Chapter7_Part_I_of_II

Chapter7_Part_I_of_II - Problems at the end of chapter of...

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Unformatted text preview: Problems at the end of chapter of the textbook particularly, 6.29, 6.36, 6.64, 6.54, Recap: Kinetic and thermodynamic control: Bronsted acid-base are entirely thermodynamically controlled. Recall, I mention that they are all equalibrium. So the above implies that the stability of the products (conjugate base only, think about it) entirely dominates the reactivity, the shift of the equilibrium, how strong the acid (or the base) is, etc. Catalytic hydrogenation and SN2 reactions are kinetic controlled, which implies how easy the reactant react with each other matters. Think steric effect, strains, electron rich/poor, but all for the reactants. All reactions are affected or controlled by both, but some reactions are more dominated by one over the other. Which of the following undergoes catalytic hydrogenation much faster? (And why?) 1. A 2. B 77% A Answer: 2.B B 23% 1 2 For catalytic hydrogenation, one of the following give a mixture of two products, the other one gives predominantly one product. What are the product structures? And indicate which products are for which reactant. A B Which is the correct product for the following reaction? 1. 2. 3. 4. A B C D O3, Zn/H2O 28% O O O O 16% O Answer: 4.D 48% A O 8% B O O D C 1 2 3 4 Which is the correct product for the following reaction? 1. 2. 3. 4. A B C D 47% 44% O O O Answer: 3.C O3, Zn/H2O A O O O 8O % O B 0% D C 1 2 3 4 Match them. H O O H H O O 1 H O O HO 2 O 3 4 O O A O O O O O O B C H D How many chiralty centers are there in this molecule? 1. 2. 3. 4. One Two Three Four Answer: 2.Two OH NH2 55% 25% 17% 3% 1 2 3 4 How many stereoisomers are there in this molecule? 1. 2. 3. 4. One Two Three Four OH NH4% 32 39% 15% Answer: 4. Four 11% 1 2 3 4 How many chiralty centers is there in this molecule? 1. 2. 3. 4. One Two Three Four OH OH 46% 35% Answer: 2.Two 10% 1 2 3 9% 4 How many stereoisomers does this molecule have? 1. 2. 3. 4. One Two Three Four OH H3CO 52% OCH3 21% Answer: 3.Three OH 24% 3% 1 2 3 4 How many chiralty centers is there in this molecule? 1. 2. 3. 4. One Two Three Four Answer: 2.Two O 80% 7% 7% 1 O 2 3 6% 4 How many stereoisomers does this molecule have? 1. 2. 3. 4. One Two Three Four O O 53% 30% Answer: 3.Three 9% 8% 1 2 3 4 When all the chirality centers are different on a molecule, then there will be 2n isomers. When there are some chirality centers that are the same, then the number of stereoisomers maybe less than 2n depending whether there is meso compound or not. Organic Chemistry CHE 275 Chapter 7 Stereochemistry The most interesting part of organic chemistry – just me though. Chirality • A molecule or anything is chiral if its two mirror image forms are not superimposable upon one another. • A molecule or anything is achiral if its two mirror image forms are superimposable. Bromochlorofluoromethane is Chiral It cannot be superimposed point for point on its mirror image. Bromochlorofluoromethane is Chiral To show nonsuperimposability, rotate this model 180° around a vertical axis. around Bromochlorofluoromethane is Chiral Another Look Enantiomers nonsuperimposable mirror images are nonsuperimposable called enantiomers called and are enantiomers with respect to each other are Conformations (same molecules with different bond rotations) Isomers Isomers constitutional isomers stereoisomers Cis-trans isomer enantiomers diastereomers Chirality in Nature • Stereoisomers are readily distinguished by chiral proteins (receptors) in nature • Properties of drugs depend on stereochemistry • Think of biological recognition as equivalent to 3-point interaction Many Drugs Are Chiral H N O H OO H S N CH3 H CO2H CH3 CH3 CO2H Penicillin V (S) - Ibuprofin • When drugs are made and sold only one enantiomer is marketed • This is because the other enantiomer sometimes has toxic properties • An exception is Ibuprofin, which is sold as a racemate • Only one enantiomer is active, but it racemizes in the body anyway Odor CH3 O H3C CH3 O CH2 (–)-Carvone (–)-Carvone spearmint oil H3C CH2 (+)-Carvone caraway seed oil Chlorodifluoromethane is Achiral Chlorodifluoromethane is Achiral The two structures are mirror images, but are not enantiomers, because they can be superimposed on each other. The Chirality Center a carbon atom with four carbon different groups attached to it w x C z y also called: chiral center asymmetric center stereocenter stereogenic center Chirality and Chirality Centers A molecule with a single chirality center is chiral. Bromochlorofluoromethane is an example. H Cl C Br F Chirality and Chirality Centers A molecule with a single chirality center is chiral. is 2-Butanol is another example. H CH3 C OH CH2CH3 One Chirality Center CH3 CH CH3CH2CH2 C CH2CH2CH2CH3 CH2CH3 a chiral alkane One Chirality Center OH OH Linalool, a naturally occurring chiral alcohol Linalool, One Chirality Center H2C CHCH3 O 1,2-Epoxypropane: a chirality center 1,2-Epoxypropane: can