8Chapter 10

8Chapter 10 - Chapter 10 Conjugation in Alkadienes and...

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Unformatted text preview: Chapter 10 Conjugation in Alkadienes and Allylic Systems conjugare is a Latin verb meaning "to conjugare link or yoke together" link Dr. Wolf's CHM 201 & 202 10-1 The Double Bond as a Substituent The Double Bond as a Substituent C C C+ allylic carbocation Dr. Wolf's CHM 201 & 202 10-2 The Double Bond as a Substituent The Double Bond as a Substituent C C C+ allylic carbocation Dr. Wolf's CHM 201 & 202 C C C • allylic radical 10-3 The Double Bond as a Substituent The Double Bond as a Substituent C C C+ C allylic carbocation C C C • allylic radical C C C conjugated diene Dr. Wolf's CHM 201 & 202 10-4 10.1 The Allyl Group H H H C C C H H Dr. Wolf's CHM 201 & 202 10-5 Vinylic versus Allylic Vinylic versus Allylic C C C vinylic carbons Dr. Wolf's CHM 201 & 202 allylic carbon 10-6 Vinylic versus Allylic Vinylic versus Allylic H H C C C H vinylic hydrogens are attached to vinylic carbons Dr. Wolf's CHM 201 & 202 10-7 Vinylic versus Allylic Vinylic versus Allylic H C C C H H allylic hydrogens are attached to allylic carbons Dr. Wolf's CHM 201 & 202 10-8 Vinylic versus Allylic Vinylic versus Allylic X X C C C X vinylic substituents are attached to vinylic carbons Dr. Wolf's CHM 201 & 202 10-9 Vinylic versus Allylic Vinylic versus Allylic X X C C C X allylic substituents are attached to allylic carbons Dr. Wolf's CHM 201 & 202 10-10 10.2 Allylic Carbocations C+ C C Dr. Wolf's CHM 201 & 202 10-11 Allylic Carbocations Allylic Carbocations the fact that a tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide alkyl CH3 CH3 H2C CH C CH3 123 Cl CH3 C Cll C CH3 1 relative rates: (ethanolysis, 45°C) Dr. Wolf's CHM 201 & 202 10-12 Allylic Carbocations Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations other CH3 CH3 CH H2C CH C+ CH3 CH3 C+ CH3 formed faster Dr. Wolf's CHM 201 & 202 10-13 Allylic Carbocations Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations other CH3 CH3 H2C CH C+ CH3 CH3 CH C+ CH3 H2C=CH— stabilizes C+ better than CH3— Dr. Wolf's CHM 201 & 202 10-14 Stabilization of Allylic Carbocations Stabilization of Allylic Carbocations Delocalization of electrons in the double bond stabilizes the carbocation resonance model orbital overlap model Dr. Wolf's CHM 201 & 202 10-15 Resonance Model Resonance Model CH3 H2C CH C+ CH3 Dr. Wolf's CHM 201 & 202 + H2C CH3 CH C CH3 10-16 Resonance Model Resonance Model CH3 H2C CH + H2C C+ CH3 δ+ H2C CH3 CH C CH3 CH3 CH CH C δ+ CH3 Dr. Wolf's CHM 201 & 202 10-17 Orbital Overlap Model δ+ Dr. Wolf's CHM 201 & 202 δ+ 10-18 Orbital Overlap Model Dr. Wolf's CHM 201 & 202 10-19 Orbital Overlap Model Dr. Wolf's CHM 201 & 202 10-20 Orbital Overlap Model Dr. Wolf's CHM 201 & 202 10-21 10.3 SN1 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202 10-22 Hydrolysis of an Allylic Halide Hydrolysis of an Allylic Halide CH3 H2C CH C Cl CH3 H2O Na2CO3 CH3 CH3 H2C CH (85%) Dr. Wolf's CHM 201 & 202 C CH3 OH + HOCH2 CH (15%) C CH3 10-23 Corollary Experiment Corollary Experiment CH3 ClCH2 CH C CH3 H2O Na2CO3 CH3 CH3 H2C CH (85%) Dr. Wolf's CHM 201 & 202 C CH3 OH + HOCH2 CH (15%) C CH3 10-24 CH3 CH3 H2C CH C Cl and ClCH2 CH CH3 C CH3 give the same products because they give form the same