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Chapter 14- Conjugated Compounds and Ultraviolet Spectroscopy

AshleyPiekarski,Ph.D.

Conjugated and Nonconjugated Dienes

•  Compoundscanhavemorethanonedoubleortriplebond•  Iftheyareseparatedbyonlyonesinglebondtheyare

conjugatedandtheirorbitalsinteract•  Theconjugateddiene1,3-butadienehasproperFesthatare

verydifferentfromthoseofthenonconjugateddiene,1,5-pentadiene

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Why this Chapter?

•  Conjugatedcompoundsarecommoninnature•  ExtendedconjugaFonleadstoabsorpFonofvisiblelight,

producingcolor•  Conjugatedhydrocarbonswithmanydoublebondsare

polyenes(lycopeneisresponsibleforredcolorintomatoes)•  ExamineproperFesofconjugatedmoleculesandreasonsfor

theproperFes

14.1 Stability of Conjugated Dienes: Molecular Orbital Theory

•  TypicallyproducedbyeliminaFonofallylichalide•  SpecificindustrialprocessesforlargescaleproducFonof

commodiFesbycatalyFcdehydrogenaFonanddehydraFon

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Measuring Stability

•  ConjugateddienesaremorestablethannonconjugatedbasedonheatsofhydrogenaFon

•  HydrogenaFng1,3-butadieneproduces16kJ/molless

heatthan1,4-pentadiene

Stability

•  ElectrostaFcpotenFalmapsshowelectrondensitydelocalizedthroughouttheenFrepibondingframework

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14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

•  Review:addiFonofelectrophiletoC=C•  Markovnikov regiochemistry via more stable

carbocation

Carbocations from Conjugated Dienes

•  AddiFonofHBrleadstodelocalizedsecondaryallyliccarbocaFon

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Products of Addition to Delocalized Carbocation

•  NucleophilecanaddtoeithercaFonicsite•  ThetransiFonstatesforthetwopossibleproductsarenot

equalinenergy

14.3 Kinetic vs. Thermodynamic Control of Reactions

•  AtcompleFon,allreacFonsareatequilibriumandtherelaFveconcentraFonsarecontrolledbythedifferencesinfreeenergiesofreactantsandproducts(ThermodynamicControl)

•  IfareacFonisirreversibleorifareacFonisfarfromequilibrium,thentherelaFveconcentraFonsofproductsdependsonhowfasteachforms,whichiscontrolledbytherelaFvefreeenergiesofthetransiFonstatesleadingtoeachproduct(Kine7cControl)

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14.3 Kinetic vs. Thermodynamic Control of Reactions

Kinetic and Thermodynamic Control Example

•  AddiFontoaconjugateddieneatorbelowroomtemperaturenormallyleadstoamixtureofproductsinwhichthe1,2adductpredominatesoverthe1,4adduct

•  Athighertemperature,productraFochangesand1,4adductpredominates

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Kinetic and Thermodynamic Control Example

kineFc

thermodynamic

14.4 The Diels-Alder Cycloaddition Reaction

•  Conjugatedienescancombinewithalkenestoformsix-memberedcycliccompounds

•  TheformaFonoftheringinvolvesnointermediate(concertedformaFonoftwobonds)

•  DiscoveredbyO\oPaulHermannDielsandKurtAlderinGermanyinthe1930’s

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View of the Diels-Alder Reaction

•  WoodwardandHoffmanshowedthisshowntobeanexampleofthegeneralclassofpericyclicreac1ons

•  Involvesorbitaloverlap,changeofhybridizaFonandelectrondelocalizaFonintransiFonstate

•  ThereacFoniscalledacycloaddi1on

14.5 Characteristics of the Diels-Alder Reaction

•  Thealkenecomponentiscalledadienophile•  C=C is conjugated to an electron withdrawing

group, such as C=O or CN (nitrile) •  Alkynes can also be dienophiles

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Stereospecificity of the Diels-Alder Reaction

•  ThereacFonisstereospecific,maintainingrelaFverelaFonshipsfromreactanttoproduct•  There is a one-to-one relationship between

stereoisomeric reactants and products

Regiochemistry of the Diels-Alder Reaction

•  Reactantsaligntoproduceendo(ratherthanexo)product•  endo and exo indicate relative stereochemistry in

bicyclic structures •  Substituent on one bridge is exo if it is anti (trans)

to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges

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Conformations of Dienes in the Diels-Alder Reaction

•  TherelaFveposiFonsofthetwodoublebondsinthedieneare“cis”or“trans”toeachotheraboutthesinglebond(beinginaplanemaximizesoverlap)

•  TheseconformaFonsarecalleds-cisands-trans(“s”standsfor“singlebond”)

•  Dienesreactinthes-cisconforma1onintheDiels-AlderreacFon

14.7 Structure Determination in Conjugated Systems: UV Spectroscopy

•  Conjugatedcompoundscanabsorblightintheultravioletregionofthespectrum

•  Theelectronsinthehighestoccupiedmolecularorbital(HOMO)undergoatransiFontothelowestunoccupiedmolecularorbital(LUMO)

•  Theregionfrom2x10-7mto4x10-7m(200to400nm)ismostusefulinorganicchemistry

•  Aplotofabsorbance(logoftheraFooftheintensityoflightinoverlighttransmi\ed)againstwavelengthinthisregionisanultravioletspectrum

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UV-vis spectrum

UV-vis spectrum

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Quantitative Use of UV Spectra

•  AbsorbanceforaparFcularcompoundinaspecificsolventataspecifiedwavelengthisdirectlyproporFonaltoitsconcentraFon

•  YoucanfollowchangesinconcentraFonwithFmebyrecordingabsorbanceatthewavelength

•  Beers’law:absorbance=εcl•  “ε” is molar absorptivity (extinction coefficient) •  “c” is concentration in mol/L •  “l” is path of light through sample in cm

14.8 Interpreting UV Spectra: The Effect of Conjugation

•  λmax:wavelengthwhereUVabsorbanceforacompoundisgreatest

•  EnergydifferencebetweenHOMOandLUMOdecreasesastheextentofconjugaFonincreases

•  λmaxincreasesasconjugaFonincreases(lowerenergy)•  1,3-butadiene: 217 nm, 1,3,5-hexatriene: 258

nm

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14.9 Conjugation, Color and the Chemistry of Vision

•  Visibleregionisabout400to800nm•  ExtendedsystemsofconjugaFonabsorbinvisibleregion•  β-Carotene,11doublebondsinconjugaFon,λmax=455

nm•  Visualpigmentsareresponsibleforabsorbinglightineye

andtriggeringnervestosendsignaltobrain