Chapter 10 Conjugation in Alkadienes and Allylic Systems Conjugare is a Latin verb meaning "to link or yoke together" Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

10.1 The Allyl Group C H R 3

Vinylic versus Allylic carbon vinylic carbons 4

Vinylic versus Allylic H H C C H C H Vinylic hydrogens are attached to vinylic carbons. Allylic hydrogens are attached to allylic carbons. 4

Vinylic versus Allylic X X X C C C C X C X C X Vinylic substituents are attached to vinylic carbons. Allylic substituents are attached to allylic carbons. 4

The Double Bond as a Substituent allylic carbocation C + allylic radical • C allylic carbanion C - a conjugated diene C 2

Allylic Molecular Orbitals A linear combination of three p orbitals results in the formation of three new molecular orbitals. C H + 3

Molecular Orbitals of the Allylic Group 2

Electrons in the Allylic MOs Highest energy Lowest energy 2

10.2 Allylic Carbocations C + 5

relative rates: (ethanolysis, 45°C) Allylic Carbocations A tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide. Cl CH3 C H2C CH CH3 C CH3 Cl CH3 123 1 relative rates: (ethanolysis, 45°C) 6

Provides good evidence that allylic carbocations are more stable than alkyl carbocations. CH3 H2C CH + CH3 C CH3 C + CH3 H2C=CH— stabilizes C+ better than does CH3— 6

Stabilization of Allylic Carbocations Extra stabilization comes from delocalization of electrons in the double bond. This stabilizes the carbocation. Two models: resonance model orbital overlap model 9

Resonance stabilization of C+. Resonance Model Resonance stabilization of C+. CH3 H2C CH + C CH3 H2C CH + C C CH3 H2C CH + 10

Orbital Overlap Model Empty orbital of C+. 12

Filled pi bond orbitals. Orbital Overlap Model Filled pi bond orbitals. 12

Orbital overlap stabilization of C+. Orbital Overlap Model Orbital overlap stabilization of C+. 12

10.3 SN1 Reactions of Allylic Halides 5

Hydrolysis of an Allylic Halide Cl CH3 C H2C CH Carbonate binds the proton from water. H2O Na2CO3 (15%) + CH3 HOCH2 CH C (85%) OH CH3 C H2C CH 16

Corollary Experiment C CH3 ClCH2 CH Same products. H2O Na2CO3 (15%) + HOCH2 CH C (85%) OH CH3 C H2C CH 16

Give the same products because they form the same carbocation. Cl CH3 C H2C CH CH3 ClCH2 CH C and Give the same products because they form the same carbocation. H2C CH3 CH + C 16

(15%) + CH3 HOCH2 CH C (85%) OH CH3 C H2C CH More positive charge on tertiary carbon; therefore more tertiary alcohol in product. H2C CH3 CH + C 16

10.4 SN2 Reactions of Allylic Halides 5

Allylic halides also undergo SN2 reactions Allylic SN2 Reactions Allylic halides also undergo SN2 reactions faster than simple primary alkyl halides. Cl CH2 H2C CH H3C CH2 CH2 Cl 1 80 relative rates In reaction with I-, a good nucleophile, and run in acetone, an SN2 solvent. 6

relative rates: (I-, acetone) Allylic SN2 reactions Two factors: Steric: Trigonal carbon smaller than tetrahedral carbon. Cl CH2 H2C CH H3C CH2 CH2 Cl 80 1 relative rates: (I-, acetone) 6

relative rates: (I-, acetone) Allylic SN2 reactions Two factors: Electronic: Electron delocalization lowers LUMO energy which means lower activation energy. See pg. 329 of the text. Cl CH2 H2C CH H3C CH2 CH2 Cl 80 1 relative rates: (I-, acetone) 6

10.5 Allylic Free Radicals C • 5

Allylic Free Radicals are Stabilized by Electron Delocalization • C • 22

Free Radical Stabilities are Related to Bond-dissociation Energies 410 kJ/mol • CH3CH2CH2—H CH3CH2CH2 + H• 368 kJ/mol • CHCH2—H H2C CHCH2 H2C + H• C—H bond is weaker in propene because resulting radical (allyl) is more stable than the radical (propyl) from propane. 23