carbocation form Dr. Wolf's CHM 201 & 202 10-25 CH3 CH3 H2C CH Cl C and ClCH2 CH C CH3 CH3 give the same products because they give form the same carbocation form CH3 H2C CH C+ CH3 Dr. Wolf's CHM 201 & 202 + H2C CH3 CH C CH3 10-26 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH3 H2C CH C+ CH3 Dr. Wolf's CHM 201 & 202 + H2C CH3 CH C CH3 10-27 (85%) H2C (15%) CH3 CH OH + C HOCH2 CH CH3 C CH3 CH3 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH3 H2C CH C+ CH3 Dr. Wolf's CHM 201 & 202 + H2C CH3 CH C CH3 10-28 10.4 SN2 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202 10-29 Allylic SN2 Reactions •Allylic halides also undergo SN2 reactions •faster than simple primary alkyl halides. H2C CH2 Cl CH H3C CH2 80 CH2 Cl 1 relative rates: (I-, acetone) Dr. Wolf's CHM 201 & 202 10-30 Allylic SN2 Reactions •Two factors: •Steric • Trigonal carbon smaller than tetrahedral carbon. H2C CH2 Cl CH H3C CH2 80 CH2 Cl 1 relative rates: (I-, acetone) Dr. Wolf's CHM 201 & 202 10-31 Allylic SN2 reactions •Two factors: •Electronic • Electron delocalization lowers LUMO energy • which means lower activation energy. H3C CH2 CH2 Cl H2C CH CH2 Cl 80 1 relative rates: (I-, acetone) Dr. Wolf's CHM 201 & 202 10-32 10.5 Allylic Free Radicals C• C C Dr. Wolf's CHM 201 & 202 10-33 Allylic free radicals are stabilized by Allylic free radicals are stabilized by electron delocalization electron delocalization C• C C Dr. Wolf's CHM 201 & 202 C •C C 10-34 Free-radical stabilities are related to Free-radical stabilities are related to bond-dissociation energies bond-dissociation energies CH3CH2CH2—H H2C CHCH2—H 410 kJ/mol • CH3CH2CH2 + H• 368 kJ/mol • CHCH2 + H• H2C C—H bond is weaker in propene because C—H resulting radical (allyl) is more stable than radical (propyl) from propane radical Dr. Wolf's CHM 201 & 202 10-35 10.6 Allylic Halogenation Dr. Wolf's CHM 201 & 202 10-36 Chlorination of Propene Chlorination of Propene addition ClCH2CHCH3 H2C Cl CHCH3 + Cl2 H2C 500 °C CHCH2Cl + HCl substitution Dr. Wolf's CHM 201 & 202 10-37 Allylic Halogenation Allylic Halogenation selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate Dr. Wolf's CHM 201 & 202 10-38 Hydrogen-atom abstraction step Hydrogen-atom abstraction step H H H C C 410 kJ/mol C H H H .. . Cl: .. 368 kJ/mol allylic C—H bond weaker than vinylic allylic chlorine atom abstracts allylic H in chlorine propagation step propagation Dr. Wolf's CHM 201 & 202 10-39 Hydrogen-atom abstraction step Hydrogen-atom abstraction step H H H C• C 410 kJ/mol Dr. Wolf's CHM 201 & 202 C H H .. H : Cl: .. 368 kJ/mol 10-40 N-Bromosuccinimide N-Bromosuccinimide reagent used (instead of Br2) for allylic for bromination Br O O NBr + O Dr. Wolf's CHM 201 & 202 heat + NH CCl4 (82-87%) O 10-41 Limited Scope Limited Scope Limited Allylic halogenation is only used when: all of the allylic hydrogens are equivalent all and and the resonance forms of allylic radical are equivalent are Dr. Wolf's CHM 201 & 202 10-42 Example Example HH Cyclohexene Cyclohexene satisfies both requirements requirements All allylic hydrogens are equivalent H Dr. Wolf's CHM 201 & 202 H 10-43 Example Example HH Cyclohexene Cyclohexene satisfies both requirements requirements All allylic hydrogens are equivalent H H • H H H H H • H Both resonance forms are equivalent Dr. Wolf's CHM 