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NCSU CH 221 - 221cam8-L12-skeletal

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11Objectives©Dr. Kay SandbergLast lectureThis lectureAlkene preparationsNMR involving pi-systemsIRE1 mechanism1) dehydration of alcoholsE2 mechanism2) dehydrohalogenation of alkyl halides2DeuteriumOHDH2SO4ΔMOP© Kay SandbergH = 1H (protium)D = 2H (deuterium)same chemicalreactivity sinceboth are hydrogens(only difference is in rate)Unlike the protium isotope of hydrogen (H), deuterium (D) is NOT understoodand MUST be shown explicitly. CouchUse the X-tool on JME toolbar to input D –draw the stick bond first, then click X-tool (change H to D)and then click on the end of the bond where you want the D.3Predict MOPPredict major organic product (draw skeletal structure)OHH+BrB-_heatheatAB© Kay Sandberg4Mechanism A© Kay Sandberg3214OHHHHHHHHHHH CBA mechanismStep 1For 1oROH (hydride shift &water leave simultaneously)Regioselective (____________ bond)Stereoselective (____)5Mechanism B© Kay SandbergB mechanism3214HHHHHHHHHBrONo carbocation so no rearrangement!!!!6E1 vs E2© Kay SandbergE1 vsE2Section 5.15HXΔHOEt + +XOCCH XΔC CHαβHOCH2CH3HXHOCH2CH3δ+δ+δ+δ+Weak baseE2E1“Grab, slam, boot”δ+δ+In absence ofstrong base(only solvent)HOCH2CH327E1 kineticsCCH Xαβ© Kay SandbergSection 5.17β - Elimination Reactions:Elimination reactions that exhibit 1st order kineticsE1 mechanismR = k[RX]CCHαβH OHCH2CH3RearrangementsAlkyl halide reactivity:__________slowXHOCH2CH3RI >RBr >RCl >> RFConditions favoring E1: 3oRX, weak baseHOCH2CH3δ+δ+δ+δ+Weak base8E1 comparisonOHHBrSection 5.17β - Elimination Reactions:E1 mechanismOHHDehydration Dehydrohalogenation© Kay SandbergRDS:9Aromatic proton resonance peaksSection 13.7Integration2:3CH3CH3HHHHδ,ppm263745No splitting is observed between protons with identical chemical shifts. ©Dr. Kay SandbergSpin-spin splitting rules for 1H NMR 1) Chemically equivalent protons donot show spin-spin splitting10Aryl vs AlkylSection 13.7CCCCCCCCHHHHHHHHHHδ,ppm263745©Dr. Kay SandbergDownfieldAryl proton (H)(bonded to the aromatic ring)Alkyl protonUpfield________11Vinyl vs AlkylSection 13.7δ,ppm263745©Dr. Kay SandbergDownfieldvinyl proton (H)(bonded to sp2carbon of double bond)Alkyl protonUpfield_________HHHHHHHH12Protons in pi-systemsHHHHHHHHHHHoHCHOHHCHHHoHoaryl protonδ ~ 6.5 – 8.5 ppmvinyl protonδ ~ 4.5 – 6.5 ppmalkyl protonδ ~ 0.9 – 1.8 ppm313Proton comparisonsSection 13.5HHHvinylallylicalkyl©Dr. Kay Sandberg14MultiplicitiesSection 13.7OOOCH3OHHHHCH2CH3A) methoxyB) ortho to OC) ortho to carbonylD) methyleneE) methylGive the multiplicity of the following proton signals.©Dr. Kay Sandberg15Non-equivalent vicinal protonsSection 13.11© Kay SandbergClClClWhat happens when protons on adjacent carbons arenot equivalent?H3CCH3CCClC ClClHHHabcdHcJbc= 6.3 HzHbCoupling w/JbcHdCoupling w/Jcd= 3.8 HzJcdJcd16Doublet of doublets vs quartetSection 13.11© Kay SandbergNonequivalent vicinal proton couplingH3CCH3CCClC ClClHHHabcdCHdClClCHaHcHbEquivalent vicinal proton couplingPeak intensity_________Peak intensityin 1:3:3:1ratioHcJbcJcdJcddoublet of doubletsSpacing betweenlines ________HdquartetSpacing betweenlines the same17MultiplicitiesSection 13.10© Kay SandbergOOJbgJgb= 7 Hz=JbpJpb= 1.5 Hz=HpHbOHgOCH3CH3CH3JgpJpg= 14 Hz=What happens when protonson adjacent carbons arenot equivalent?18MultiplicitiesSection 13.10© Kay SandbergJbgJgb= 7 Hz=JbpJpb= 1.5 Hz=HpHbOHgOCH3CH3CH3JgpJpg= 14 Hz=Hgchemical shiftdownfield upfieldHpHb14 Hz14 Hz7 Hzdoublet of doublets doublet of doubletsdoublet of doublets419MultiplicitiesSection 13.11© Kay SandbergCCClHbCHgClHpHpJbgJgb= 10 Hz=JgpJpg= 4 Hz=Give the multiplicity of the following signals.B) HgC) HbA) Hp20Multiplicities© Kay SandbergCCClHbCHgClHpHpJbgJgb= 10 Hz=JgpJpg= 4 Hz=chemical shiftdownfield upfieldHgHpHb21Unresolved multipletsSection 13.141H NMR spectrumHo, Hr, Hbare unresolved multipletsHpδ,ppm0243 15678HgHoHr& HbHow many differentprotons are there in1-chloropentane?©Dr. Kay Sandberg22C-13 spectrumSection 13.14δ,ppmNMR spectrumCClCCCCHgHgHoHoHrHrHbHbHpHpHpA separate, distinct peak is observed for each carbon.04080 60 20100120140160©Dr. Kay Sandberg13C23IR vibrationPhysical basis of IR spectroscopy©Dr. Kay Sandberg424ComparisonPhysical basis of IR spectroscopy©Dr. Kay Sandberg4525Frequency & amplitude of ground state vibrationTimeFrequency (ν)26Frequency & amplitude of excited vibrationTimeFrequency (ν)27ComparisonTimeFrequency (ν) ________Absorbed E _______________28Stretching modesSection 13.20©Dr. Kay SandbergSymmetric stretch vibrational modeAsymmetric stretch vibrational mode29In-plane bending modesSection 13.20©Dr. Kay SandbergAsymmetric in-plane bend vibrational mode (rock)symmetric in-plane bend vibrational mode (scissor)30Out-of-plane bending modesSection 13.20©Dr. Kay Sandbergsymmetric out-of-plane bend vibrational mode (wag)asymmetric out-of-plane bend vibrational mode (twist)631IR spectrum - wavenumberSection 13.20E = hνc = λνIR spectrum of butan-2-one©Dr. Kay SandbergO~1680-1850 cm-1~3000 cm-1wavenumber (cm-1)32IR spectraSection 13.20©Dr. Kay


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