10.6 Allylic Halogenation 5

Chlorination of Propene Normal addition ClCH2CHCH3 Cl CHCH3 H2C + Cl2 CHCH2Cl H2C 500 °C + HCl Substitution, Allylic chlorination 25

It is selective for replacement of an allylic hydrogen. Allylic Halogenation It is selective for replacement of an allylic hydrogen. Proceeds by a free radical mechanism. The allylic radical is an intermediate in this process. 26

Hydrogen-atom Abstraction Step Cl : . .. H 410 kJ/mol 368 kJ/mol H Allylic C—H bond is weaker than vinylic. Chlorine atom abstracts allylic H in propagation step. 27

Hydrogen-atom Abstraction Step Cl : .. • C C H H C 410 kJ/mol 368 kJ/mol H 27

Allylic bromination uses N-Bromosuccinimide NBS is the reagent used (instead of Br2 directly) for allylic bromination. CCl4 Br + heat (82-87%) O NH O NBr 29

Allylic halogenation is best used when: Limited Scope Allylic halogenation is best used when: 1. All of the allylic hydrogens are equivalent and 2. The resonance forms of allylic radical are equivalent. 30

Cyclohexene satisfies both requirements. Example H Cyclohexene satisfies both requirements. All allylic hydrogens are equivalent. H • H H • H Both resonance forms are equivalent. 31

All allylic hydrogens are equivalent. Example All allylic hydrogens are equivalent. • CH3CH CH CH2 2-Butene CH3CH CHCH3 But Two resonance forms are not equivalent; so a mixture of isomeric allylic bromides forms. Br Br CH3CH CH CH2 and CH3CH CH CH2 31

10.7 Allylic Anions C - 5

Acidity of Propene H3C CH CH2 H3C CH2 CH3 pKa ~ 43 pKa ~ 62 H2C CH - Allylic hydrogens of propene are significantly more acidic than hydrogens of propane and are easier to remove. 10

Charge is delocalized to both terminal carbons, Resonance Model - H2C CH - CH2 H2C CH CH2 CH2 H2C CH - Charge is delocalized to both terminal carbons, stabilizing the conjugate base. 10

10.8 Classes of Dienes 5

Classification of Dienes Isolated diene Conjugated diene Cumulated diene C 2

Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene

10.9 Relative Stabilities of Dienes 5

Heats of Hydrogenation, conjugated vs isolated 1,3-pentadiene is 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. 252 kJ/mol 226 kJ/mol 4

Heats of Hydrogenation, conjugated vs isolated Δ = 126 kJ/mol Δ = 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol 5

Heats of Hydrogenation, conjugated vs isolated Δ = 126 kJ/mol Δ = 111 kJ/mol When a terminal double bond is conjugated with another double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated. 5

Heats of Hydrogenation, conjugated vs isolated Δ = 126 kJ/mol Δ = 111 kJ/mol This extra 15 kJ/mol is known by several terms: conjugation energy or delocalization energy or resonance energy. 5

Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation. H2C C + CH2 2H2 CH3CH2CH3 H° = -295 kJ H° = -125 kJ + H2C CH2CH3 H2 CH3CH2CH3 7

10.10 Bonding in Conjugated Dienes 5

Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene 10

 bonds are independent of each other. Isolated diene  bonds are independent of each other. p orbitals overlap to extend  bond to encompass four carbons. Conjugated diene 10

less electron delocalization; less stable Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene 10

Conformations of Dienes, s-trans vs s-cis H H s-trans s-cis s prefix designates conformation around single bond. s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case). 11

Conformations of Dienes s-trans s-cis Both conformations allow electron delocalization via overlap of p orbitals to give extended  system. 11

s-trans is More Stable Than s-cis Interconversion of conformations involves the two  bonds being at right angles to each other and prevents conjugation during the process. 12 kJ/mol 12

No conjugation Conjugated s-trans Conjugated s-cis 12

No conjugation Conjugated s-trans Conjugated s-cis 16 kJ/mol 12 kJ/mol 15

10.11 Bonding in Allenes 5

Cumulated dienes are less stable than isolated and conjugated dienes. (see Problem 10.10) 17