201 & 202 10-44 Example Example 2-Butene CH3CH CHCH3 All allylic hydrogens are equivalent But CH3CH CH • CH2 • CH3CH CH CH2 Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. Dr. Wolf's CHM 201 & 202 10-45 10.7 10.7 Allylic Anions Allylic Dr. Wolf's CHM 201 & 202 10-46 Allylic anions are stabilized by electron delocalization CH3 H2C CH CCH3 Dr. Wolf's CHM 201 & 202 - H2C CH3 CH C CH3 10-47 H3C CH2 CH Acidity of PropeneC H 3 pKa ~ 62 pKa ~ 43 H2C CH CH2 CH2 CH3 H2C CH2 CH3 Propene is significantly more acidic than propane. Dr. Wolf's CHM 201 & 202 10-48 Resonance Model H2C CH H2C CH2 δH2C CH CH CH2 δCH2 CH Charge is delocalized to both terminal carbons, stabilizing the conjugate base. Dr. Wolf's CHM 201 & 202 10-49 10.8 10.8 Classes of Dienes Classes Dr. Wolf's CHM 201 & 202 10-50 Classification of Dienes Classification of Dienes Classification isolated diene conjugated diene C Dr. Wolf's CHM 201 & 202 cumulated diene 10-51 Nomenclature Nomenclature Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene C Dr. Wolf's CHM 201 & 202 3,4-heptadiene 10-52 10.9 Relative Stabilities of Dienes Dr. Wolf's CHM 201 & 202 10-53 Heats of Hydrogenation Heats of Hydrogenation 252 kJ/mol Dr. Wolf's CHM 201 & 202 1,3-pentadiene is 1,3-pentadiene 26 kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond double 226 kJ/mol 10-54 Heats of Hydrogenation Heats of Hydrogenation 126 kJ/mol 252 kJ/mol Dr. Wolf's CHM 201 & 202 115 kJ/mol 226 kJ/mol 10-55 Heats of Hydrogenation Heats of Hydrogenation 126 kJ/mol 126 kJ/mol 252 kJ/mol Dr. Wolf's CHM 201 & 202 111 kJ/mol 115 kJ/mol 226 kJ/mol 10-56 Heats of Hydrogenation Heats of Hydrogenation 126 kJ/mol 111 kJ/mol when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated is Dr. Wolf's CHM 201 & 202 10-57 Heats of Hydrogenation Heats of Hydrogenation 126 kJ/mol 111 kJ/mol this extra 15 kJ/mol is known by several terms this stabilization energy delocalization energy resonance energy Dr. Wolf's CHM 201 & 202 10-58 Heats of Hydrogenation Heats of Hydrogenation Cumulated double bonds have relatively Cumulated high heats of hydrogenation high H2C C CH2 + 2H2 CH3CH2CH3 ∆ H° = -295 kJ H2C CH2CH3 + H2 CH3CH2CH3 ∆ H° = -125 kJ Dr. Wolf's CHM 201 & 202 10-59 10.10 Bonding in Conjugated Dienes Dr. Wolf's CHM 201 & 202 10-60 Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202 10-61 Isolated diene π bonds are independent of each other each 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202 10-62 Isolated diene π bonds are independent of each other each Conjugated diene Dr. Wolf's CHM 201 & 202 p orbitals overlap to give extended π bond encompassing four carbons four 10-63 Isolated diene lless electron ess delocalization; less stable less more electron more delocalization; more stable more Conjugated diene Dr. Wolf's CHM 201 & 202 10-64 Conformations of Dienes Conformations of Dienes H H H H H H H s-trans H H H HH s-cis s prefix designates conformation around single bond conformation s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper Prelog configuration case) case) Dr. Wolf's CHM 201 & 202 10-65 Conformations of Dienes Conformations of Dienes H H H H H H H s-trans H H H HH s-cis s prefix designates conformation around single bond conformation s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper Prelog configuration