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SolutionstoProblemsDienesBuildingBridgestoKnowledge

PhotoofarailwaytrackenteringAquasCalientes,thepathwaytoMachu

Picchu

1. In1,3-butadiene,four2porbitalslinearlycombine.Threesetsof2porbitals

couldlinearlycombineinthefollowingmanner:C1toC2;C2toC3andC3toC4.

Thefollowingfourbondingmolecularorbitalsarepossiblein1,3-butadiene:

π1;π2;π3;andπ4Ifthelowestenergylevelforthemolecularorbitalsformedfromthelinearcombinationof2porbitalsisrepresentedas:

Howwouldyourepresentπ2,π3andπ4?

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2 34

CC

C C++

++

--

--

Π1

2

π2

π3

π42. Completethefollowingreactions:

(a)H2C=CHCH=CH2+Br2→

(b)H2C=CHCH=CH2+Br2+(CH3)3COOC(CH3)3→

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(c)

H2C=CHCH=CH2+HBr

3. Showhowtheheatofhydrogenationcanbeusedtocalculatetheresonance

energyofstabilizationofaconjugateddiene.

Theenergyreleaseduponhydrogenatingadoublebondisapproximately126kJmol-1.Theheatofhydrogenationofanisolateddienesuchas1,4-pentadieneis254kJ/molabouttwicethatforacompoundcontainingonedoublebond.Therefore,everythingbeingequal,onewouldsuspectthattheheatofhydrogenationofaconjugateddienesuchas1,3-butadienewouldbeabout254kJmol-1,buttheactualheatofhydrogenationofthedieneis238kJmol-1.Thedifferenceof16kJmol-1isattributedtothestabilizationoftheconjugateddiene,i.e.,thefour2patomicorbitalsareparalleltoeachother,andtheyoverlaptoformamorestableconjugatedsystem.Apictureofthisoverlapisdescribedbyfigure3.2.

C CH H + Cu2Cl22

313K⎯⎯⎯→

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4. Drawandgiveanexplanationoftheenergyprofilediagramofthefollowingreaction:

Themechanismcorrelateswiththeenergyprofilediagram.

CH3CH=CHCH=CHCH3Br2

CH3CHCHCH=CHCH3

Br Br

CH3CHCH=CHCHCH3

Br Br

193K

313K

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1.

2.

3.

4.

5.Calculatetheequilibriumconstantforthefollowingreaction,if∆Go=-3.608x103J/mol

BrCH2CH(Br)CH=CH2-------->BrCH2CH=CHCH2Br

Whatdoesthevalueoftheequilibriumconstantsuggestaboutthedirectionoftheequilibriumreaction?

ΔGo = -2.303 RT ln Keq

Keq = e- ΔGo

2.303RT

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Thevaluesuggeststhattheequilibriumfavorstheformationofproduct.6. Writeamechanismtoaccountforthefollowingtransformation:

Excludingtheterminationsteps,themechanismwouldrationalizetheformationof1-bromo-2-buteneinthreesteps.1.

2.

Keq = e-

−3.608x103 Jmol

2.303 x 8.314 JKmol

x 298 K

Keq = e0.632

Keq = 1.88

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3.

1-bromo-2-buteneShowhowisopentenylpyrophosphatecouldbetheprecursortothebiosynthesisofvitaminA,compoundVI.

OH

VI

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TheformationofcompoundVimaybeenvisionedasresultingfromtail-to-headcombinationoffourisopentenylpyrophosphates. 8.Drawstructuresforthefollowingcompounds:(a) 2,2,5,5-tetramethyl-3-hexene

(b) 2,5-heptadiene

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(c) 2,4,6,8-decatetraene

(d) 1,4-cyclohexadiene

(e) bicyclo[2.2.1]-2,5-heptadieneorbicyclo[2.2.1]hepta-2,5-diene

or