Chapter 10 Conjugation in Alkadienes and Allylic Systems conjugare is a Latin verb meaning "to link or yoke together" Dr. Wolf's CHM 201 & 202 1

The Double Bond as a Substituent C + allylic carbocation Dr. Wolf's CHM 201 & 202 2

The Double Bond as a Substituent C + • C allylic carbocation allylic radical Dr. Wolf's CHM 201 & 202 2

The Double Bond as a Substituent C + • C allylic carbocation allylic radical C conjugated diene Dr. Wolf's CHM 201 & 202 2

The Allyl Group C H Dr. Wolf's CHM 201 & 202 3

Vinylic versus Allylic carbon vinylic carbons Dr. Wolf's CHM 201 & 202 4

Vinylic versus Allylic H C C H C H vinylic hydrogens are attached to vinylic carbons Dr. Wolf's CHM 201 & 202 4

Vinylic versus Allylic H allylic hydrogens are attached to allylic carbons Dr. Wolf's CHM 201 & 202 4

Vinylic versus Allylic X C C X C X vinylic substituents are attached to vinylic carbons Dr. Wolf's CHM 201 & 202 4

Vinylic versus Allylic X X C C X C allylic substituents are attached to allylic carbons Dr. Wolf's CHM 201 & 202 4

Allylic Carbocations C + Dr. Wolf's CHM 201 & 202 5

relative rates: (ethanolysis, 45°C) Allylic Carbocations the fact that 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) Dr. Wolf's CHM 201 & 202 6

Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH3 CH3 H2C CH C + + CH3 C CH3 CH3 formed faster Dr. Wolf's CHM 201 & 202 6

H2C=CH— stabilizes C+ better than CH3— Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH3 H2C CH + CH3 C C + CH3 CH3 H2C=CH— stabilizes C+ better than CH3— Dr. Wolf's CHM 201 & 202 6

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 9

Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C 10 Dr. Wolf's CHM 201 & 202 10

Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C CH3 d+ d+ H2C CH C Dr. Wolf's CHM 201 & 202 10

Orbital Overlap Model + + Dr. Wolf's CHM 201 & 202 12

Orbital Overlap Model Dr. Wolf's CHM 201 & 202 12

Orbital Overlap Model Dr. Wolf's CHM 201 & 202 12

Orbital Overlap Model Dr. Wolf's CHM 201 & 202 12

SN1 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202 5

Hydrolysis of an Allylic Halide Cl CH3 C H2C CH H2O Na2CO3 CH3 OH CH3 C H2C CH + HOCH2 CH C CH3 (15%) (85%) Dr. Wolf's CHM 201 & 202 16

Corollary Experiment CH3 ClCH2 CH C H2O Na2CO3 CH3 HOCH2 CH C OH CH3 C + (15%) (85%) Dr. Wolf's CHM 201 & 202 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 Dr. Wolf's CHM 201 & 202 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 CH3 CH3 + H2C + CH C H2C CH C CH3 CH3 Dr. Wolf's CHM 201 & 202 16

more positive charge on tertiary carbon; therefore more tertiary alcohol in product + H2C CH C + H2C CH C CH3 CH3 Dr. Wolf's CHM 201 & 202 16

(85%) (15%) CH3 CH3 C + H2C CH OH HOCH2 CH C CH3 CH3 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH3 CH3 + H2C + CH C H2C CH C CH3 CH3 Dr. Wolf's CHM 201 & 202 16

SN2 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202 5

relative rates: (I-, acetone) Allylic SN2 Reactions Allylic halides also undergo SN2 reactions faster than simple primary alkyl halides. Cl CH2 H2C CH H3C CH2 CH2 Cl 80 1 relative rates: (I-, acetone) Dr. Wolf's CHM 201 & 202 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) Dr. Wolf's CHM 201 & 202 6

relative rates: (I-, acetone) Allylic SN2 reactions Two factors: Electronic Electron delocalization lowers LUMO energy which means lower activation energy. Cl CH2 H2C CH H3C CH2 CH2 Cl 80 1 relative rates: (I-, acetone) Dr. Wolf's CHM 201 & 202 6

Allylic Free Radicals C • Dr. Wolf's CHM 201 & 202 5

Allylic free radicals are stabilized by electron delocalization • C • Dr. Wolf's CHM 201 & 202 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 radical (propyl) from propane Dr. Wolf's CHM 201 & 202 23

Allylic Halogenation Dr. Wolf's CHM 201 & 202 5

Chlorination of Propene addition ClCH2CHCH3 Cl CHCH3 H2C + Cl2 CHCH2Cl H2C 500 °C + HCl substitution Dr. Wolf's CHM 201 & 202 25

selective for replacement of allylic hydrogen free radical mechanism Allylic Halogenation selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate Dr. Wolf's CHM 201 & 202 26

