Chapter 10Conjugation in Alkadienes andAllylic Systemsconjugare is a Latin verb meaning "to link or yoke together"

The Double Bond as a Substituent

10.1The Allyl Group

Vinylic versus AllylicCCC

Vinylic hydrogens are attached to vinylic carbons.Vinylic versus AllylicCCCHHH

Allylic hydrogens are attached to allylic carbons.Vinylic versus Allylic

Vinylic versus AllylicCCCXXXVinylic substituents are attached to vinylic carbons.

Vinylic versus AllylicCCCXXXAllylic substituents are attached to allylic carbons.

10.2Allylic Carbocations

The fact that a tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide ClCH3CH3CH3relative rates: (ethanolysis, 45C)1231Allylic Carbocations

provides good evidence that allylic carbocationsare more stable than other carbocations. CH3H2C=CH stabilizes C+ better than does CH3Allylic CarbocationsCC+CH3CH3

Delocalization of electrons in the double bond stabilizes the carbocation.resonance model orbital overlap modelStabilization of Allylic Carbocations

Resonance Model

Orbital Overlap Model++

Orbital Overlap Model

Orbital Overlap Model

Orbital Overlap Model

10.3SN1 Reactions of Allylic Halides

H2ONa2CO3Hydrolysis of an Allylic Halide

Corollary ExperimentH2ONa2CO3

andGive the same products because they form the same carbocation.

More positive charge on tertiary carbon; therefore more tertiary alcohol in product.

10.4SN2 Reactions of Allylic Halides

Allylic halides also undergo SN2 reactions faster than simple primary alkyl halides.relative rates: (I-, acetone)801Allylic SN2 ReactionsClCH2H3CCH2

Two factors:StericTrigonal carbon smaller than tetrahedral carbon.relative rates: (I-, acetone)801ClCH2H3CCH2Allylic SN2 reactions

Two factors:ElectronicElectron delocalization lowers LUMO energy which means lower activation energy.relative rates: (I-, acetone)801Allylic SN2 reactionsClCH2H3CCH2

10.5Allylic Free Radicals

Allylic Free Radicals are Stabilized byElectron Delocalization

Allylic Free Radicals are Stabilized byElectron DelocalizationSpin density is a measure of the unpaired electron distribution in a molecule.The picture on the next slide shows the unpaired electron in allyl radical "divides its time" equally between C-1 and C-3.

Allylic Free Radicals are Stabilized byElectron DelocalizationSpin density in allyl radical

Free Radical Stabilities are Related toBond-dissociation EnergiesCH3CH2CH2H410 kJ/molCH3CH2CH2+H368 kJ/mol+HCH bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane.

10.6Allylic Halogenation

addition500 Csubstitution+Cl2+ HClChlorination of Propene

Selective for replacement of allylic hydrogenFree radical mechanismAllylic radical is intermediateAllylic Halogenation

410 kJ/molHydrogen-atom Abstraction StepHHH368 kJ/molAllylic CH bond is weaker than vinylic.Chlorine atom abstracts allylic H in propagation step.

410 kJ/molHydrogen-atom Abstraction StepCCCHHHHH368 kJ/mol

Reagent used (instead of Br2) for allylic bromination. N-Bromosuccinimide

all of the allylic hydrogens are equivalent andthe resonance forms of allylic radical are equivalent. Limited ScopeAllylic halogenation is only used when:

ExampleCyclohexene satisfies both requirements.Both resonance forms are equivalent.HHHAll allylic hydrogens are equivalent.

Example2-ButeneAll allylic hydrogens are equivalent.

Example2-ButeneAll allylic hydrogens are equivalent.Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides.formsCH3CHCHCH2CH3CHCHCH2BrBrand

10.7Allylic Anions

Acidity of PropenePropene is significantly more acidic than propane.H3CCH2CH3H2CCH2-CH3pKa ~ 43pKa ~ 62

Resonance ModelH2CCH-CH2Charge is delocalized to both terminal carbons,stabilizing the conjugate base.

10.8 Classes of Dienes

Isolated dieneConjugated dieneCumulated dieneClassification of Dienes

(2E,5E)-2,5-heptadiene(2E,4E)-2,4-heptadiene3,4-heptadieneNomenclature

10.9Relative Stabilitiesof Dienes

252 kJ/mol226 kJ/mol1,3-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.Heats of Hydrogenation

252 kJ/mol226 kJ/mol126 kJ/mol115 kJ/molHeats of Hydrogenation

126 kJ/mol111 kJ/molHeats of HydrogenationWhen terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated.

126 kJ/mol111 kJ/molHeats of HydrogenationThis extra 15 kJ/mol is known by several terms: conjugation energy delocalization energy resonance energy

Cumulated double bonds have relatively high heats of hydrogenation.Heats of HydrogenationH2CCH2C+2H2CH3CH2CH3+H2CH3CH2CH3

10.10Bondingin Conjugated Dienes

Isolated dieneConjugated diene1,4-pentadiene1,3-pentadiene

Isolated dieneConjugated diene bonds are independent of each other.1,3-pentadiene

Isolated dieneConjugated diene bonds are independent of each other.p orbitals overlap to give extended bond encompassing four carbons.

