8Chapter 07

8Chapter 07 - Chapter 7 Stereochemistry Dr. Wolf's CHM 201...

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Unformatted text preview: Chapter 7 Stereochemistry Dr. Wolf's CHM 201 & 202 7-1 7.1 Molecular Chirality: Molecular Enantiomers Enantiomers Dr. Wolf's CHM 201 & 202 7-2 Chirality Chirality A molecule is chiral if its two mirror image molecule chiral forms are not superposable upon one another. are ASYMMETRIC! ASYMMETRIC! A molecule is achiral if its two mirror image molecule achiral forms are superposable. SYMMETRIC! are Dr. Wolf's CHM 201 & 202 7-3 Bromochlorofluoromethane is chiral Bromochlorofluoromethane is chiral Cl Br H It cannot be It superposed point for point on its mirror image. mirror F Dr. Wolf's CHM 201 & 202 7-4 Bromochlorofluoromethane is chiral Bromochlorofluoromethane is chiral Cl Cl Br Br H F H F To show To nonsuperposability, rotate this model 180° around a vertical axis. vertical Dr. Wolf's CHM 201 & 202 7-5 Bromochlorofluoromethane is chiral Bromochlorofluoromethane is chiral Cl Cl Br Br H F Dr. Wolf's CHM 201 & 202 H F 7-6 Another look Another look Dr. Wolf's CHM 201 & 202 7-7 Enantiomers Enantiomers nonsuperposable mirror images are nonsuperposable called enantiomers called and are enantiomers with respect to each other Dr. Wolf's CHM 201 & 202 7-8 Isomers constitutional isomers Dr. Wolf's CHM 201 & 202 stereoisomers 7-9 Isomers constitutional isomers enantiomers Dr. Wolf's CHM 201 & 202 stereoisomers diastereomers 7-10 Chlorodifluoromethane Chlorodifluoromethane is achiral is achiral Dr. Wolf's CHM 201 & 202 7-11 Chlorodifluoromethane Chlorodifluoromethane is achiral is achiral The two The structures are mirror images, but are not enantiomers, because they can be superposed on each other. each Dr. Wolf's CHM 201 & 202 7-12 7.2 The Chirality Center Dr. Wolf's CHM 201 & 202 7-13 The Chirality Center a carbon atom with four different groups attached to it w x C y z Dr. Wolf's CHM 201 & 202 also called: chiral center asymmetric center stereocenter stereogenic center 7-14 Chirality and chirality centers A molecule with a single chirality center molecule is chiral. is Bromochlorofluoromethane is an example. H Cl C F Br Dr. Wolf's CHM 201 & 202 7-15 Chirality and chirality centers A molecule with a single chirality center molecule is chiral. is 2-Butanol is another example. H CH3 C CH2CH3 OH Dr. Wolf's CHM 201 & 202 7-16 Examples of molecules with 1 chirality center CH3 CH3CH2CH2 C CH2CH2CH2CH3 CH2CH3 a chiral alkane Dr. Wolf's CHM 201 & 202 7-17 Examples of molecules with 1 chirality center OH OH Linalool, a naturally occurring chiral alcohol Dr. Wolf's CHM 201 & 202 7-18 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 Dr. Wolf's CHM 201 & 202 7-19 Examples of molecules with 1 chirality center Limonene: a chirality Limonene: center can be part of a ring ring CH3 CH H C CH3 CH2 attached to the chirality center are: —H —CH2CH2 —CH2CH=C —C=C Dr. Wolf's CHM 201 & 202 7-20 Examples of molecules with 1 chirality center H D C CH3 T Chiral as a result of isotopic substitution Dr. Wolf's CHM 201 & 202 7-21 A molecule with a single chirality center 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). Dr. Wolf's CHM 201 & 202 7-22 7.3 Symmetry in Achiral Symmetry Structures Structures Dr. Wolf's CHM 201 & 202 7-23 SSymmetrytests for achiral sstructures ymmetry Symmetry tests foraachiral tructures chiral Symmetry achiral Any molecule with a plane of symmetry or a center of symmetry must be achiral. or achiral Dr. Wolf's CHM 201 & 202 7-24 Plane of symmetry A plane of symmetry bisects a molecule into two plane mirror image halves. Chlorodifluoromethane has a plane of symmetry. has Dr. Wolf's CHM 201 & 202 7-25 Plane of symmetry A plane of symmetry