Hydrogen-atom abstraction step Cl : . .. H 410 kJ/mol 368 kJ/mol H allylic C—H bond weaker than vinylic chlorine atom abstracts allylic H in propagation step Dr. Wolf's CHM 201 & 202 27

Hydrogen-atom abstraction step Cl : .. • C C H H C 410 kJ/mol 368 kJ/mol H Dr. Wolf's CHM 201 & 202 27

reagent used (instead of Br2) for allylic bromination N-Bromosuccinimide reagent used (instead of Br2) for allylic bromination O NBr Br O NH heat + + CCl4 (82-87%) Dr. Wolf's CHM 201 & 202 29

Allylic halogenation is only used when: Limited Scope Allylic halogenation is only used when: all of the allylic hydrogens are equivalent and the resonance forms of allylic radical are equivalent Dr. Wolf's CHM 201 & 202 30

Cyclohexene satisfies both requirements Example H Cyclohexene satisfies both requirements All allylic hydrogens are equivalent Dr. Wolf's CHM 201 & 202 31

Cyclohexene satisfies both requirements Example H Cyclohexene satisfies both requirements All allylic hydrogens are equivalent H • H • Both resonance forms are equivalent Dr. Wolf's CHM 201 & 202 31

All allylic hydrogens are equivalent 2-Butene CH3CH CHCH3 Example All allylic hydrogens are equivalent 2-Butene CH3CH CHCH3 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 31

Allylic Anions Dr. Wolf's CHM 201 & 202 1

Allylic anions are stabilized by electron delocalization CH3 H2C CH - C CH3 CH3 H2C CH - C Dr. Wolf's CHM 201 & 202

Acidity of Propene H3C CH CH2 H3C CH2 CH3 pKa ~ 43 pKa ~ 62 H2C CH - Propene is significantly more acidic than propane. Dr. Wolf's CHM 201 & 202 10

Resonance Model - H2C CH - CH2 H2C CH CH2 CH2 H2C CH - Charge is delocalized to both terminal carbons, stabilizing the conjugate base. Dr. Wolf's CHM 201 & 202 10

Classes of Dienes Dr. Wolf's CHM 201 & 202 1

Classification of Dienes isolated diene conjugated diene cumulated diene C Dr. Wolf's CHM 201 & 202 2

Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene Dr. Wolf's CHM 201 & 202 2

Relative Stabilities of Dienes Dr. Wolf's CHM 201 & 202 3

Heats of Hydrogenation 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 Dr. Wolf's CHM 201 & 202 4

Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202 5

Heats of Hydrogenation 126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202 5

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 Dr. Wolf's CHM 201 & 202 5

Heats of Hydrogenation 126 kJ/mol 111 kJ/mol this extra 15 kJ/mol is known by several terms stabilization energy delocalization energy resonance energy Dr. Wolf's CHM 201 & 202 5

Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation H2C C + CH2 2H2 CH3CH2CH3 DH° = -295 kJ + H2C CH2CH3 H2 CH3CH2CH3 DH° = -125 kJ Dr. Wolf's CHM 201 & 202 7

Bonding in Conjugated Dienes Dr. Wolf's CHM 201 & 202 8

Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene 10 Dr. Wolf's CHM 201 & 202 10

p bonds are independent of each other Isolated diene p bonds are independent of each other 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202 10

p bonds are independent of each other Isolated diene p bonds are independent of each other p orbitals overlap to give extended p bond encompassing four carbons Conjugated diene Dr. Wolf's CHM 201 & 202 10

less electron delocalization; less stable Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene Dr. Wolf's CHM 201 & 202 10

Conformations of Dienes 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) Dr. Wolf's CHM 201 & 202 11

Conformations of Dienes 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) Dr. Wolf's CHM 201 & 202 11

Conformations of Dienes s-trans s-cis Both conformations allow electron delocalization via overlap of p orbitals to give extended p system Dr. Wolf's CHM 201 & 202 11

s-trans is more stable than s-cis Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol Dr. Wolf's CHM 201 & 202 12

Dr. Wolf's CHM 201 & 202 12

16 kJ/mol 12 kJ/mol Dr. Wolf's CHM 201 & 202 15

Bonding in Allenes Dr. Wolf's CHM 201 & 202 16

cumulated dienes are less stable than isolated and conjugated dienes (see Problem 10.7 on p 375) Dr. Wolf's CHM 201 & 202 17

linear arrangement of carbons Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 19

linear arrangement of carbons Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 19

