Chapter10(3)

Chapter10(3) - Which carbon has an allylic H? 1. 2. 3. 4....

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Unformatted text preview: Which carbon has an allylic H? 1. 2. 3. 4. 5. b O a 41% O a b c d e c O d Br e28% 12% 10% 9% Answer : 4. d 1 2 3 4 5 Which carbon has an vinylic H? 1. 2. 3. 4. 5. b O a 62% O a b c d e Br c O d e 14% 11% Answer : 2. b 11% 2% 1 2 3 4 5 What is the product for the following reaction if the reagents are H2/Pt cat? 1. 2. 3. 4. a b c d 48% 26% A C Answer : 4. d B 22% D 3% 1 2 3 4 What is the product for the following reaction if the reagents are H2/Lindlar Pd cat? 1. 2. 3. 4. a b c d 48% 27% A B 14% D 11% C Answer : 2. b 1 2 3 4 What is the product for the following reaction if the reagents are Na/NH3 ? 1. 2. 3. 4. a b c d 83% A C Answer : 3. c B D 10% 5% 2% 1 2 3 4 What is the product for the following reaction ? 1. 2. 3. 4. a b c d H2O/H+ 35% O 25% OH 20A % 20% C O 1 B D O 2 3 4 Organic Chemistry CHE 275 Chapter 10 Conjugation in Dienes and Allylic Systems C The Double Bond as a Substituent C C C+ C allylic carbocation C C • allylic radical C C C conjugated diene The Allyl Group H H R H C C C H H Vinylic versus Allylic C C C vinylic carbons allylic carbon Vinylic versus Allylic H H C C C H vinylic hydrogens are attached to vinylic carbons vinylic Vinylic versus Allylic H C C C H H allylic hydrogens are attached to allylic carbons allylic Vinylic versus Allylic X X C C C X vinylic substituents are attached to vinylic carbons Vinylic versus Allylic X X X C C C X allylic substituents are attached to allylic carbons Allylic Carbocations C C C + Allylic a tertiary allylicCarbocations halide undergoes tertiary solvolysis (SN1) faster than a simple tertiary alkyl halide alkyl CH3 CH3 H2C CH C CH3 123 Cl CH3 C CH3 1 relative rates: (ethanolysis, 45°C) Cll C Allylic provides good Carbocations evidence for the conclusion that provides allylic carbocations are more stable than other carbocations other CH3 CH3 CH H2C CH C+ CH3 formed faster CH3 C+ CH3 Allylic provides good Carbocations evidence for the conclusion that provides 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 does CH3— Stabilization of Allylic Carbocations Delocalization of electrons in the double bond stabilizes the carbocation “resonance model” Resonance Model CH3 H2C CH + H2C C+ CH3 δ+ H2C CH3 CH C CH3 CH3 CH C δ+ CH3 SN1 Hydrolysis of an Allylic Halide CH3 H2C CH C Cl CH3 H2O Na2CO3 CH3 CH3 H2C CH (85%) C CH3 OH + HOCH2 CH (15%) C CH3 Corollary Experiment CH3 ClCH2 CH C CH3 H2O Na2CO3 CH3 CH3 H2C CH (85%) C CH3 OH + HOCH2 CH (15%) C CH3 CH3 CH3 H2C CH Cl C and ClCH2 CH C CH3 CH CH3 give the same products because they give form the same carbocation form CH3 H2C CH C+ CH3 + H2C CH3 CH C CH3 (85%) H2C (15%) CH3 CH OH + C HOCH2 CH CH3 C CH3 CH CH3 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH3 H2C CH C+ CH3 + H2C CH3 CH C CH3 Allylic Free Radicals C• C C Allylic free radicals are stabilized by electron delocalization C• C C •C C C Radicals are Stabilized by Electron Delocalization Spin density is a measure of the unpaired Spin electron distribution in a molecule. electron The unpaired electron in allyl radical "divides it The time" equally between C-1 and C-3. time" Radicals are Stabilized by Electron Delocalization Spin density in allyl radical Spin Free-radical Stabilities are Related to 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 Allylic Halogenation: Chlorination of Propene addition ClCH2CHCH3 ClCH H2C Cl CHCH3 + Cl2 H2C 500 °C CHCH2Cl + HCl substitution Allylic Halogenation selective for replacement of allylic hydrogen selective free radical mechanism allylic radical is intermediate Hydrogen-Atom Abstraction Step H H H C C 410 kJ/mol 410 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 Hydrogen-Atom Abstraction Step H H H C• C 410 kJ/mol 410 C H H .. H : Cl: .. 