bisects a molecule into two plane mirror image halves. mirror 1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry. Dr. Wolf's CHM 201 & 202 7-26 Center of symmetry Center A point in the center of the molecule is a center of molecule symmetry if a line drawn from it to any element, when extended an equal distance in the opposite direction, encounters an identical element. Dr. Wolf's CHM 201 & 202 7-27 7.4 Properties of Chiral Molecules: Optical Activity Dr. Wolf's CHM 201 & 202 7-28 Optical Activity Optical Activity A substance is optically active if it rotates substance 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 Dr. Wolf's CHM 201 & 202 7-29 Light Light has wave properties periodic increase and decrease in amplitude of periodic wave wave Dr. Wolf's CHM 201 & 202 7-30 Light Light optical activity is usually measured using light optical having a wavelength of 589 nm having this is the wavelength of the yellow light from a this sodium lamp and is called the D line of sodium sodium Dr. Wolf's CHM 201 & 202 7-31 Polarized light ordinary 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 Dr. Wolf's CHM 201 & 202 7-32 Polarization of light Nicol prism Dr. Wolf's CHM 201 & 202 7-33 Rotation of plane-polarized light α Dr. Wolf's CHM 201 & 202 7-34 Specific rotation observed rotation (α) depends on the number depends of molecules encountered and is proportional to: of path length (l), and concentration (c) ), and therefore, define specific rotation [α] as: [α] = 100 α 100 cl Dr. Wolf's CHM 201 & 202 concentration = g/100 mL length in decimeters 7-35 Racemic mixture a mixture containing equal quantities mixture 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 Dr. Wolf's CHM 201 & 202 7-36 Optical purity Optical purity an optically pure substance consists exclusively an of a single enantiomer of enantiomeric excess = enantiomeric % one enantiomer – % other enantiomer one % optical purity = enantiomeric excess optical e.g. 75% (-) – 25% (+) = 50% opt. pure (-) e.g. Dr. Wolf's CHM 201 & 202 7-37 7.5 Absolute and Relative Configuration Dr. Wolf's CHM 201 & 202 7-38 Configuration Configuration Relative configuration compares the 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 Dr. Wolf's CHM 201 & 202 7-39 Relative configuration Relative configuration CH3CHCH CH2 OH [α] + 33.2° H2, Pd Pd CH3CHCH2CH3 OH [α] + 13.5° No bonds are made or broken at the stereogenic center in this experiment. Therefore, when (+)-3-buten-2-ol in and (+)-2-butanol have the same sign of rotation, the arrangement of atoms in space is analogous. The two arrangement have the same relative configuration. Dr. Wolf's CHM 201 & 202 7-40 Two possibilities Two possibilities HO H H H2, Pd HO OH H2, Pd H H OH But in the absence of additional information, we can't tell But which structure corresponds to which (+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol. Dr. Wolf's CHM 201 & 202 7-41 Two possibilities Two possibilities HO H H H2, Pd HO OH H2, Pd H H OH Nor can we tell which structure corresponds to (+)-2-butanol, and which one to (–)-2-butanol. Dr. Wolf's CHM 201 & 202 7-42 Absolute configurations Absolute configurations HO H H2, Pd OH [α] –33.2° Dr. Wolf's CHM 201 & 202 H [α] +13.5° [α] +33.2° H HO H2, Pd H OH [α] –13.5° 7-43 Relative configuration Relative configuration CH3CH2CHCH2OH CH3 [α] -5.8° HBr CH3CH2CHCH2Br 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 stereogenic center in the reaction shown, so the relative positions of the atoms reaction are the same. Yet the sign of rotation changes. are Dr. Wolf's CHM 201 & 202 7-44 7.6 7.6 The Cahn-Ingold-Prelog R-S R-S Notational System Notational Dr. Wolf's CHM 201 & 202 7-45 Two requirements for aasystem Two requirements for system for specifying absolute configuration for specifying absolute configuration 1. 