case) case) Dr. Wolf's CHM 201 & 202 10-66 Conformations of Dienes Conformations of Dienes s-trans s-cis Both conformations allow electron delocalization via Both overlap of p orbitals to give extended π system Dr. Wolf's CHM 201 & 202 10-67 ss-transis more stable than ss-cis -trans is more stable than -cis -trans -trans Interconversion of conformations requires two Interconversion π bonds to be at right angles to each other and prevents conjugation and 12 kJ/mol Dr. Wolf's CHM 201 & 202 10-68 Dr. Wolf's CHM 201 & 202 10-69 16 kJ/mol 12 kJ/mol Dr. Wolf's CHM 201 & 202 10-70 10.11 Bonding in Allenes Dr. Wolf's CHM 201 & 202 10-71 Cumulated Dienes Cumulated Dienes C C C cumulated dienes are less stable than isolated and conjugated dienes (see Problem 10.7 on p 375) Dr. Wolf's CHM 201 & 202 10-72 Structure of Allene Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 10-73 Structure of Allene Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 10-74 Bonding in Allene Bonding in Allene sp 2 Dr. Wolf's CHM 201 & 202 sp sp 2 10-75 Bonding in Allene Bonding in Allene Dr. Wolf's CHM 201 & 202 10-76 Bonding in Allene Bonding in Allene Dr. Wolf's CHM 201 & 202 10-77 Bonding in Allene Bonding in Allene Dr. Wolf's CHM 201 & 202 10-78 Chiral Allenes Chiral Allenes Allenes of the type shown are chiral X A C C C Y B A ≠ B; X ≠ Y Have a stereogenic axis Dr. Wolf's CHM 201 & 202 10-79 Stereogenic Axis Stereogenic Axis Stereogenic analogous to difference between: analogous a screw with a right-hand thread and one with a left-hand thread with a right-handed helix and a left-handed helix Dr. Wolf's CHM 201 & 202 10-80 10.12 Preparation of Dienes Dr. Wolf's CHM 201 & 202 10-81 1,3-Butadiene 1,3-Butadiene CH3CH2CH2CH3 590-675°C chromiaalumina H2C CHCH CH2 + 2H2 More than 4 billion pounds of 1,3-butadiene More prepared by this method in U.S. each year prepared used to prepare synthetic rubber (See "Diene used Polymers" box) Polymers" Dr. Wolf's CHM 201 & 202 10-82 Dehydration of Alcohols Dehydration of Alcohols KHSO4 OH OH Dr. Wolf's CHM 201 & 202 heat 10-83 Dehydration of Alcohols Dehydration of Alcohols KHSO4 OH OH heat major product; major 88% yield Dr. Wolf's CHM 201 & 202 10-84 Dehydrohalogenation of Alkyl Halides Dehydrohalogenation of Alkyl Halides KOH Br Br Dr. Wolf's CHM 201 & 202 heat 10-85 Dehydrohalogenation of Alkyl Halides Dehydrohalogenation of Alkyl Halides Dehydrohalogenation KOH Br Br heat major product; major 78% yield Dr. Wolf's CHM 201 & 202 10-86 Reactions of Dienes Reactions of Dienes isolated dienes: double bonds react independently double of one another of cumulated dienes: specialized topic cumulated conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own functional Dr. Wolf's CHM 201 & 202 10-87 10.13 Addition of Hydrogen Halides to Conjugated Dienes Dr. Wolf's CHM 201 & 202 10-88 Electrophilic Addition to Conjugated Dienes Electrophilic Addition to Conjugated Dienes H X + H Proton adds to end of diene system Carbocation formed is allylic Dr. Wolf's CHM 201 & 202 10-89 H Example: Example: H H H H H HCl H H Cl H H H H H Dr. Wolf's CHM 201 & 202 ? H ? H H Cl H H H H 10-90 H Example: Example: H H H H H HCl H H Cl H H H H H Dr. Wolf's CHM 201 & 202 10-91 via: H H H H + H H H H X H H H H H H Dr. Wolf's CHM 201 & 202 H H H + H H H H 10-92 and: H H H H H H