be part of a ring attached to the chirality center are: —H —CH3 —OCH2 —CH2O One Chirality Center Limonene: a chirality center can be part of a ring ring CH3 CH H C CH3 CH2 attached to the chirality center are: —H —CH2CH2 —CH2CH= —C= One Chirality Center H D C CH3 T Chiral as a result of isotopic substitution Chiral A molecule with a single chirality center molecule must be chiral. But, a molecule with two or more But, chirality centers may be chiral chirality or it may not (Sections 7.10-7.13). Symmetry tests for achiral structures structures Any molecule with a plane of symmetry or a center of symmetry must be achiral. or achiral A Plane of Symmetry A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. Plane of Symmetry • The plane has the same thing on both sides for the flask • There is no mirror plane for a hand • We can apply this same analysis to molecules A 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. Optical Activity A substance is optically active if it rotates the plane of polarized light. the In order for a substance to exhibit optical activity, it must be chiral and one enantiomer activity, must be present in excess of the other. must Light has wave properties has periodic increase and decrease in amplitude of wave Light optical activity is usually measured using light having a wavelength of 589 nm wavelength this is the wavelength of the yellow light from a sodium this lamp and is called the D line of sodium lamp Polarized Light ordinary (nonpolarized) light consists of many beams vibrating in different planes different plane-polarized plane-polarized light consists of only those beams that vibrate in the same plane same Polarization of Light Nicol prism Rotation of plane-polarized light Rotation α Specific Rotation observed rotation (α) depends on the number observed depends of molecules encountered and is proportional to: of path length (l), and concentration (c) therefore, define specific rotation [α] as: [α] = 100 α 100 cl concentration = g/100 mL length in decimeters Racemic Mixtures a mixture containing equal quantities of enantiomers is called a racemic mixture racemic a racemic mixture is optically inactive optically (α = 0 ) a sample that is optically inactive can be either an achiral substance or a racemic mixture Optical Purity an optically pure substance consists exclusively of a single enantiomer of enantiomeric excess = enantiomeric % one enantiomer – % other enantiomer one % optical purity = enantiomeric excess Configuration Relative configuration compares the Relative arrangement of atoms in space of one compound with those of another. with Absolute configuration is the precise arrangement of atoms in space. arrangement Configuration Relative configuration compares the Relative arrangement of atoms in space of one compound with those of another. with until the 1950s, all configurations were relative Absolute configuration is the precise arrangement of atoms in space. arrangement we can now determine the absolute we configuration of almost any compound configuration Relative Configuration CH3CHCH CH2 Pd OH [α] + 33.2° CH3CHCH2CH3 OH [α] + 13.5° No bonds are made or broken at the chirality 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. Two Possibilities H HO OH P d , H2 H H P d , H2 HO OH H 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. Two Possibilities H HO OH P d , H2 H H P d , H2 HO Nor can we tell which structure corresponds to Nor (+)-2-butanol, and which one to (–)-2-butanol. OH H Absolute Configurations H OH P d , H2 H [α ] = − 3 .5 ϒ 1 OH [α ] = + 3 3 . 2 ϒ [α ] = + 1 3 .5 ϒ HO H P d, H 2 HO H [α ] = − 3 . 2 ϒ 3 Relative Configuration CH3CH2CHCH2OH HBr CH3CH2CHCH2Br CH3 [α] -5.8° CH3 [α] + 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 chirality center in the reaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes. Two requirements for a system for specifying absolute configuration 1. need rules for ranking substituents at chirality 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. Cahn-Ingold-Prelog Rules 1. Rank the substituents at the chirality center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked 2. substituent points away from you. Example Order of decreasing rank: Order 4>3>2 >1 The CIP Rules 1. Rank the substituents at the chirality 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 counterclockwise, the configuration is S. Example Order of decreasing rank: Order 4 →3 →2 clockwise clockwise R counterclockwise S R-Configuration at Chiral Center • Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn S-Configuration at Chiral Center • Lowest priority group is pointed away and direction of higher 3 is counterclockwise, or left turn Enantiomers of 2-Butanol CH3CH2 H C H OH H3C (S)-2-Butanol HO CH2CH3 C CH3 (R)-2-Butanol Chirality Center in a Ring H3C H R H H —CH2C=C > —CH2CH2 > —CH3 > —H —CH Please make your selection... 1. Choice One 2. Choice Two 0% 1 0% 2 Please make your selection... 1. Choice One 2. Choice Two 0% 1 0% 2 ...
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