Stereochemistry 2024 PDF

Stereochemistry Prof. Dr. Bahlul Zayed Awen 2024 7-1  Isomerism There are three major types of isomerism: constitutional isomerism...

Stereochemistry Prof. Dr. Bahlul Zayed Awen 2024 7-1  Isomerism There are three major types of isomerism: constitutional isomerism geometrical isomerism stereoisomerism optical isomerism Prof. Dr. Bahlul Zayed Awen 2024 7-2  Constitutional isomers Constitutional isomers (also known as structural isomers) have the same molecular formula but differ in the sequence in which the individual atoms are bonded (connectivity). skeletal isomerism C C C C C and C C C positional isomerism OH OH NO2 and NO2 Prof. Dr. Bahlul Zayed Awen 2024 7-3  Constitutional isomers Functional isomers: - compounds of identical molecular formula but which have different functional groups. e.g. C2H5OH and CH3OCH3 Prof. Dr. Bahlul Zayed Awen 2024 7-4  Molecular Chirality: Enantiomers Prof. Dr. Bahlul Zayed Awen 2024 7-5  Chirality A molecule is chiral if its two mirror image forms are not superposable upon one another. ASYMMETRIC! A molecule is achiral if its two mirror image forms are superposable. SYMMETRIC! Prof. Dr. Bahlul Zayed Awen 2024 7-6  Bromochlorofluoromethane is chiral Cl Br It cannot be superposed point H for point on its mirror image. F Prof. Dr. Bahlul Zayed Awen 2024 7-7  Bromochlorofluoromethane is chiral Cl Cl Br Br H H To show F F nonsuperposability, rotate this model 180° around a vertical axis. Prof. Dr. Bahlul Zayed Awen 2024 7-8  Bromochlorofluoromethane is chiral Cl Br Br Cl H H F F Prof. Dr. Bahlul Zayed Awen 2024 7-9  Another look Prof. Dr. Bahlul Zayed Awen 2024 7-10  Enantiomers nonsuperposable mirror images are called enantiomers and are enantiomers with respect to each other Prof. Dr. Bahlul Zayed Awen 2024 7-11  Isomers constitutional stereoisomers isomers Prof. Dr. Bahlul Zayed Awen 2024 7-12  Isomers constitutional stereoisomers isomers enantiomers diastereomers Prof. Dr. Bahlul Zayed Awen 2024 7-13  Chlorodifluoromethane is achiral Prof. Dr. Bahlul Zayed Awen 2024 7-14  Chlorodifluoromethane is achiral The two structures are mirror images, but are not enantiomers, because they can be superposed on each other. Prof. Dr. Bahlul Zayed Awen 2024 7-15  The Chirality Center Prof. Dr. Bahlul Zayed Awen 2024 7-16  The Chirality Center a carbon atom with four w different groups attached to it x C y also called: chiral center z asymmetric center stereocenter stereogenic center Prof. Dr. Bahlul Zayed Awen 2024 7-17  Chirality and chirality centers A molecule with a single chirality center is chiral. Bromochlorofluoromethane is an example. H Cl C F Br Prof. Dr. Bahlul Zayed Awen 2024 7-18  Chirality and chirality centers A molecule with a single chirality center is chiral. 2-Butanol is another example. H CH3 C CH2CH3 OH Prof. Dr. Bahlul Zayed Awen 2024 7-19  Examples of molecules with 1 chirality center CH3 CH3CH2CH2 C CH2CH2CH2CH3 CH2CH3 a chiral alkane Prof. Dr. Bahlul Zayed Awen 2024 7-20  Examples of molecules with 1 chirality center OH Linalool, a naturally occurring chiral alcohol Prof. Dr. Bahlul Zayed Awen 2024 7-21  Examples of molecules with 1 chirality center H2C CHCH3 O 1,2-Epoxypropane: a chirality center can be part of a ring attached to the chirality center are: —H —CH3 —OCH2 —CH2O Prof. Dr. Bahlul Zayed Awen 2024 7-22  Examples of molecules with 1 chirality center Limonene: a chirality CH3 center can be part of a ring attached to the chirality center are: H C CH2 —H —CH2CH2 CH3 —CH2CH=C —C=C Prof. Dr. Bahlul Zayed Awen 2024 7-23  Examples of molecules with 1 chirality center H D C CH3 T Chiral as a result of isotopic substitution Prof. Dr. Bahlul Zayed Awen 2024 7-24  A molecule with a single chirality center must be chiral. But, a molecule with two or more chirality centers may be chiral or it may not Prof. Dr. Bahlul Zayed Awen 2024 7-25  Symmetry in Achiral Structures Prof. Dr. Bahlul Zayed Awen 2024 7-26  Symmetry tests for achiral structures Any molecule with a plane of symmetry or a center of symmetry must be achiral. Prof. Dr. Bahlul Zayed Awen 2024 7-27  Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. Prof. Dr. Bahlul Zayed Awen 2024 7-28  Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves. 1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry. Prof. Dr. Bahlul Zayed Awen 2024 7-29  Properties of Chiral Molecules: Optical Activity Prof. Dr. Bahlul Zayed Awen 2024 7-30  Optical Activity A substance is optically active if it rotates the