1. need rules for ranking substituents at ranking stereogenic center in order of decreasing precedence precedence 2. 2. need convention for orienting molecule so orienting 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. V. Dr. Wolf's CHM 201 & 202 7-46 The Cahn-Ingold-Prelog Rules The Cahn-Ingold-Prelog Rules (Table 7.1) (Table 7.1) 1. Rank the substituents at the stereogenic 1. center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked 2. substituent points away from you. Dr. Wolf's CHM 201 & 202 7-47 Example Example 1 1 3 4 4 3 2 2 Order of decreasing rank: 4>3>2 >1 Dr. Wolf's CHM 201 & 202 7-48 The Cahn-Ingold-Prelog Rules The Cahn-Ingold-Prelog Rules (Table 7.1) (Table 7.1) • • • 1. Rank the substituents at the stereogenic 1. center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked 2. substituent points away from you. substituent 3. If the order of decreasing precedence traces 3. a clockwise path, the absolute configuration is R. If the path is anticlockwise, the If configuration is S. Dr. Wolf's CHM 201 & 202 7-49 Example Example 1 1 3 4 4 3 2 2 Order of decreasing rank: 4 →3 →2 clockwise clockwise R Dr. Wolf's CHM 201 & 202 anticlockwise S 7-50 Enantiomers of 2-butanol Enantiomers of 2-butanol CH3CH2 H C H OH H3C (S)-2-Butanol Dr. Wolf's CHM 201 & 202 HO CH2CH3 C CH3 (R)-2-Butanol 7-51 Very important! Very important! Two different compounds with the Two same sign of rotation need not have the same configuration. the Dr. Wolf's CHM 201 & 202 7-52 Chirality center in aaring Chirality center in ring H3C H R H H —CH2C=C > —CH2CH2 > —CH3 > —H —CH Dr. Wolf's CHM 201 & 202 7-53 7.7 7.7 Fischer Projections Fischer • Purpose of Fischer projections is to show Purpose configuration at chirality center without necessity of drawing wedges and dashes or using models. Dr. Wolf's CHM 201 & 202 7-54 Rules for Fischer projections Rules for Fischer projections H Cl Br F Arrange the molecule so that horizontal Arrange bonds at chirality center point toward you and vertical bonds point away from you. and Dr. Wolf's CHM 201 & 202 7-55 Rules for Fischer projections Rules for Fischer projections H Br Cl F Projection of molecule on page is a cross. When Projection represented this way it is understood that horizontal bonds project outward, vertical bonds are back. are Dr. Wolf's CHM 201 & 202 7-56 Rules for Fischer projections Rules for Fischer projections H Br Cl F Projection of molecule on page is a cross. When Projection represented this way it is understood that horizontal bonds project outward, vertical bonds are back. are Dr. Wolf's CHM 201 & 202 7-57 7.8 Physical Properties of Physical Enantiomers Enantiomers Dr. Wolf's CHM 201 & 202 7-58 Physical properties of enantiomers Physical properties of enantiomers Physical Same: melting point, boiling point, density, etc melting Different: Different: properties that depend on shape of molecule (biological-physiological properties) can be (biological-physiological different different Dr. Wolf's CHM 201 & 202 7-59 Odor Odor CH3 O H3C O CH2 (–)-Carvone (–)-Carvone spearmint oil Dr. Wolf's CHM 201 & 202 CH3 H3C CH2 (+)-Carvone caraway seed oil 7-60 Chiral drugs Chiral drugs Chiral Ibuprofen is chiral, but normally sold as a racemic mixture. The S enantiomer racemic is the one responsible for its analgesic and antiinflammatory properties. H3C H CH2CH(CH3)2 C HO HO Dr. Wolf's CHM 201 & 202 C O 7-61 7.10 Reactions That Create A Reactions Chiral Center Chiral Dr. Wolf's CHM 201 & 202 7-62 Many reactions convert achiral Many reactions convert achiral reactants to chiral products. reactants to chiral products. It is important to recognize, however, that if all of the It components of