368 kJ/mol NBromosuccinimide O reagent used (instead of Br2) for allylic reagent for bromination Br NBr + O heat + CCl4 (82-87%) O NH NH O Limited Scope Allylic halogenation is only used when: all of the allylic hydrogens are equivalent and and the resonance forms of allylic radical are equivalent are Example H H Cyclohexene Cyclohexene satisfies both requirements requirements All allylic All hydrogens are equivalent H H • H H H H H • Both resonance forms are equivalent H Example 2-Butene CH3CH CHCH3 All allylic hydrogens are equivalent But CH3CH CH • CH2 • CH3CH CH Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. CH2 Classification of Dienes isolated diene isolated conjugated diene C cumulated diene Nomenclatur e (2E,5E)-2,5-heptadiene (2 (2E,4E)-2,4-heptadiene C 3,4-heptadiene Heats of 1,3-pentadiene is Hydrogenatio 1,3-pentadiene 26 kJ/mol more n stable than 252 kJ/mol 252 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond double 226 kJ/mol Heats of Hydrogenatio n 126 kJ/mol 126 kJ/mol 252 kJ/mol 252 111 kJ/mol 115 kJ/mol 226 kJ/mol Heats of Hydrogenatio n 126 kJ/mol 126 111 kJ/mol when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated 15 Heats of Hydrogenatio n 126 kJ/mol 126 111 kJ/mol this extra 15 kJ/mol is known by several terms this stabilization energy delocalization energy resonance energy Heats of Cumulated Hydrogenatio relatively double bonds have high heats of hydrogenation high n H2C C CH2 + 2H2 CH3CH2CH3 ∆ H° = -295 kJ H2C CH2CH3 + H2 CH3CH2CH3 ∆ H° = -125 kJ Bonding in Conjugated Dienes Isolated diene Isolated 1,4-pentadiene 1,3-pentadiene Conjugated diene Isolated diene Isolated π bonds are independent of each other each 1,3-pentadiene Conjugated diene Isolated diene Isolated π bonds are independent of each other each Conjugated diene p orbitals overlap to give extended π bond encompassing four carbons four Isolated diene Isolated lless electron ess delocalization; less stable less more electron more delocalization; more stable more Conjugated diene Conformations of Dienes H H H H H H H s-trans s- 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) Conformations of Dienes s-trans s- s-cis Both conformations allow electron delocalization via Both overlap of p orbitals to give extended π system s-trans is more stable than s-cis Interconversion of conformations requires Interconversion two π bonds to be at right angles to each other and prevents conjugation other 12 kJ/mol 16 kJ/mol 16 12 kJ/mol Cumulated Dienes C C C cumulated dienes are less stable than cumulated isolated and conjugated dienes Structure of Allene 131 pm 118.4° linear arrangement of carbons nonplanar geometry Bonding in Allene sp 2 sp sp 2 Bonding in Allene Bonding in Allene Bonding in Allene Chiral Allenes Allenes of the type shown are chiral Allenes X A C C C Y B A ≠ B; X ≠ Y Have a chirality axis Chirality Axis analogous to difference between: a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix Preparation of Dienes CH3CH2CH2CH3 CH Preparati on of 590-675°C H C Dienes 2 chromiaalumina 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" Dehydration of Alcohols KHSO4 KHSO OH OH heat major product; 88% yield 88% Dehydrohalogenati on of Alkyl Halides KOH KOH Br Br heat major product; 78% yield 78% Reactions of Dienes isolated dienes: double bonds react isolated double independently 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 Electrophilic Addition to Conjugated Dienes H X + H Proton adds to end of diene system Proton Carbocation formed is allylic Exampl e: H H H H H H HCl H H Cl H H H H H ? H ? H H Cl H H Minor product H H Exampl e: H H H H H H HCl HCl H H Cl H H H H H via: via: H H H H + H H H H X H H H H Protonation of the end of the diene unit gives an allylic carbocation. H H H H H + H H H H and: and: H H H H H H Cl H H 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 1,2-Addition versus 1,4Addition 1,2-addition of XY 1,2-addition 1,4-addition of XY 1,4-addition Y Y X via + X X HBr Addition to 1,3Butadiene H2C CH2 CHCH HBr CH3CHCH CH2 + CH3CH CHCH2Br Br electrophilic