Cl H H Dr. Wolf's CHM 201 & 202 H 3-Chlorocyclopentene H H H H H – H + H H H H H H Cl + H H Cll C H H H H H 10-93 1,2-Addition versus 1,41,2-Addition versus 1,4Addition Addition 1,2-addition of XY 1,2-addition Y X Dr. Wolf's CHM 201 & 202 10-94 1,2-Addition versus 1,41,2-Addition versus 1,4Addition Addition 1,2-addition of XY 1,2-addition 1,4-addition of XY Y Y X Dr. Wolf's CHM 201 & 202 X 10-95 1,2-Addition versus 1,41,2-Addition versus 1,4Addition Addition 1,2-addition of XY 1,2-addition 1,4-addition of XY Y Y X via X + X Dr. Wolf's CHM 201 & 202 10-96 HBr Addition to 1,3-Butadiene HBr Addition to 1,3-Butadiene H2C CH2 CHCH HBr CH3CHCH CH2 + CH3CH CHCH2Br Br electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature Dr. Wolf's CHM 201 & 202 10-97 Rationale Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations 1-bromo-2-butene react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. CH3CHCH CH2 + CH3CH CHCH2Br CH3CH + CHCH2 Br via: + CH3CHCH Dr. Wolf's CHM 201 & 202 CH2 10-98 Rationale Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations 1-bromo-2-butene react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. CH3CHCH CH2 + CH3CH CHCH2Br Br formed faster Dr. Wolf's CHM 201 & 202 10-99 Rationale Rationale 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond. CH3CHCH CH2 + CH3CH CHCH2Br Br more stable Dr. Wolf's CHM 201 & 202 10-100 Rationale Rationale The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. CH3CHCH CH2 Br major product at -80°C (formed faster) Dr. Wolf's CHM 201 & 202 CH3CH CHCH2Br major product at 25°C (more stable) 10-101 Kinetic Control Kinetic Control versus versus Thermodynamic Control Thermodynamic Control • Kinetic control: major product is the one Kinetic formed at the fastest rate formed • Thermodynamic control: major product is the Thermodynamic one that is the most stable one Dr. Wolf's CHM 201 & 202 10-102 + CH3CHCH CH3CH CH2 + CHCH2 HBr H2C Dr. Wolf's CHM 201 & 202 CHCH CH2 10-103 + CH3CHCH CH3CH higher activation energy CH2 + CHCH2 CH3CHCH formed more slowly CH2 Br Dr. Wolf's CHM 201 & 202 CH3CH CHCH2Br 10-104 Addition of hydrogen chloride to Addition 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this product? amounts + Dr. Wolf's CHM 201 & 202 HCl ? 10-105 Think mechanistically. + Protonation occurs: at end of diene system in direction that gives most stable carbocation HCl Kinetically controlled product corresponds to attack by Kinetically chloride ion at carbon that has the greatest share of positive charge in the carbocation positive Dr. Wolf's CHM 201 & 202 10-106 Think mechanistically Think H + Cl + one resonance form is one tertiary carbocation; other is primary other Dr. Wolf's CHM 201 & 202 10-107 Think mechanistically Think H + Cl Cll C H + + + one resonance form is one tertiary carbocation; other is primary other Dr. Wolf's CHM 201 & 202 one resonance form is one secondary carbocation; other is primary other 10-108 Think mechanistically Think H Cl More stable carbocation + + one resonance form is one tertiary carbocation; other is primary other Dr. Wolf's CHM 201 & 202 Is attacked by chloride ion Is at carbon that bears greater share of positive charge charge 10-109 Think mechanistically Think H + Cl + one resonance form is one tertiary carbocation; other is primary other Dr. Wolf's CHM 