the starting state (reactants, catalysts, solvents, etc.) are achiral, any chiral product will be formed as a racemic mixture. formed This generalization can be more simply stated as This "Optically inactive starting materials can't give optically active products." (Remember: In order for a optically substance to be optically active, it must be chiral and one enantiomer must be present in greater amounts than the other. other. Dr. Wolf's CHM 201 & 202 7-63 Example Example O CH3CH H CH3COOH CH2 H3C CH2 C O Achiral Dr. Wolf's CHM 201 & 202 Chiral, but racemic 7-64 epoxidation from this direction epoxidation gives R epoxide R Dr. Wolf's CHM 201 & 202 7-65 epoxidation from this direction epoxidation gives R epoxide R S epoxidation from this direction epoxidation gives S epoxide Dr. Wolf's CHM 201 & 202 7-66 epoxidation from this direction epoxidation gives R epoxide 50% R 50% S epoxidation from this direction epoxidation gives S epoxide Dr. Wolf's CHM 201 & 202 7-67 Example Example Br2, H2O CH3CH CH2 CH3CHCH2Br OH Achiral Dr. Wolf's CHM 201 & 202 Chiral, but racemic 7-68 Example Example HBr CH3CH CHCH3 CH3CHCH2CH3 Br Achiral Dr. Wolf's CHM 201 & 202 Chiral, but racemic 7-69 Many reactions convert chiral Many reactions convert chiral reactants to chiral products. reactants to chiral products. However, if the reactant is racemic, the product will be However, racemic also. racemic Remember: "Optically inactive starting materials can't Remember: give optically active products." give Dr. Wolf's CHM 201 & 202 7-70 Example Example HBr CH3CHCH2CH3 OH Chiral, but racemic Dr. Wolf's CHM 201 & 202 CH3CHCH2CH3 Br Chiral, but racemic 7-71 Many biochemical reactions convert Many biochemical reactions convert an achiral reactant to a single an achiral reactant to a single enantiomer of a chiral product enantiomer of a chiral product Reactions in living systems are catalyzed by enzymes, Reactions which are enantiomerically homogeneous. which The enzyme (catalyst) is part of the reacting system, so The such reactions don't violate the generalization that "Optically inactive starting materials can't give optically active products." optically Dr. Wolf's CHM 201 & 202 7-72 Example Example H HO2C H C C fumarase CO2H H H2O HO2C C OH HO2CCH2 Fumaric acid (S)-(–)-Malic acid Achiral Single enantiomer Dr. Wolf's CHM 201 & 202 7-73 7.11 Chiral Molecules with Two Chirality Centers How many stereoisomers when How a particular molecule contains two chiral centers? two Dr. Wolf's CHM 201 & 202 7-74 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH What are all the possible R and S combinations of What the two chirality centers in this molecule? the Dr. Wolf's CHM 201 & 202 7-75 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH What are all the possible R and S combinations of What the two chirality centers in this molecule? the Carbon-2 R Carbon-3 R Dr. Wolf's CHM 201 & 202 R S S R S S 7-76 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH 4 Combinations = 4 Stereoisomers Carbon-2 R Carbon-3 R Dr. Wolf's CHM 201 & 202 R S S R S S 7-77 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH 4 Combinations = 4 Stereoisomers What is the relationship between these stereoisomers? Carbon-2 R Carbon-3 R Dr. Wolf's CHM 201 & 202 R S S R S S 7-78 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH enantiomers: 2R,3R and 2S,3S 2R,3S and 2S,3R Carbon-2 R Carbon-3 R Dr. Wolf's CHM 201 & 202 R S S R S S 7-79 CO2H HO H [α] = -9.5° CO2H [α] = +9.5° R H OH R enantiomers enantiomers H HO S H S CH3 CH3 CO2H CO2H S R HO HO H H S CH3 OH Dr. Wolf's CHM 201 & 202 enantiomers enantiomers [α] = +17.8° [α] = -17.8° OH H H OH R CH3 7-80 2,3-Dihydroxybutanoic acid 2,3-Dihydroxybutanoic acid O 3 2 CH3CHCHCOH HO OH but not all