plane of polarized light. In order for a substance to exhibit optical activity, it must be chiral and one enantiomer must be present in excess of the other. Prof. Dr. Bahlul Zayed Awen 2024 7-31  Light has wave properties periodic increase and decrease in amplitude of wave Prof. Dr. Bahlul Zayed Awen 2024 7-32  Light optical activity is usually measured using light having a wavelength of 589 nm this is the wavelength of the yellow light from a Prof. Dr.sodium lamp Bahlul Zayed Awen 2024 and is called the D line of sodium 7-33  Polarized light ordinary (nonpolarized) light consists of many beams vibrating in different planes plane-polarized light consists of only those beams that vibrate in the same plane Prof. Dr. Bahlul Zayed Awen 2024 7-34  Polarization of light Nicol prism Prof. Dr. Bahlul Zayed Awen 2024 7-35  Rotation of plane-polarized light  Prof. Dr. Bahlul Zayed Awen 2024 7-36  Specific rotation observed rotation () depends on the number of molecules encountered and is proportional to: path length (l), and concentration (c) therefore, define specific rotation [] as: 100  concentration = g/100 mL [] = length in decimeters cl Prof. Dr. Bahlul Zayed Awen 2024 7-37  Racemic mixture a mixture containing equal quantities of enantiomers is called a racemic mixture a racemic mixture is optically inactive ( = 0) a sample that is optically inactive can be either an achiral substance or a racemic mixture Prof. Dr. Bahlul Zayed Awen 2024 7-38  Optical purity an optically pure substance consists exclusively of a single enantiomer enantiomeric excess = % one enantiomer – % other enantiomer % optical purity = enantiomeric excess e.g. 75% (-) – 25% (+) = 50% opt. pure (-) Prof. Dr. Bahlul Zayed Awen 2024 7-39  Absolute and Relative Configuration Prof. Dr. Bahlul Zayed Awen 2024 7-40  Configuration Relative configuration compares the arrangement of atoms in space of one compound with those of another. until the 1950s, all configurations were relative Absolute configuration is the precise arrangement of atoms in space. we can now determine the absolute configuration of almost any compound Prof. Dr. Bahlul Zayed Awen 2024 7-41  Relative configuration H2, Pd CH3CHCH CH2 CH3CHCH2CH3 OH OH [] + 33.2° [] + 13.5° No bonds are made or broken at the stereogenic center in this experiment. Therefore, when (+)-3-buten-2-ol and (+)-2-butanol have the same sign of rotation, the arrangement of atoms in space is analogous. The two have the same relative configuration. Prof. Dr. Bahlul Zayed Awen 2024 7-42  Two possibilities H HO H HO H2, Pd H OH H OH H2, Pd But in the absence of additional information, we can't tell which structure corresponds to (+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol. Prof. Dr. Bahlul Zayed Awen 2024 7-43  Two possibilities H HO H HO H2, Pd H OH H OH H2, Pd Nor can we tell which structure corresponds to (+)-2-butanol, and which one to (–)-2-butanol. Prof. Dr. Bahlul Zayed Awen 2024 7-44  Absolute configurations H HO H HO H2, Pd [] +33.2° [] +13.5° H OH H OH H2, Pd [] –33.2° [] –13.5° Prof. Dr. Bahlul Zayed Awen 2024 7-45  Relative configuration HBr CH3CH2CHCH2OH CH3CH2CHCH2Br CH3 CH3 [] -5.8° [] + 4.0° Not all compounds that have the same relative configuration have the same sign of rotation. No bonds are made or broken at the stereogenic center in the reaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes. Prof. Dr. Bahlul Zayed Awen 2024 7-46  The Cahn-Ingold-Prelog R-S Notational System Prof. Dr. Bahlul Zayed Awen 2024 7-47  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 The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog. Prof. Dr. Bahlul Zayed Awen 2024 7-48  The Cahn-Ingold-Prelog Rules 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. Prof. Dr. Bahlul Zayed Awen 2024 7-49  Example 1 1 4 3 3 4 2 2 Order of decreasing rank: 4>3>2 >1 Prof. Dr. Bahlul Zayed Awen 2024 7-50  The Cahn-Ingold-Prelog Rules 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. Prof. Dr. Bahlul Zayed Awen 2024 7-51  Example 1 1 4 3 3 4 2 2 Order of decreasing rank: 432 clockwise anticlockwise R S Prof. Dr. Bahlul Zayed Awen 2024 7-52  Enantiomers of 2-butanol H H CH3CH2 CH2CH3 C OH HO C H3C CH3 (S)-2-Butanol (R)-2-Butanol Prof. Dr. Bahlul Zayed Awen 2024 7-53  Very important! Two different compounds with the same sign of rotation need not have the same configuration. Prof. Dr. Bahlul Zayed Awen 2024 7-54  Chirality center in a ring H3C H R H H —CH2C=C > —CH2CH2 > —CH3 > —H Prof. Dr. Bahlul Zayed Awen 2024 7-55