Structure of Allene, CH2=C=CH2 118.4° 131 pm Linear arrangement of carbons Nonplanar geometry 19

p-orbitals on the sp2 carbons are orthogonal. Bonding in Allene p-orbitals on the sp2 carbons are orthogonal. sp 2 sp sp 2 19

p-orbitals on the sp2 carbons are orthogonal. Bonding in Allene p-orbitals on the sp2 carbons are orthogonal. 19

Bonding in Allene 19

Bonding in Allene 19

Allenes of the type shown are chiral Chiral Allenes Allenes of the type shown are chiral A X C B Y A  B; X  Y Have a chirality axis (Section 7.9) 25

Analogous to difference between: 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. 26

10.12 Preparation of Dienes 5

Used to prepare synthetic rubber (See "Diene Polymers" box, p. 406). 1,3-Butadiene 590-675°C CH3CH2CH2CH3 H2C CHCH CH2 chromia- alumina + 2H2 More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each year. Used to prepare synthetic rubber (See "Diene Polymers" box, p. 406). 2

Dehydration of Alcohols KHSO4 OH heat major product; 88% yield Conjugated, more stable 3

Dehydrohalogenation of Alkyl Halides KOH Br heat major product; 78% yield Conjugated, more stable 3

Isolated dienes: double bonds react independently of one another. Reactions of Dienes Isolated dienes: double bonds react independently of one another. Cumulated dienes: a specialized topic. Conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own. 6

10.13 Addition of Hydrogen Halides to Conjugated Dienes 5

Electrophilic Addition to Conjugated Dienes + H X H Proton adds to end of diene system. Carbocation formed is allylic. 8

H Example HCl Cl H ? H Cl ? Which 9

The favored reaction proceeds via: + H H X H + Protonation of the end of the diene unit (above) gives an allylic carbocation. The opposing protonation would give an isolated secondary carbocation. 10

The Allylic C+ gives Two Products: Cl H 1,2-addition of HCl Cl– H + 3-Chlorocyclopentene 1,4-addition of HCl 10

1,2-Addition vs 1,4-Addition 1,2-addition of XY 1,4-addition of XY X Y X Y via X + 12

Addition of HBr to 1,3-Butadiene H2C CHCH CH2 HBr Br CH2 CH3CHCH + CHCH2Br CH3CH 3-Bromo-1-butene 1-bromo-2-butene This is electrophilic addition. 1,2 and 1,4-addition both observed. Product ratio depends on temperature. 13

Rationale: 3-Bromo-1-butene (left) is formed faster than 1-bromo-2-butene (right) because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br CH2 CH3CHCH + CHCH2Br CH3CH (formed faster) CH2 CH3CHCH CHCH2 CH3CH via: + 13

Rationale: However, 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond. Br CH2 CH3CHCH + CHCH2Br CH3CH (formed faster) (more stable) 13

Rationale: The two products equilibrate at 25°C. At equilibrium, the more stable isomer predominates. Br CH2 CH3CHCH CHCH2Br CH3CH major product at -80°C major product at 25°C (more stable) (formed faster) 1,2 addition product 1,4 addition product 13

Kinetic Control vs Thermodynamic Control Kinetic control: the major product is the one formed at the fastest rate. Thermodynamic control: the major product is the one that is the most stable. 16

Energy Diagram for Addition of HBr to 1,3-Butadiene H2C CHCH CH2 HBr Br CH2 CH3CHCH + CHCH2Br CH3CH 17

higher activation energy + formed more slowly Br CH2 CH3CHCH CHCH2Br CH3CH Kinetic product Thermodynamic product 17

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 product? + HCl ? 19

Example Problem + HCl Think mechanistically. Protonation occurs: at end of diene system such that the most stable carbocation forms. Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation. + HCl 19

Think mechanistically: Example Problem Think mechanistically: H Cl Cl H + + One resonance form is a tertiary carbocation; other is primary. One resonance form is a secondary carbocation; other is primary. 20

Think mechanistically: Example Problem Think mechanistically: H Cl The more stable tertiary carbocation is attacked by chloride ion since this carbon bears a greater share of positive charge. + + One resonance form is a favored tertiary carbocation. 20

Think mechanistically: Example Problem Think mechanistically: H Cl Cl– Cl + + One resonance form is tertiary carbocation; other is primary. major product 20