relationships are enantiomeric stereoisomers that are not enantiomers are: stereoisomers diastereomers……. diastereomers……. similar but not identical chemical and physical properties Carbon-2 R Carbon-3 R Dr. Wolf's CHM 201 & 202 R S S R S S 7-81 Isomers constitutional isomers enantiomers Dr. Wolf's CHM 201 & 202 stereoisomers diastereomers 7-82 CO2H HO H [α] = -9.5° CO2H [α] = +9.5° R H OH R enantiomers enantiomers H HO S OH H S CH3 CH3 diastereomers diastereomers CO2H CO2H S R HO HO H H S CH3 Dr. Wolf's CHM 201 & 202 enantiomers enantiomers [α] = +17.8° [α] = -17.8° OH H H OH R CH3 7-83 Fischer Projections Fischer Projections recall for Fischer recall projection: horizontal bonds point toward you; vertical bonds point away staggered conformation does not have correct orientation of bonds for Fischer projection Fischer Dr. Wolf's CHM 201 & 202 CO2H CH3 7-84 Fischer projections Fischer projections Fischer transform molecule to eclipsed conformation in order to construct Fischer projection projection Dr. Wolf's CHM 201 & 202 7-85 Fischer projections Fischer projections Fischer CO2H CO H OH H OH CH3 Dr. Wolf's CHM 201 & 202 7-86 Erythro and Threo Erythro and Threo stereochemical prefixes used to specify relative stereochemical configuration in molecules with two chirality centers centers easiest to apply using Fischer projections orientation: vertical carbon chain Dr. Wolf's CHM 201 & 202 7-87 Erythro Erythro when carbon chain is vertical, same (or when analogous) substituents on same side of same Fischer projection Fischer CO2H CO2H H OH HO H H OH HO H –9.5° CH3 Dr. Wolf's CHM 201 & 202 CH3 +9.5° 7-88 Threo Threo when carbon chain is vertical, same (or when analogous) substituents on opposite sides of opposite Fischer projection Fischer CO2H OH H HO +17.8° H CH3 Dr. Wolf's CHM 201 & 202 CO2H H HO OH H CH3 –17.8° 7-89 Two chirality centers in aaring Two chirality centers in ring R R S S trans-1-Bromo-2-chlorocyclopropane nonsuperposable mirror images; enantiomers Dr. Wolf's CHM 201 & 202 7-90 Two chirality centers in aaring Two chirality centers in ring S R S R cis-1-Bromo-2-chlorocyclopropane nonsuperposable mirror images; enantiomers Dr. Wolf's CHM 201 & 202 7-91 Two chirality centers in aaring Two chirality centers in ring S R S R cis-1-Bromo-2-chlorotrans-1-Bromo-2-chlorocyclopropane cyclopropane stereoisomers that are not enantiomers; diastereomers Dr. Wolf's CHM 201 & 202 7-92 7.12 Achiral Molecules with Two Chirality Centers It is possible for a molecule to have It chirality centers yet be achiral. chirality Dr. Wolf's CHM 201 & 202 7-93 2,3-Butanediol 2,3-Butanediol 2 3 CH3CHCHCH3 HO OH Consider a molecule with two equivalently substituted Consider chirality centers such as 2,3 butanediol. chirality Dr. Wolf's CHM 201 & 202 7-94 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol 2R,3R 2S,3S 2R,3S chiral chiral achiral Dr. Wolf's CHM 201 & 202 7-95 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol CH3 CH3 H HO OH H CH3 OH HO H OH H H H OH CH3 CH3 CH3 2R,3R 2S,3S 2R,3S chiral chiral achiral Dr. Wolf's CHM 201 & 202 7-96 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol these two are enantiomers 2R,3R 2S,3S chiral chiral Dr. Wolf's CHM 201 & 202 7-97 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol CH3 CH3 H HO OH H OH H HO H CH3 CH3 2R,3R 2S,3S chiral these two are enantiomers chiral Dr. Wolf's CHM 201 & 202 7-98 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol the third structure is the superposable on its superposable mirror image 2R,3S achiral Dr. Wolf's CHM 201 & 202 7-99 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol therefore, this structure therefore, and its mirror image and are the same are it is called a meso form it a meso form is an achiral