addition electrophilic 1,2 and 1,4-addition both observed product ratio depends on temperature Rationale 3-Bromo-1-butene (left) is formed faster than 3-Bromo-1-butene 1-bromo-2-butene (right) 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 (formed faster) via: + CH3CHCH CH2 Rationale 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond. CH3CHCH CH2 Br (formed faster) + CH3CH CHCH2Br (more stable) (more 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 major (formed faster) CH3CH CHCH2Br major product at 25°C (more stable) Kinetic Control versus Thermodynamic Control Kinetic control: major product is the one formed at the fastest rate (or easier to form) Thermodynamic control: major product is the one that is the most stable + CH3CHCH CH3CH CH2 HBr H2C CHCH CH2 + CHCH2 CH3CH + CHCH2 + CH3CHCH higher activation energy CH2 CH3CHCH formed more slowly CH2 Br CH3CH CHCH2Br Example Problem Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this + HCl ? product? product? Example Problem Think mechanistically. Think + HCl Protonation occurs: at end of diene system in direction that gives most stable carbocation Kinetically controlled product corresponds to attack by Kinetically chloride ion at carbon that has the greatest share of positive charge in the carbocation positive Example Think mechanisticallyProblem H + Cl Cll C H + + + one resonance form is one tertiary carbocation; other is primary other one resonance form is one secondary carbocation; other is primary other Example Problem Think mechanistically Think H Cl More stable carbocation + + one resonance form is one tertiary carbocation; other is primary other Is attacked by chloride ion Is at carbon that bears greater share of positive charge charge Example Problem Think mechanistically Think H + Cl + one resonance form is one tertiary carbocation; other is primary other Cl– Cll C major product Halogen Addition to Dienes gives mixtures of 1,2 and 1,4-addition products 1,4-addition Example H2C CH2 CHCH Br2 BrCH2CHCH CH2 + BrCH2CH CHCH2Br Br (37%) (63%) The Diels-Alder Reaction Synthetic method for preparing Synthetic compounds containing a cyclohexene ring compounds The Diels-Alder Reaction Synthetic method for preparing Synthetic compounds containing a cyclohexene ring compounds + conjugated alkene alkene diene (dienophile) diene (dienophile) cyclohexene via transition state transition Mechanisti c features concerted mechanism concerted cycloaddition pericyclic reaction a concerted reaction that proceeds concerted through a cyclic transition state through Recall the general reaction... + alkene conjugated alkene (dienophile) diene (dienophile) diene cyclohexene The equation as written is somewhat The misleading because ethylene is a relatively unreactive dienophile. unreactive What Makes a Reactive dienophile? The most reactive dienophiles have an The electron-withdrawing group (EWG) directly attached to the double bond. attached EWG C C Typical EWGs Typical C O C N Example H2C CHCH CH2 + H2C benzene via: O CH 100°C O O CH CH CH CH (100%) CH O Example H2C CH2 CHC CH3 benzene via: O H3C + 100°C O O H3C O O O O (100%) O Acetylenic Dienophile O H2C CHCH CH2 + CH3CH2OCC benzene O CCOCH2CH3 100°C O COCH2CH3 COCH (98%) COCH2CH3 O Diels-Alder Reaction is *A stereospecific reaction is one in which *A Stereospecific* stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. inversion Diels-Alder: syn addition to alkene Diels-Alder: cis-trans relationship of substituents on alkene cis-trans retained in cyclohexene product retained O Example H2C CHCH C6H5 CH2 + COH C H H C6H5 only product (& enantiomer) COH H O C H O Example H2C CHCH COH H CH2 + C C6H5 C6H5 only product (& enantiomer) H COH H O C H Cyclic dienes yield bridged bicyclic Diels-Alder adducts. O COCH3 H C + CH3OC C H H O O COCH3 H (& enantiomer) COCH3 O H O COCH3 H O is the is same as H COCH3 COCH3 O COCH3 H (& enantiomer) O ...
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