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NCSU CH 221 - 221cam8-L8-skeletal

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11Lecture objectives©Dr. Kay SandbergLast lectureThis lectureSN1 & SN2 mechanismsLewis Acid-base chemistryalcohol alkyl halideHXHalogenation of alkaneselectrophile & nucleophile2SN1 reviewR Clδ−δ+Section 4.11SN1 mechanismROH, HClROH2, Cl-R OH ClHRCl,H2Oδ+δ−R OHHδ+δ++3oReaction CoordinateGo2o© Dr. Kay Sandberg1o__________intermediateH2O3Flash card componentsR-OH2) functional groupsFlash card components•reactant•product3) reagent4) mechanism1) transformationR-Xalcohol alkyl halide©Dr. Kay Sandberg4Alcohol to alkyl halide flash cardsR-ClR-OH1o& 2oR-OH3oalcohols alkyl halidesR-OH1o& 2oR-BrR-OH1o,2o,3o©Dr. Kay Sandberg5Electron flow arrow review©Dr. Kay SandbergO+HClHElectron____(red region)Electron____(blue region)O+ClHH6Alkane halogenation flash cardSection 4.14Halogenation of alkanesR—H R—XF2ΔHo= -426 kJ/molI2ΔHo= +54 kJ/molCl2ΔHo= -103 kJ/molBr2ΔHo= -30 kJ/mol©Dr. Kay Sandberg_______X2R—H + X2R—X + HX__________27Carbon radicalSection 4.16Halogenation of alkanesDifferent mechanism than ROH conversion to RX(carbocation & alkyloxonium ion (polar) intermediates)Mechanism for RH conversion to RX involvesfree radicalsRCRRSpecies that contains _______________.©Dr. Kay SandbergO+ClHHC: _________________ intermediatesCRRR8Electronic description of carbon radicalMechanism for RH conversion to RX involvesfree radicalsRCRRSpecies that contains an unpaired electron.More sp_ ishthan sp_ ishmore ____(flattened pyramid)©Dr. Kay Sandberg: neutral (nonpolar) intermediatesCRRRFC = family # - possession #9Radical stabilitiesMost stable?Section 4.16Free radicals Species that contain unpaired electrons.HCHHHCH3CHH3CCH3CHH3CCH3CCH3C•are like carbocations (C+) in that they are also electron deficient.1omethyl2o3oAlkyl groups _______ e-density to C•.<<<©Dr. Kay Sandberg“_________” “_________”10BDE_____lytic bond cleavageSection 4.17Free radicals Evidence for this stabilityX YXY+X YXY+One sided arrow(single barbed)Two sided arrow(double barbed)_______lytic bond cleavageBDE: bond dissociation E©Dr. Kay SandbergBond broken_____lytically11BDE comparison©Dr. Kay Sandberg1o2oCHHCCHHHHHH410 kJ/mol13 kJ/mol397 kJ/molE12BDE listHomolytic bond cleavage Section 4.17Free radicals Evidence for this stabilityR HRH+One sided arrow1o : +410 kJ/molmethyl: +435 kJ/mol2o : +397 kJ/mol3o : +380 kJ/mol©Dr. Kay SandbergRadical classifications313Free radical reactionSection 4.18Free radicals: Methane monochlorination•Initiation step•Propagation steps•Chain terminating stepCH3—H + Cl2CH3—Cl + HCl_______________ substitution rxn: Cl is substituted for an alkane HCl ClCHHHH+CHHH Cl HCl+Hi T orlighthνhν: breaks the weakest bondC—H BDE: 435 kJ/molCl—Cl BDE: 242 kJ/mol________chemical rxn©Dr. Kay Sandberg14Initiation stepCl ClHomolytic bond cleavage One sided arrowSection 4.18Free radicals: Methane monochlorinationStep 1initiationhνDissociation of chlorine molecule into ________©Dr. Kay Sandberg15Propagation stepCHHHHCl+Section 4.18Free radicals: Methane monochlorinationStep 1 initiationCl ClCl+ClhνStep 2 propagation__________ abstraction from alkane by Cl atom©Dr. Kay SandbergH atom (H.) different from proton (H+)16Propagation stepSection 4.18Free radicals: Methane monochlorinationStep 1 initiationCHHHHCl+Step 2 propagationCHHHHCl+Chain propagationCHHHCl Cl+H3C• reacts with molecular Cl creating __________ ...