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NCSU CH 221 - 221cam8-L19-skeletal (1)

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11Objectives©Dr. Kay SandbergLast lectureThis lectureDifferent factors that affect the reactionSN2Mechanism comparisonHow to predict which of the 4 competingmechanisms will dominateE1 vsE2vsSN1 vsSN22TS of SN1 and SN2©Dr. Kay SandbergSN2 vsSN1Rate = k[nucleophile][alkyl halide]Rate = k[alkyl halide]HO BrCδ- δ-HHHBrCδ-H3CH3CH3Cδ+Major factor:how _________ is the “backdoor”Major factor:how stable is the forming ______________!!!!__________!!!!3Substrate trends for SN1 and SN2© Kay SandbergSection 8.10Nucleophilic Substitution (NS)_____ effectsR3CX R2CHX RCH2X CH3XIncreasing rate ofsubstitution by SN2 mechanism methyl3o___ crowded ____ crowded2o1oIncreasing rate ofsubstitution by SN1 mechanism ________ effects____ stablecarbocation____ stablecarbocationno ___no ___yesboth4SN1 example© Kay SandbergSection 8.9Nucleophilic Substitution (NS)C BrCH3H3CCH3CCH3H3CCH3Br+slowOHHCH3CH3CCH3OHHOHHfastfastCH3CH3CCH3OHHOHH+5SN1 E profileBr(CH3)3C(CH3)3C+,Br-Section 8.9SN1 mechanismOHHBr-(CH3)3CEReaction CoordinateOHHOHHBr-(CH3)3C(CH3)3CBr© Kay SandbergH2O(CH3)3COH2, Br-+(CH3)3COH, Br-+H3O,Eact, H2O6Relative rates of SNSection 8.10© Kay SandbergR—X + H2OR—OH + H—X formic acidCH3BrCH3CH2Br(CH3)2CHBr(CH3)3CBrH3CBrBrBrBrmethyl Br1oBr2oBr3oBrRelative rate<<1<<14327Stereochemistry of the SN2 mechanismCHBrSR© Kay Sandberg8CHBrδ−SRδ−© Kay Sandberg9CHBrδ−SRδ−© Kay Sandberg10CHBrδ−SRδ−© Kay Sandberg11SRCHBr© Kay Sandberg12Stereochemical consequence of SN2CHBr© Kay SandbergCHBrSRSRSN2 yields inversion of configuration313ClCHINaF14Stereochemistry of the SN1 mechanismCH3CH2CH2CCH3CH3CH2Br© Kay Sandberg15CH3CCH3CH2CH2CH3CH2Brδ+δ-© Kay Sandberg16CH3CCH3CH2CH2CH2CH3Br© Kay SandbergBrOHOHOH17CH3CCH3CH2CH2CH2CH3Br© Kay SandbergOHOH18CH3CCH2CH2CH3CH2CH3δ+© Kay SandbergOHδ+419CH3CCH2CH2CH3CH2CH3© Kay SandbergOH20Stereochemistry analysis of SN1CZYXXCZYLGchiralSection 8.11Stereochemistry of SN1 reactions© Kay SandberginversionLG:-Nu:-Nu:-retentionXCNuZYXCZYNu50%50%achiral21Incomplete racemizationCZYXXCZYLGchiralSection 8.11Stereochemistry of SN1 reactions© Kay SandbergLG:-inversionretentionXCNuZYXCZYNu> 50%< 50%_________racemizationNu:-Nu:-Ion pair: LG-carbocation22SN1 exampleBrSection 8.12Rearrangements in SN1 reactions© Kay SandbergBrCCCH3HH3CCH3HCCCH3HH3CCH3HOHslowCCCH3HH3CCH3Hfast-H+fastH2ORearrangements indicate _________ intermediates.Rearrangements are not observed in ___ reactions.23Mechanism determinationHBr+654321HOH54321HBr654321Br654321HBr6+© Kay SandbergMajor very minorminor24Reaction practiceH3CCH3BrDraw the 2 stereoisomeric substitution products you would expect to isolate from the hydrolysis of cis-1,4-dimethylcyclohexyl bromide? Section 8.11Stereochemistry of SN1 reactions© Kay SandbergH2O525SN vs E© Kay SandbergSection 8.14Nucleophilic Substitution (NS)vsElimination (E)CCHXY+CC+ HY + XCCHYX+NSECompeting reactions: can it be predicted which will dominate?