201 & 202 Cl– Cll C major product 10-110 10.14 Halogen Addition to Dienes gives mixtures of 1,2 and gives 1,4-addition products 1,4-addition Dr. Wolf's CHM 201 & 202 10-111 Example Example H2C CH2 CHCH Br2 BrCH2CHCH CH2 + BrCH2CH CHCH2Br Br (37%) Dr. Wolf's CHM 201 & 202 (63%) 10-112 10.15 10.15 The Diels-Alder Reaction The Synthetic method for preparing Synthetic compounds containing a cyclohexene ring compounds Dr. Wolf's CHM 201 & 202 10-113 In general... In general... + conjugated alkene conjugated alkene diene (dienophile) diene (dienophile) Dr. Wolf's CHM 201 & 202 cyclohexene 10-114 via via via transition state Dr. Wolf's CHM 201 & 202 10-115 Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 10-116 Mechanistic features Mechanistic features concerted mechanism cycloaddition pericyclic reaction a concerted reaction that proceeds concerted through a cyclic transition state through Dr. Wolf's CHM 201 & 202 10-117 Recall the general reaction... Recall the general reaction... + alkene conjugated alkene conjugated (dienophile) diene (dienophile) diene cyclohexene The equation as written is somewhat The misleading because ethylene is a relatively unreactive dienophile. unreactive Dr. Wolf's CHM 201 & 202 10-118 What makes a reactive dienophile? What makes a reactive dienophile? The most reactive dienophiles have an The electron-withdrawing group (EWG) directly electron-withdrawing directly attached to the double bond. attached EWG C C Typical EWGs Typical O C Dr. Wolf's CHM 201 & 202 C N 10-119 Example Example O H2C CHCH CH2 + H2C benzene CH CH 100°C O CH CH (100%) Dr. Wolf's CHM 201 & 202 10-120 Example Example O H2C CHCH CH2 + H2C benzene CH 100°C O O CH CH via: CH CH CH (100%) Dr. Wolf's CHM 201 & 202 10-121 Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 10-122 Example Example H2C O CH2 CHC CH3 benzene + O 100°C O O H3C O (100%) Dr. Wolf's CHM 201 & 202 O 10-123 Example Example H2C O CH2 CHC CH3 benzene via: via: O H3C + 100°C O O H3C O O O Dr. Wolf's CHM 201 & 202 O (100%) O 10-124 Acetylenic Dienophile Acetylenic Dienophile O O H2C CHCH CH2 + CH3CH2OCC benzene CCOCH2CH3 100°C O COCH2CH3 COCH (98%) COCH2CH3 Dr. Wolf's CHM 201 & 202 O 10-125 Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 10-126 Diels-Alder Reaction is Stereospecific* Diels-Alder Reaction is Stereospecific* syn addition to alkene cis-trans relationship of substituents on alkene cis-trans retained in cyclohexene product retained *A stereospecific reaction is one in which *A stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. inversion Dr. Wolf's CHM 201 & 202 10-127 Example Example O C6H5 H2C CHCH CH2 + COH C H C H H C6H5 COH only product Dr. Wolf's CHM 201 & 202 H O 10-128 Example Example O COH H H2C CHCH CH2 + C C6H5 C H C6H5 H COH only product Dr. Wolf's CHM 201 & 202 H O 10-129 Cyclic dienes yield bridged bicyclic Cyclic dienes yield bridged bicyclic Diels-Alder adducts. Diels-Alder adducts. Dr. Wolf's CHM 201 & 202 10-130 Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 Dr. Wolf's CHM 201 & 202 10-131 Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 10-132 O COCH3 H C + CH3OC O C H H O COCH3 H COCH3 Dr. Wolf's CHM 201 & 202 O 10-133 H O COCH3 H O COCH3 H is the same as COCH3 H COCH3 O O Dr. Wolf's CHM 201 & 202 10-134 10.16 10.16 The π Molecular Orbitals