10.14 Halogen Addition to Conjugated Dienes Gives mixtures of 1,2 and 1,4-addition products 5

Example H2C CHCH CH2 Br2 Br CH2 BrCH2CHCH + CHCH2Br BrCH2CH (37%) (63%) 13

10.15 The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring 5

an alkene (dienophile) Diels-Alder reaction + a conjugated diene an alkene (dienophile) cyclohexene Electron flow: transition state 2

Mechanistic Features of Diels-Alder It has a concerted mechanism, the addition is syn and an endo product is formed (called the endo rule or the Alder rule). It is a (4 + 2) cycloaddition reaction (forms a cyclic structure). Pericyclic reaction: A class of reactions that use pi electrons, have concerted mechanisms and proceed through a cyclic transition state (includes cycloadditions). 4

What Makes a Reactive Dienophile? Ethylene itself is not an effective dienophile. The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond. Typical EWGs C O N C EWG 5

Example CH O H2C CHCH CH2 + H2C CH benzene 100°C (100%) CH O CH O via: 6

Example O CHC CH2 H2C CH3 + benzene 100°C H3C via: O (100%) H3C O 6

Acetylenic Dienophile CCOCH2CH3 CH3CH2OCC H2C CHCH CH2 + benzene 100°C (98%) COCH2CH3 O 6

Diels-Alder Reaction is Stereospecific* *A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. Diels-Alder: occurs by syn addition to alkene. The cis-trans relationship of substituents on alkene retained in cyclohexene product. 9

Example O C6H5 COH C + H2C CHCH CH2 H H cis diene heat H C6H5 COH O + enantiomer 10

Example C C6H5 COH H O + H2C CHCH CH2 heat trans diene H C6H5 COH O + enantiomer 10

Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts COCH3 H O CH3OC + H COCH3 O Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts + enantiomer 13

H COCH3 O H COCH3 O is the same as 14

Endo product formation 10

10.16 The  Molecular Orbitals of Ethylene and 1,3-Butadiene 5

Orbitals and Chemical Reactions A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals (HOMO & LUMO) of the reactants. Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other. 6

Orbitals and Chemical Reactions HOMO-LUMO interactions can be illustrated by way of the Diels-Alder reaction between ethylene and 1,3-butadiene. Consider only the  electrons of ethylene and 1,3-butadiene and ignore the framework of  bonds in each molecule. 6

Red and blue colors distinguish sign of wave function. The  MOs of Ethylene Red and blue colors distinguish sign of wave function. Bonding  MO is antisymmetric with respect to plane of molecule. Bonding  orbital of ethylene; two electrons in this orbital. 6

Antibonding  orbital of ethylene; no electrons in this orbital. The  MOs of Ethylene Antibonding  orbital of ethylene; no electrons in this orbital. LUMO HOMO Bonding  orbital of ethylene; two electrons in this orbital. 6

Two of these orbitals are bonding; two are antibonding. __ The  MOs of 1,3-Butadiene __ Four p orbitals contribute to the  system of 1,3-butadiene; therefore, there are four  molecular orbitals. Two of these orbitals are bonding; two are antibonding. __ LUMO __ HOMO __ 6

The Two Bonding  MOs of 1,3-Butadiene HOMO 4  electrons; 2 in each orbital. Lowest energy orbital 6

The Two Antibonding  MOs of 1,3-Butadiene Highest energy orbital LUMO Both antibonding orbitals are vacant. 6

10.17 A  Molecular Orbital Analysis of the Diels-Alder Reaction 5

MO Analysis of Diels-Alder Reaction Since electron-withdrawing groups increase the reactivity of a dienophile, it is proposed that electrons flow from the HOMO of the diene to the LUMO of the dienophile. 6

MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another. HOMO of 1,3-butadiene This allows  bond formation between the alkene and the diene. LUMO of ethylene (dienophile) 6

A “Forbidden" Reaction H2C CH2 heat + H2C CH2 The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? 6

HOMO of one ethylene molecule. A “Forbidden" Reaction H2C CH2 heat + H2C CH2 HOMO of one ethylene molecule. HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new bonds. LUMO of other ethylene molecule. 6

End of Chapter 10 Conjugation in Alkadienes and Allylic Systems 5