© Prentice Hall 2001Chapter 41 Naming Enantiomers: The R,S System of Nomenclature 1.Rank groups by atomic number of the atom bonded to the chirality center. Use the same system that was used for the E and Z isomers of alkenes

© Prentice Hall 2001Chapter 42 2.Orient the molecule so that the group (or atom) of lowest priority is directed away from you Draw a curve from group of highest priority to the group of second priority Naming Enantiomers: The R,S System of Nomenclature

© Prentice Hall 2001Chapter 43 A similar set of rules exists for use with Fischer Projections 1.If the group of lowest priority is on a vertical line, just draw the curve from the group of highest priority to the group of second highest priority. If clockwise, configuration is R. If counterclockwise, configuration is S. (S)-2-bromobutane Naming Enantiomers: The R,S System of Nomenclature

© Prentice Hall 2001Chapter 44 2.If the group of lowest priority is on a horizontal line, follow step 1, but reverse the R & S. Rule of thumb: If lowest priority is on a Vertical bond, configuration is Very true. If lowest priority is on a Horizontal bond, configuration is Horribly wrong. (R)-2-bromobutane Naming Enantiomers: The R,S System of Nomenclature

© Prentice Hall 2001Chapter 45 3.In drawing the curve from the group of highest priority to the group of second highest priority, it is ok to go past the group of lowest priority, but never past the group of third priority. Naming Enantiomers: The R,S System of Nomenclature

© Prentice Hall 2001Chapter 46 Naming Enantiomers: The R,S System of Nomenclature In working with a Fischer Projection, a rotation of 90° or a horizontal flip will switch the chirality Rotation of a Fischer projection by 180° in the plane of the paper does not switch the chirality (R)-2-bromobutane (S)-2-bromobutane (R)-2-bromobutane

© Prentice Hall 2001Chapter 47 Drill on Problem 8 Indicate whether the following structure has R or S configuration R Switch CH 3 and CH 3 CH 2 groups

© Prentice Hall 2001Chapter 48 Drill on Problem 8 Indicate whether the following structure has R or S configuration R

© Prentice Hall 2001Chapter 49 Optical Rotation  is the measured rotation in degrees l is the path length in decimeters c is the concentration in grams per mL T is the temp in °C is the wavelength

© Prentice Hall 2001Chapter 410 Optical Rotation and Absolute Configuration In general there is no relationship between the R or S configuration of an enantiomer and the direction it rotates polarized light This must be determined by experiment (S)-(+)-lactic acid(S)-(–)-sodium lactate A mixture of equal amounts of two enantiomers is called a racemic mixture

© Prentice Hall 2001Chapter 411 Optical Purity For example, if a sample of 2-bromobutane has an observed specific rotation of +9.2°, and the specific rotation of pure (S)-(+) enantiomer is +23.1°, then

© Prentice Hall 2001Chapter 412 Compounds with More Than One Chirality Center

© Prentice Hall 2001Chapter 413 Meso Compounds

© Prentice Hall 2001Chapter 414 Drill on Meso Compounds Problem 27 Does the following compound have a stereoisomer that is a meso compound? No possibility of a plane of symmetry No meso form

© Prentice Hall 2001Chapter 415 Drill on Meso Compounds Problem 27 Does the following compound have a stereoisomer that is a meso compound? Plane of symmetry Meso form possible