Bonding in Allene sp 2 sp sp 2 Dr. Wolf's CHM 201 & 202 19

Bonding in Allene Dr. Wolf's CHM 201 & 202 19

Bonding in Allene Dr. Wolf's CHM 201 & 202 19

Bonding in Allene Dr. Wolf's CHM 201 & 202 19

Allenes of the type shown are chiral Chiral Allenes Allenes of the type shown are chiral A X C B Y A ¹ B; X ¹ Y Have a stereogenic axis Dr. Wolf's CHM 201 & 202 25

analogous to difference between: Stereogenic 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 Dr. Wolf's CHM 201 & 202 26

Preparation of Dienes Dr. Wolf's CHM 201 & 202 1

used to prepare synthetic rubber (See "Diene Polymers" box) 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) Dr. Wolf's CHM 201 & 202 2

Dehydration of Alcohols KHSO4 OH heat Dr. Wolf's CHM 201 & 202 3

Dehydration of Alcohols KHSO4 OH heat major product; 88% yield Dr. Wolf's CHM 201 & 202 3

Dehydrohalogenation of Alkyl Halides KOH Br heat Dr. Wolf's CHM 201 & 202 3

Dehydrohalogenation of Alkyl Halides KOH Br heat major product; 78% yield Dr. Wolf's CHM 201 & 202 3

isolated dienes: double bonds react independently of one another Reactions of Dienes isolated dienes: double bonds react independently of one another cumulated dienes: specialized topic conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own Dr. Wolf's CHM 201 & 202 6

Addition of Hydrogen Halides to Conjugated Dienes Dr. Wolf's CHM 201 & 202 7

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 8

Example: H HCl Cl H H Cl ? ? Dr. Wolf's CHM 201 & 202 9

Example: H HCl Cl H Dr. Wolf's CHM 201 & 202 9

via: H + H H X H + Dr. Wolf's CHM 201 & 202 10

and: H + Cl H Cl– 3-Chlorocyclopentene H + H H Cl H H H H H 10 Dr. Wolf's CHM 201 & 202 10

1,2-Addition versus 1,4-Addition 1,2-addition of XY X Y Dr. Wolf's CHM 201 & 202 12

1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY X Y X Y Dr. Wolf's CHM 201 & 202 12

1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY X Y X Y via X + Dr. Wolf's CHM 201 & 202 12

HBr Addition to 1,3-Butadiene H2C CHCH CH2 HBr Br CH2 CH3CHCH + CHCH2Br CH3CH electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature Dr. Wolf's CHM 201 & 202 13

Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br CH2 CH3CHCH + CHCH2Br CH3CH via: + + CH2 CH3CHCH CHCH2 CH3CH Dr. Wolf's CHM 201 & 202 13

Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene 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 Dr. Wolf's CHM 201 & 202 13

Rationale 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 more stable Dr. Wolf's CHM 201 & 202 13

Rationale The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. Br CH2 CH3CHCH CHCH2Br CH3CH major product at -80°C major product at 25°C (formed faster) (more stable) Dr. Wolf's CHM 201 & 202 13

Kinetic Control versus Thermodynamic Control Kinetic control: major product is the one formed at the fastest rate Thermodynamic control: major product is the one that is the most stable Dr. Wolf's CHM 201 & 202 16

CH2 CH3CHCH CHCH2 CH3CH + HBr H2C CHCH CH2 Dr. Wolf's CHM 201 & 202 17

higher activation energy CH3CHCH CH2 + higher activation energy CH3CHCH CH2 + CH3CH CHCH2 formed more slowly Br CH2 CH3CHCH CHCH2Br CH3CH Dr. Wolf's CHM 201 & 202 17

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 ? Dr. Wolf's CHM 201 & 202 19

+ HCl Think mechanistically. Protonation occurs: at end of diene system in direction that gives most stable carbocation Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation + HCl Dr. Wolf's CHM 201 & 202 19

Think mechanistically Cl + + one resonance form is tertiary carbocation; other is primary Dr. Wolf's CHM 201 & 202 20

Think mechanistically Cl Cl H + + + + one resonance form is tertiary carbocation; other is primary one resonance form is secondary carbocation; other is primary Dr. Wolf's CHM 201 & 202 20

Think mechanistically Cl More stable carbocation Is attacked by chloride ion at carbon that bears greater share of positive charge + + one resonance form is tertiary carbocation; other is primary Dr. Wolf's CHM 201 & 202 20

Think mechanistically Cl Cl– Cl + + one resonance form is tertiary carbocation; other is primary major product Dr. Wolf's CHM 201 & 202 20

Halogen Addition to Dienes gives mixtures of 1,2 and 1,4-addition products Dr. Wolf's CHM 201 & 202 23

Example H2C CHCH CH2 Br2 Br CH2 BrCH2CHCH + CHCH2Br BrCH2CH (37%) (63%) Dr. Wolf's CHM 201 & 202 13

The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring Dr. Wolf's CHM 201 & 202 1