δ+Electrons remain_Electrons go_Both result in a better negative charge “handler”___________ favored__________ favored26Strategy steps© Kay SandbergSection 8.14Nucleophilic Substitution (NS)vsElimination (E)CCHXY+CC+ HY + XCCHYX+NSEδ+δ+Step 1: classify the _________Step 2: classify the _________Competing reactions: How can we predict which will dominate?StrategyStrategyStep 3: classify the _________27Primary substrate© Kay SandbergHCHCBrHHHSN vsE1oSN1E1SN2E2Step 1: Classify substrateStep 1: Classify substrate28Tertiary substrate© Kay SandbergSN vsENuCCCCBrHHHHHHHHHBecause of sterically hindered “backdoor”tertiary alkyl halides cannot undergo ____SN1E1SN2E23oStep 1: Classify substrateStep 1: Classify substrate29Secondary substrate© Kay SandbergSN vsEHas some sterics, but not as ___ as 3o.Can form a carbocation, but not as ____ as 3o.SN1E1SN2E22oHCH3CCBrHHHThere are some nucleophiles that can get to it(those nucleophiles that have __________)NuStep 1: Classify substrateStep 1: Classify substrate30Summary chart for substrate classifcation© Kay SandbergSubstrate classificationSecondaryPrimary TertiarySN1E1SN2E2No-can-dodue to _______SN1E1SN2E2No-can-dodue to ____SN1E1SN2E2631Nucleophile chart© Kay SandbergNucleophile classificationWeakWeakGoodGood_______ ____________________*NH3is the exceptionwater & alcoholsBig atoms small atoms (2ndperiod)NuNu“long arm” “short arm”I1-, HS1-,Br1-HO1-,RO1-Step 2: Classify nucleophile32Good nucleophile characteristics© Kay SandbergNucleophile classificationGood Good nucleophilesnucleophilesPolarizable Non-polarizableAnionicBig atoms small atoms (2ndperiod)NuNu“long arm” “short arm”I1-, HS1-,Br1-HO1-,RO1-____ ____ BronstedBronstedbasesbases______ ______ BronstedBronstedbasesbasespKa’s of conj acids less than pKaof water (15.7)SN1E1SN2E2SN1E1SN2E233Weak nucleophile characteristics© Kay SandbergNucleophile classificationWeak Weak nucleophilesnucleophilesNeutralwater & alcoholsSN1E1SN2E2ConjugateConjugateacidacidBaseBaseppKKaaHO1-H2OBase adjectiveBase adjective15.7 strongI1-HI -10 very weakH2OH3O1+-1.7 weakHOR H2OR1+-2 weakWeak Weak BronstedBronstedbasesbases“short arms”OCCHHHHHH34Solvent chart© Kay SandbergOHStep 3: Classify solventStep 3: Classify solventSolvent classification______ ___________OHHH3CSCH3ONCHOH3CH3CCNH3CDMSODMFMeCN35Role of solvent animationCH3CH2CH2CCH3CH3CH2Br© Kay SandbergOHOHOHOHOH36CH3CH2CH2CCH3CH3CH2Brδ−δ+© Kay SandbergOHOHOHOHOH737CH3CH2CH2CCH3CH3CH2Br© Kay SandbergOHOHOHOHOH38CH3CH2CH2CCH3CH3CH2Br© Kay SandbergOHOHOHOHOHOHOHOHOH39Solvent summary© Kay SandbergOHStep 3: Classify solventStep 3: Classify solventSolvent classificationProtic AproticPolarOHHH3CSCH3ONCHOH3CH3CCNH3CSN1E1SN2E2SN1E1SN2E240Strong bases can bond to either protons or electrophilic carbons (but they have “short arms”)Primary substrate, non-polarizable Nu© Kay SandbergPredict the reaction:HCHCBrHHHSN vsE1o: bimolecularOCCCCHHHHHHHHHpKaHOC(CH3)3= 18SN1E1SN2E241Primary substrate, non-polarizable Nu© Kay SandbergPredict the reaction:HCHCBrHHHSN vsE1o: bimolecular_______________ base(“short arms” and bulky)OCCCCHHHHHHHHH42OCCHHHHHHCHCBrHHHPrimary substrate, non-polarizable Nu© Kay SandbergPredict the reaction:SN vsE1o: bimolecularStrong, unhindered base(“short arms”, but skinny)Strong bases can bond to either protonsor electrophilic carbonspKaHOCH2CH3= 16e-’s stay σ843Primary substrate, polarizable Nu© Kay SandbergPredict the reaction:HCHCBrHHHSN vsESCCHHHHH1o:


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