©Dr. Kay SandbergCl ClCl+Clhν17Propagation stepSection 4.18Free radicals: Methane monochlorinationStep 1 initiationRadical reacts with non-radical producing radical & non-radical. ©Dr. Kay SandbergCHHHCl Cl+CHHClHCl+CHHHHCl+Step 2 propagationCHHHHCl+Chain propagationCl ClCl+Clhν18Termination stepSection 4.18Free radicals: Methane monochlorinationStep 1 initiationCHHHCl Cl+CHHClHCl+Step 3 chain-termination2 radicals react forming non-radical products©Dr. Kay SandbergCHHHHCl+Step 2 propagationCHHHHCl+Chain propagationCl ClCl+Clhν419Termination stepCHHHCl+Section 4.18CHHClHFree radicals: Methane monochlorinationStep 1: initiationStep 2: chain propagationStep 3: chain-terminationCl+ClCl Cl©Dr. Kay Sandberg20Chlorination E diagramΔ = -21 kJ/molEReaction CoordinateRH + Cl•R HClδδSection 4.18©Dr. Kay SandbergCl•bond “____age”(+410 kJ/mol)bond “_____age”(-431 kJ/mol)bond _____age (-338 kJ/mol)Free radical substitutionHClR•+ Cl-ClR ClClδδbond _____age (+242 kJ/mol)Δ = -96 kJ/molΔtotal= -117 kJ/molRCl21RHBrδδReaction CoordinateR HClδδSection 4.18©Dr. Kay SandbergCl•Free radical substitution+410BDE in kJ/mol+ = bond “breakage”-= bond “makeage”-431+410 -366EΔ = -21Δ = +44R BrBrδδ-282 +192Δ = -90RClR ClClδδ-338 +242RBr Br•Δtotal= -46Δtotal= -117Hammond’s postulateC has ___ rad. characterMoundsΔ = -9622Chlorination vs brominationReaction CoordinateSection 4.18©Dr. Kay SandbergCl•Free radical substitutionERClRBr Br•H-Cl bond_______ thanH-Br bondR HClδδR HBrδδR BrBrδδR ClClδδ23vs iodinationReaction CoordinateSection 4.18©Dr. Kay SandbergFree radical substitutionECl•RClRBr Br•R HClδδR HBrδδR BrBrδδR ClClδδ24MonochlorinationSection 4.19What do these and all other unsubstituted cycloalkanes have in common?CHHH CHHHFree radicals: monochlorinationCHHHHH2CH2CCH2CH2All hydrogens are __________©Dr. Kay SandbergCl2hν525MonochlorinationSection 4.19Methyl HCHHH CHHCCHHHHHFree radicals: higher alkane monochlorinationMethylene HCHHH CClHCCHHHHHCHHCl CHHCCHHHHH+Cl2+2-chlorobutane1-chlorobutaneStatisically 6:4hν©Dr. Kay Sandberg26Transition states of monochlorinationSection 4.19CHHH CHHCCHHHHHClδδ1ofree radicalCHHH CHHCCHHHHHδClδ2ofree radical____ stable2oH abstraction occurs~______________1oH abstractionH-abstraction coordinateGo©Dr. Kay Sandberg27Rates of halogenationSection 4.19Free radicals: higher alkane monochlorinationRelative rate of chlorination: 3o> 2o> 1oBromination shows _____selectivity__ __ 1 Relative rate of bromination: 3o> 2o> 1o___ ____ 1 When one of several possible isomers predominatesSince chlorination is not very _______selective, it is _________ to prepare monochloro-substitutedalkanes unless all hydrogens are equivalentBromination is _____selective for the formationof ___ alkyl bromides.©Dr. Kay Sandberg(constitutional)“What” slideProbability could “outweigh” reactivity________ FAR “outweighs” _________28E diagramReaction CoordinateR HClδδSection 4.18©Dr. Kay SandbergCl•Free radical


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