atomic number gets 1. 2. Orient the molecule so that lowest-ranked substituent points away from you.

  3. 4 4 1 2 2 1 3 3 Example Order of decreasing rank:1 > 2 > 3 > 4

  4. The Cahn-Ingold-Prelog Rules 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.

  5. H H CH2CH3 CH3CH2 C HO C OH CH3 H3C Enantiomers of 2-butanol (S)-2-Butanol (R)-2-Butanol

  6. H3C H R H H Stereogenic center in a ring —CH2C=C > —CH2CH2 > —CH3 > —H

  7. Physical Properties of Enantiomers

  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

  9. Odor CH3 CH3 O O H3C H3C CH2 CH2 (–)-Carvonespearmint oil (+)-Carvonecaraway seed oil

  10. H H3C CH2CH(CH3)2 C C HO O Chiral drugs Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.

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