Isolated dieneConjugated dieneless electron delocalization; less stablemore electron delocalization; more stable

s-transs-cisConformations of Dieness prefix designates conformation around single bond.s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case).

s-transs-cisConformations of DienesBoth conformations allow electron delocalization via overlap of p orbitals to give extended system.

s-trans is More Stable Than s-cis12 kJ/molInterconversion of conformations requires two bonds to be at right angles to each other and prevents conjugation.

16 kJ/mol12 kJ/mol

10.11Bonding in Allenes

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

Structure of Allene131 pmLinear arrangement of carbonsNonplanar geometry118.4

sp 2spBonding in Allenesp 2

Bonding in Allene

Bonding in Allene

Bonding in Allene

Allenes of the type shown are chiralABXYA B; X YHave a chirality axis (Section 7.9)Chiral Allenes

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. Chirality Axis

10.12Preparation of Dienes

CH3CH2CH2CH3590-675Cchromia-aluminaMore 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).1,3-Butadiene+2H2

KHSO4heatDehydration of Alcohols

KOHheatDehydrohalogenation of Alkyl Halides

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.Reactions of Dienes

10.13Addition of Hydrogen HalidestoConjugated Dienes

Proton adds to end of diene system.Carbocation formed is allylic.Electrophilic Addition to Conjugated DienesHXH+

Example:

HClExample:

via:HXProtonation of the end of the diene unit gives an allylic carbocation.

and:3-Chlorocyclopentene

1,2-addition of XY1,2-Addition versus 1,4-Addition

Electrophilic addition1,2 and 1,4-addition both observedProduct ratio depends on temperatureHBr Addition to 1,3-ButadieneHBr+

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. Rationale+(formed faster)

(more stable)Rationale+1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond.(formed faster)

major product at -80CRationaleThe two products equilibrate at 25C. Once equilibrium is established, the more stable isomer predominates.(formed faster)1,2 addition product1,4 addition product

Kinetic ControlversusThermodynamic ControlKinetic control: major product is the one formed at the fastest rate.Thermodynamic control: major product is the one that is the most stable.

HBr

+higher activation energyformed more slowly

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?Example Problem+HCl?

Think mechanistically.Protonation occurs: at end of diene system in direction that gives most stable carbocationKinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation.+HClExample Problem

One resonance form is secondary carbocation; other is primary.One resonance form is tertiary carbocation; other is primary.ClHExample ProblemThink mechanistically

++One resonance form is tertiary carbocation; other is primary.More stable carbocation Is attacked by chloride ion at carbon that bears greater share of positive charge.Example ProblemThink mechanistically

++One resonance form is tertiary carbocation; other is primary.ClClmajor productExample ProblemThink mechanistically

Gives mixtures of 1,2 and 1,4-addition products10.14Halogen Addition to Dienes

ExampleBr2+(37%)(63%)

10.15 The Diels-Alder ReactionSynthetic method for preparing compounds containing a cyclohexene ring

conjugated dienealkene (dienophile)cyclohexene+In General...

transition statevia

Concerted mechanismCycloadditionPericyclic reactionA concerted reaction that proceeds through a cyclic transition state.Mechanistic Features

conjugated dienealkene (dienophile)cyclohexene+Recall the General Reaction...The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.

What Makes a Reactive Dienophile?The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.

+H2CCHExample

+benzene100CExample

+Acetylenic Dienophile

Diels-Alder: syn addition to alkenecis-trans relationship of substituents on alkene retained in cyclohexene productDiels-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.

ExampleC6H5COHHHO+ enantiomer

Example+ enantiomer

Cyclic Dienes Yield Bridged BicyclicDiels-Alder Adducts

+ + enantiomer

is the same as

10.16 The Molecular OrbitalsofEthylene and 1,3-Butadiene

Orbitals and Chemical ReactionsA 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.

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 electrons of ethylene and 1,3-butadiene. We can ignore the framework of bonds in each molecule.Orbitals and Chemical Reactions

The MOs of EthyleneRed 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.

The MOs of EthyleneBonding orbital of ethylene; two electrons in this orbital.Antibonding orbital of ethylene; no electrons in this orbital. LUMO HOMO

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.The MOs of 1,3-Butadiene

The Two Bonding MOs of 1,3-ButadieneLowest energy orbital4 electrons; 2 in each orbital.HOMO

The Two Antibonding MOs of 1,3-ButadieneHighest energy orbitalBoth antibonding orbitals are vacant.LUMO

10.17A Molecular Orbital Analysisof theDiels-Alder Reaction

MO Analysis of Diels-Alder ReactionSince 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.

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

LUMO of ethylene (dienophile)HOMO of 1,3-butadieneMO Analysis of Diels-Alder Reaction

A Forbidden" ReactionThe dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?+

A Forbidden" Reaction+HOMO of one ethylene molecule.LUMO of other ethylene molecule.HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new bonds.

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