of Ethylene and 1,3-Butadiene Dr. Wolf's CHM 201 & 202 10-135 Orbitals and Chemical Reactions Orbitals and Chemical Reactions • A deeper understanding of chemical reactivity deeper can be gained by focusing on the frontier orbitals of the reactants. of • Electrons flow from the highest occupied Electrons molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other. (LUMO) Dr. Wolf's CHM 201 & 202 10-136 Orbitals and Chemical Reactions Orbitals and Chemical Reactions • We can illustrate HOMO-LUMO interactions We by way of the Diels-Alder reaction between ethylene and 1,3-butadiene. ethylene • We need only consider only the π electrons of We ethylene and 1,3-butadiene. We can ignore the framework of σ bonds in each molecule. Dr. Wolf's CHM 201 & 202 10-137 The π MOs of Ethylene The π MOs of Ethylene The The • red and blue colors red distinguish sign of wave function wave • bonding π MO is bonding antisymmetric with respect to plane of molecule molecule Bonding π orbital of ethylene; Bonding two electrons in this orbital Dr. Wolf's CHM 201 & 202 10-138 The π MOs of Ethylene The π MOs of Ethylene The The Antibonding π orbital of ethylene; Antibonding no electrons in this orbital LUMO HOMO Bonding π orbital of ethylene; Bonding two electrons in this orbital Dr. Wolf's CHM 201 & 202 10-139 The π MOs of 1,3-Butadiene The π MOs of 1,3-Butadiene The The • Four p orbitals contribute to the π system of 1,3-butadiene; therefore, there are four π four molecular orbitals. molecular • Two of these orbitals are bonding; two are Two antibonding. antibonding. Dr. Wolf's CHM 201 & 202 10-140 The Two Bonding π MOs of 1,3-Butadiene The Two Bonding π MOs of 1,3-Butadiene The The HOMO 4 π electrons; 2 in each orbital Lowest energy orbital Dr. Wolf's CHM 201 & 202 10-141 The Two Antibonding π MOs of 1,3-Butadiene The Two Antibonding π MOs of 1,3-Butadiene The The Highest energy orbital LUMO Dr. Wolf's CHM 201 & 202 Both antibonding orbitals are vacant 10-142 10.17 A π Molecular Orbital Analysis of the Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 10-143 MO Analysis of Diels-Alder Reaction MO Analysis of Diels-Alder Reaction • Inasmuch as electron-withdrawing groups Inasmuch increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile. diene Dr. Wolf's CHM 201 & 202 10-144 MO Analysis of Diels-Alder Reaction MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene HOMO of 1,3-butadiene HOMO and LUMO of ethylene are in phase with one another another allows σ bond formation allows between the alkene and the diene the LUMO of ethylene (dienophile) Dr. Wolf's CHM 201 & 202 10-145 MO Analysis of Diels-Alder Reaction MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene LUMO of ethylene (dienophile) Dr. Wolf's CHM 201 & 202 10-146 A "forbidden" reaction A "forbidden" reaction H2C H2C + CH2 CH2 • The dimerization of ethylene to give The cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? of Dr. Wolf's CHM 201 & 202 10-147 A "forbidden" reaction A "forbidden" reaction H2C H2C + CH2 CH2 HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new σ bonds bonds Dr. Wolf's CHM 201 & 202 HOMO of HOMO one ethylene one molecule LUMO of LUMO other ethylene other molecule 10-148 End of Chapter 10 Dr. Wolf's CHM 201 & 202 10-149 ...
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