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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