meso molecule that has chirality centers centers Dr. Wolf's CHM 201 & 202 2R,3S achiral 7-100 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol CH3 CH3 HO therefore, this structure therefore, and its mirror image H are the same H OH HO H H OH CH3 Dr. Wolf's CHM 201 & 202 it is called a meso form it a meso form is an achiral meso molecule that has chirality centers centers CH3 2R,3S achiral 7-101 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol meso forms have a plane of meso symmetry and/or a center of symmetry symmetry plane of symmetry is most plane common case common top half of molecule is mirror top image of bottom half image 2R,3S achiral Dr. Wolf's CHM 201 & 202 7-102 Three stereoisomers of 2,3-butanediol Three stereoisomers of 2,3-butanediol CH3 HO H HO H CH3 A line drawn the center of the Fischer the projection of a projection meso form bisects it into two mirrorimage halves. CH3 H OH H OH CH3 2R,3S achiral Dr. Wolf's CHM 201 & 202 7-103 Cyclic compounds Cyclic compounds chiral meso S R R R There are three stereoisomers of 1,2-dichlorocyclopropane; the achiral (meso) cis isomer and cyclopropane; two enantiomers of the trans isomer. two Dr. Wolf's CHM 201 & 202 7-104 7.13 Molecules with Multiple Chirality Centers Dr. Wolf's CHM 201 & 202 7-105 How many stereoisomers? How many stereoisomers? maximum number of stereoisomers = 2n where n = number of structural units capable of where stereochemical variation stereochemical structural units include chirality centers and cis structural and/or trans double bonds and/or number is reduced to less than 2n if meso forms are possible are Dr. Wolf's CHM 201 & 202 7-106 Example Example O HOCH2CH—CH—CH—CHCH OH OH OH OH 4 chirality centers 16 stereoisomers Dr. Wolf's CHM 201 & 202 7-107 Cholic acid Cholic acid Cholic HO H CH3 H H Dr. Wolf's CHM 201 & 202 CH2CH2CO2H H H3C HO HO CH3 H OH 11 chirality centers 211 = 2048 stereoisomers one is "natural" cholic acid a second is the enantiomer of second natural cholic acid natural 2046 are diastereomers of cholic 2046 acid acid 7-108 How many stereoisomers? How many stereoisomers? maximum number of stereoisomers = 2n where n = number of structural units capable of where stereochemical variation stereochemical structural units include chirality centers and cis structural and/or trans double bonds and/or number is reduced to less than 2n if meso forms are possible are Dr. Wolf's CHM 201 & 202 7-109 How many stereoisomers? How many stereoisomers? 3-Penten-2-ol R E E HO H R Z HO Dr. Wolf's CHM 201 & 202 H Z H S OH S H OH 7-110 7.14 7.14 Chemical Reactions That Produce Diastereomers Dr. Wolf's CHM 201 & 202 7-111 Stereochemistry of Addition to Alkenes Stereochemistry of Addition to Alkenes C C + E—Y E C C Y In order to know understand stereochemistry of In product, you need to know two things: product, (1) stereochemistry of alkene (cis or trans; Z or E) (1) (2) stereochemistry of mechanism (syn or anti) (2) Dr. Wolf's CHM 201 & 202 7-112 Bromine Addition to trans-2-Butene Bromine Addition to trans-2-Butene Br2 S R S R meso anti addition to trans-2-butene gives meso anti trans-2-butene diastereomer diastereomer Dr. Wolf's CHM 201 & 202 7-113 Bromine Addition to cis-2-Butene Bromine Addition to cis-2-Butene Br2 S R R + S 50% 50% anti addition to cis-2-butene gives racemic mixture anti cis-2-butene of chiral diastereomer of Dr. Wolf's CHM 201 & 202 7-114 Epoxidation of trans-2-Butene Epoxidation of trans-2-Butene S R RCO3H + R S 50% 50% syn addition to trans-2-butene gives racemic syn trans-2-butene mixture of chiral diastereomer mixture Dr. Wolf's CHM 201 & 202 7-115 Epoxidation of cis-2-Butene Epoxidation of cis-2-Butene S R RCO3H R S meso syn addition to cis-2-butene gives meso syn cis-2-butene