Organic Chemistry,6th Edition L.G.Wade,Jr. Chapter 5 Stereochemistry Jo Blackburn Richland College,Dallas,TX Dallas County Community College District ©2006，Prentice Hall
Chapter 5 Stereochemistry Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2006, Prentice Hall Organic Chemistry, 6th Edition L. G. Wade, Jr
Stereoisomers Same bonding sequence. Different arrangement in space. Example:HOOC-CH=CH-COOH has two geometric(cis-trans)isomers: 0 O H &-0H o C-OH C=0 C=C HO-C H H H fumaric acid,mp 287C maleic acid,mp 138 C essential metabolite Chapter5 toxic irritant 2
Chapter 5 2 Stereoisomers • Same bonding sequence. • Different arrangement in space. • Example: HOOC-CH=CH-COOH has two geometric (cis-trans) isomers: HO C O C H C H C O OH fumaric acid, mp 287 C essential metabolite o H C C C H C O OH O HO maleic acid, mp 138 C toxic irritant o =>
Chirality ·“Handedness”:right glove doesn'tfit the left hand. 。 Mirror-image object is different from the original object. => right hand left hand Copyright 2005 Pearson Prentice Hall,Inc. 3
Chapter 5 3 Chirality • “Handedness”: right glove doesn’t fit the left hand. • Mirror-image object is different from the original object. =>
Chirality in Molecules The cis isomer is achiral. The trans isomer is chiral. Enantiomers:nonsuperimposable mirror images,different molecules. => same compound different compounds H cis-1,2-dichlorocyclopentane trans-1,2-dichlorocyclopentane (achiral) (chiral) Copyright2005 Pearson Prentice Hall,Inc. 4
Chapter 5 4 Chirality in Molecules • The cis isomer is achiral. • The trans isomer is chiral. • Enantiomers: nonsuperimposable mirror images, different molecules. =>
Stereocenters Any atom at which the exchange of two groups yields a stereoisomer. ·Examples: ·Asymmetric carbons Double-bonded carbons in cis-trans isomers CH2CH3 CH2CH2CH3 CH3 CH2CH3 H3 Br CH(CH3)2 asymmetric carbon chirality centers ( stereocenters(circled) Copyright2005 Pearson Prentice Hall,Inc
Chapter 5 5 Stereocenters • Any atom at which the exchange of two groups yields a stereoisomer. • Examples: • Asymmetric carbons • Double-bonded carbons in cis-trans isomers =>
Chiral Carbons Tetrahedral carbons with 4 different attached groups are chiral. If there's only one chiral carbon in a molecule,its mirror image will be a different compound (enantiomer). mirror => 6 Copyright2005 Pearson Prentice Hall,Inc
Chapter 5 6 Chiral Carbons • Tetrahedral carbons with 4 different attached groups are chiral. • If there’s only one chiral carbon in a molecule, its mirror image will be a different compound (enantiomer). =>
Mirror Planes of Symmetry If two groups are the same,carbon is achiral.(animation) A molecule with an internal mirror plane cannot be chiral.* Caution!If there is no plane of symmetry,molecule may be chiral or achiral.See if mirror image can be internal mirror plane superimposed.= of symmetry (o) Copyngh200 Pearson Prentice Hall,Ine Chapter 5
Chapter 5 7 Mirror Planes of Symmetry • If two groups are the same, carbon is achiral. (animation) • A molecule with an internal mirror plane cannot be chiral.* Caution! If there is no plane of symmetry, molecule may be chiral or achiral. See if mirror image can be superimposed. =>
(R),(S)Nomenclature Different molecules(enantiomers)must have different names. Usually only one enantiomer will be biologically active. OH Configuration around the chiral carbon is specified 米 with (R)and (S). CH NH, Chapter 5 natural alanine =8
Chapter 5 8 (R), (S) Nomenclature • Different molecules (enantiomers) must have different names. • Usually only one enantiomer will be biologically active. • Configuration around the chiral carbon is specified with (R) and (S). =>
Cahn-Ingold-Prelog Rules Assign a priority number to each group attached to the chiral carbon. Atom with highest atomic number assigned the highest priority #1. In case of ties,look at the next atoms along the chain. Double and triple bonds are treated like bonds to duplicate atoms. Chapter 5 9
Chapter 5 9 Cahn-Ingold-Prelog Rules • Assign a priority number to each group attached to the chiral carbon. • Atom with highest atomic number assigned the highest priority #1. • In case of ties, look at the next atoms along the chain. • Double and triple bonds are treated like bonds to duplicate atoms. =>
Assign Priorities 0、20H C 3C..H4 NH2 natural alanine H CH expands to 4 H *C.CH(CH3)2 CH2OH *CCH(CH3)2 2 CH2OH 三> Chapter 5 10
Chapter 5 10 Assign Priorities C C O OH H3C NH2 H natural alanine 1 2 3 4 Cl Cl H H * 1 2 3 4 1 2 3 4 => C C O H C H CH2 CH2OH CH(CH3) 2 * expands to C C O C CH2 CH2OH CH(CH3) 2 H O H C C C *