In general... + conjugated diene alkene (dienophile) cyclohexene 2 Dr. Wolf's CHM 201 & 202 2

via transition state Dr. Wolf's CHM 201 & 202 3

Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

a concerted reaction that proceeds through a cyclic transition state Mechanistic features concerted mechanism cycloaddition pericyclic reaction a concerted reaction that proceeds through a cyclic transition state Dr. Wolf's CHM 201 & 202 4

Recall the general reaction... + conjugated diene alkene (dienophile) cyclohexene The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile. Dr. Wolf's CHM 201 & 202 2

What makes a reactive dienophile? The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond. Typical EWGs C EWG C O C N Dr. Wolf's CHM 201 & 202 5

Example CH O H2C CHCH CH2 + H2C CH benzene 100°C CH O (100%) 6 Dr. Wolf's CHM 201 & 202 6

Example CH O H2C CHCH CH2 + H2C CH benzene 100°C CH O via: CH O (100%) Dr. Wolf's CHM 201 & 202 6

Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

Example O H2C CHC CH2 CH3 + benzene 100°C H3C O (100%) 6 Dr. Wolf's CHM 201 & 202 6

Example O H2C CHC CH2 CH3 + benzene 100°C via: H3C O H3C O (100%) 6 Dr. Wolf's CHM 201 & 202 6

Acetylenic Dienophile CCOCH2CH3 CH3CH2OCC H2C CHCH CH2 + benzene 100°C COCH2CH3 O (98%) Dr. Wolf's CHM 201 & 202 6

Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

Diels-Alder Reaction is Stereospecific* syn addition to alkene cis-trans relationship of substituents on alkene retained in cyclohexene product *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. Dr. Wolf's CHM 201 & 202 9

Example O C6H5 COH C + H2C CHCH CH2 H H H C6H5 COH O only product 10 Dr. Wolf's CHM 201 & 202 10

Example C C6H5 COH H O + H2C CHCH CH2 H C6H5 COH O only product 10 Dr. Wolf's CHM 201 & 202 10

Cyclic dienes yield bridged bicyclic Diels-Alder adducts. Dr. Wolf's CHM 201 & 202 12

Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 Dr. Wolf's CHM 201 & 202

Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

C COCH3 H O CH3OC + H COCH3 O Dr. Wolf's CHM 201 & 202 13

H COCH3 O H COCH3 O is the same as Dr. Wolf's CHM 201 & 202 14

The p Molecular Orbitals of Ethylene and 1,3-Butadiene Dr. Wolf's CHM 201 & 202 1

Orbitals and Chemical Reactions A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals 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. Dr. Wolf's CHM 201 & 202 6

Orbitals and Chemical Reactions We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene. We need only consider only the p electrons of ethylene and 1,3-butadiene. We can ignore the framework of s bonds in each molecule. Dr. Wolf's CHM 201 & 202 6

red and blue colors distinguish sign of wave function The p MOs of Ethylene red and blue colors distinguish sign of wave function bonding p MO is antisymmetric with respect to plane of molecule Bonding p orbital of ethylene; two electrons in this orbital Dr. Wolf's CHM 201 & 202 6

LUMO HOMO The p MOs of Ethylene Antibonding p orbital of ethylene; no electrons in this orbital LUMO HOMO Bonding p orbital of ethylene; two electrons in this orbital Dr. Wolf's CHM 201 & 202 6

Two of these orbitals are bonding; two are antibonding. The p MOs of 1,3-Butadiene Four p orbitals contribute to the p system of 1,3-butadiene; therefore, there are four p molecular orbitals. Two of these orbitals are bonding; two are antibonding. Dr. Wolf's CHM 201 & 202 6

The Two Bonding p MOs of 1,3-Butadiene HOMO 4 p electrons; 2 in each orbital Lowest energy orbital Dr. Wolf's CHM 201 & 202 6

The Two Antibonding p MOs of 1,3-Butadiene Highest energy orbital LUMO Both antibonding orbitals are vacant Dr. Wolf's CHM 201 & 202 6

A p Molecular Orbital Analysis of the Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 1

MO Analysis of Diels-Alder Reaction Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile. Dr. Wolf's CHM 201 & 202 6

MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another allows s bond formation between the alkene and the diene LUMO of ethylene (dienophile) Dr. Wolf's CHM 201 & 202 6

MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene LUMO of ethylene (dienophile) Dr. Wolf's CHM 201 & 202 6

A "forbidden" reaction H2C CH2 + H2C CH2 The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? Dr. Wolf's CHM 201 & 202 6

HOMO of one ethylene molecule A "forbidden" reaction H2C CH2 + HOMO of one ethylene molecule HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new s bonds LUMO of other ethylene molecule Dr. Wolf's CHM 201 & 202 6

End of Chapter 10 Dr. Wolf's CHM 201 & 202 6