diastereomer diastereomer Dr. Wolf's CHM 201 & 202 7-116 Stereospecific reaction Stereospecific reaction Of two stereoisomers of a particular starting Of material, each one gives different material, stereoisomeric forms of the product Related to mechanism: terms such as syn addition and anti addition refer to stereospecificity Dr. Wolf's CHM 201 & 202 7-117 cis-2-butene bromination anti 2R,3R + 2S,3S anti meso . trans-2-butene bromination Stereospecific reactions Stereospecific reactions cis-2-butene cis epoxidation epoxidation syn syn meso meso trans-2-butene epoxidation syn 2R,3R + 2S,3S Dr. Wolf's CHM 201 & 202 7-118 Stereoselective reaction Stereoselective reaction A single starting material can give two or more stereoisomeric products, but gives one of them in greater amounts than any other H H CH3 CH2 CH3 CH H2 CH3 + CH3 Pt H 68% Dr. Wolf's CHM 201 & 202 H H CH3 32% 7-119 7.15 7.15 Resolution of Enantiomers Resolution Separation of a racemic mixture into its two Separation enantiomeric forms enantiomeric Dr. Wolf's CHM 201 & 202 7-120 Strategy Strategy enantiomers C(+) C(+) Dr. Wolf's CHM 201 & 202 C(-) C(-) 7-121 Strategy Strategy enantiomers C(+) C(+) C(-) C(-) 2P(+) 2P(+) C(+)P(+) C(+)P(+) C(-)P(+) C(-)P(+) diastereomers Dr. Wolf's CHM 201 & 202 7-122 Strategy Strategy enantiomers C(+) C(+) C(-) C(-) 2P(+) 2P(+) C(+)P(+) C(+)P(+) C(-)P(+) C(-)P(+) C(+)P(+) C(+)P(+) C(-)P(+) C(-)P(+) diastereomers Dr. Wolf's CHM 201 & 202 7-123 Strategy Strategy enantiomers C(+) C(+) C(+) C(+) P(+) P(+) C(-) C(-) 2P(+) 2P(+) C(+)P(+) C(+)P(+) C(-)P(+) C(-)P(+) C(+)P(+) C(+)P(+) C(-)P(+) C(-)P(+) diastereomers Dr. Wolf's CHM 201 & 202 P(+) P(+) C(-) C(-) 7-124 7.16 Stereoregular Polymers atactic isotactic syndiotactic Dr. Wolf's CHM 201 & 202 7-125 Atactic Polypropylene Atactic Polypropylene random stereochemistry of methyl groups random attached to main chain (stereorandom) attached properties not very useful for fibers etc. formed by free-radical polymerization Dr. Wolf's CHM 201 & 202 7-126 Isotactic Polypropylene Isotactic Polypropylene stereoregular polymer; all methyl groups on same side of main chain useful properties prepared by coordination polymerization prepared under Ziegler-Natta conditions under Dr. Wolf's CHM 201 & 202 7-127 Syndiotactic Polypropylene Syndiotactic Polypropylene stereoregular polymer; methyl groups alternate stereoregular side-to-side on main chain side-to-side useful properties prepared by coordination polymerization under prepared Ziegler-Natta conditions Ziegler-Natta Dr. Wolf's CHM 201 & 202 7-128 7.17 Chirality Centers Other Than Carbon Dr. Wolf's CHM 201 & 202 7-129 Silicon Silicon Silicon b b a a Si c d d Si c Silicon, like carbon, forms four bonds in its stable Silicon, compounds and many chiral silicon compounds have been resolved have Dr. Wolf's CHM 201 & 202 7-130 Nitrogen in amines Nitrogen in amines Nitrogen b b very fast a N c : a : N c Pyramidal geometry at nitrogen can produce Pyramidal a chiral structure, but enantiomers equilibrate too rapidly to be resolved equilibrate Dr. Wolf's CHM 201 & 202 7-131 Phosphorus in phosphines Phosphorus in phosphines Phosphorus b b slow a P c : a : P c Pyramidal geometry at phosphorus can produce a Pyramidal chiral structure; pyramidal inversion slower than for amines and compounds of the type shown have been resolved been Dr. Wolf's CHM 201 & 202 7-132 Sulfur in sulfoxides Sulfur in sulfoxides Sulfur b b slow a +S O_ : a : S+ O_ Pyramidal geometry at sulfur can produce a chiral Pyramidal structure; pyramidal inversion is slow and compounds of the type shown have been resolved compounds Dr. Wolf's CHM 201 & 202 7-133 End of Chapter 7 Dr. Wolf's CHM 201 & 202 7-134 ...
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