有機(jī)化學(xué)英文chapter9課件

OrganicChemistryWilliamH.BrownChristopherS.FooteBrentL.IversonOrganicChemistryWilliamH.BrNucleophilicSubstitutionand

-Elimination

Chapter8Chapter9NucleophilicSubstitutionandNucleophilicSubstitutionNucleophilicsubstitution:

anyreactioninwhichonenucleophilesubstitutesforanotheratatetravalentcarbonNucleophile:amoleculeorionthatdonatesapairofelectronstoanothermoleculeoriontoformanewcovalentbond;aLewisbaseNucleophilicSubstitutionNucleophilicSubstitutionSomenucleophilicsubstitutionreactionsNucleophilicSubstitutionSomeSolventsProticsolvent:asolventthatisahydrogenbonddonorthemostcommonproticsolventscontain-OHgroupsAproticsolvent:asolventthatcannotserveasahydrogenbonddonornowhereinthemoleculeisthereahydrogenbondedtoanatomofhighelectronegativitySolventsProticsolvent:asolvDielectricConstantSolventsareclassifiedaspolarandnonpolarthemostcommonmeasureofsolventpolarityisdielectricconstantDielectricconstant:ameasureofasolvent’sabilitytoinsulateoppositechargesfromoneanotherthegreaterthevalueofthedielectricconstantofasolvent,thesmallertheinteractionbetweenionsofoppositechargedissolvedinthatsolventpolarsolvent:dielectricconstant>15nonpolarsolvent:dielectricconstant<15DielectricConstantSolventsarProticSolventsProticSolventsAproticSolventsAproticSolventsMechanismsChemistsproposetwolimitingmechanismsfornucleophilicsubstitutionafundamentaldifferencebetweenthemisthetimingofbond-breakingandbond-formingstepsAtoneextreme,thetwoprocessestakeplacesimultaneously;designatedSN2S=substitutionN=nucleophilic2=bimolecular(twospeciesareinvolvedintherate-determiningstep)MechanismsChemistsproposetwoMechanism-SN2bothreactantsareinvolvedinthetransitionstateoftherate-determiningstepMechanism-SN2bothreactantsMechanism-SN2Mechanism-SN2Mechanism-SN1BondbreakingbetweencarbonandtheleavinggroupisentirelycompletedbeforebondformingwiththenucleophilebeginsThismechanismisdesignatedSN1whereS=substitutionN=nucleophilic1=unimolecular(onlyonespeciesisinvolvedintherate-determiningstep)Mechanism-SN1BondbreakingbMechanism-SN1Step1:ionizationoftheC-XbondgivesacarbocationintermediateMechanism-SN1Step1:ionizatMechanism-SN1Step2:reactionofthecarbocation(anelectrophile)withmethanol(anucleophile)givesanoxoniumionStep3:protontransfercompletesthereactionMechanism-SN1Step2:reactioMechanism-SN1Mechanism-SN1EvidenceofSNreactions1.WhatisrelationshipbetweentherateofanSNreactionand:thestructureofNu?thestructureofRLv?thestructureoftheleavinggroup?thesolvent?2.Whatisthestereochemicaloutcomeiftheleavinggroupisdisplacedfromachiralcenter?3.Underwhatconditionsareskeletalrearrangementsobserved?EvidenceofSNreactions1.WhaKineticsForanSN1reactionreactionoccursintwostepsthereactionleadingtoformationtransitionstateforthecarbocationintermediateinvolvesonlythehaloalkaneandnotthenucleophiletheresultisafirst-orderreactionKineticsForanSN1reactionKineticsForanSN2reaction,reactionoccursinonestepthereactionleadingtothetransitionstateinvolvesthehaloalkaneandthenucleophiletheresultisasecond-orderreaction;firstorderinhaloalkaneandfirstorderinnucleophileKineticsForanSN2reaction,NucleophilicityNucleophilicity:akineticpropertymeasuredbytherateatwhichaNucausesanucleophilicsubstitutionunderastandardizedsetofexperimentalconditionsBasicity:aequilibriumpropertymeasuredbythepositionofequilibriuminanacid-basereactionBecauseallnucleophilesarealsobases,westudycorrelationsbetweennucleophilicityandbasicityNucleophilicityNucleophilicityNucleophilicityNucleophilicityNucleophilicityRelativenucleophilicitiesofhalideionsinpolaraproticsolventsarequitedifferentfromthoseinpolarproticsolventsHowdoweaccountforthesedifferences?IncreasingNucleophilicitySolventPolaraproticPolarproticF-

<

Cl-

<

Br-

<

I-I-

<

Br-

<

Cl-

<

F-NucleophilicityRelativenucleoNucleophilicityAguidingprincipleisthefreerthenucleophile,thegreateritsnucleophilicityPolaraproticsolvents(e.g.,DMSO,acetone,acetonitrile,DMF)areveryeffectiveinsolvatingcations,butnotnearlysoeffectiveinsolvatinganions.becauseanionsareonlypoorlysolvated,theyparticipatereadilyinSNreactions,andnucleophilicityparallelsbasicity:F->Cl->Br->I-NucleophilicityAguidingprincNucleophilicityPolarproticsolvents(e.g.,water,methanol)anionsarehighlysolvatedbyhydrogenbondingwiththesolventthemoreconcentratedthenegativechargeoftheanion,themoretightlyitisheldinasolventshellthenucleophilemustbeatleastpartiallyremovedfromitssolventshelltoparticipateinSNreactionsbecauseF-ismosttightlysolvatedandI-theleast,nucleophilicityisI->Br->Cl->F-NucleophilicityPolarproticsoNucleophilicityGeneralizationwithinarowofthePeriodicTable,nucleophilicityincreasesfromlefttoright;thatis,itincreaseswithbasicityIncreasingNucleophilicityPeriodPeriod

2Period

3F-

<

OH-

<

NH2-

<

CH3-

Cl-

<

SH-

<

PH2-

NucleophilicityGeneralizationINucleophilicityGeneralizationinaseriesofreagentswiththesamenucleophilicatom,anionicreagentsarestrongernucleophilesthanneutralreagents;thistrendparallelsthebasicityofthenucleophileIncreasingNucleophilicityROH

<

RO-H2O

<

OH-NH3

<

NH2-RSH

<

RS-NucleophilicityGeneralizationINucleophilicityGeneralizationwhencomparinggroupsofreagentsinwhichthenucleophilicatomisthesame,thestrongerthebase,thegreaterthenucleophilicity

NucleophilicityGeneralizationStereochemistryForanSN1reactionatachiralcenter,theRandSenantiomersareformedinequalamounts,andtheproductisaracemicmixtureCHClCl-Cl-C+HClCH3OH-H+CH3OCHClClCOCH3HREnantiomerSEnantiomer+REnantiomerAracemicmixturePlanarcarbocation(achiral)StereochemistryForanSN1reacStereochemistryForSN1reactionsatachiralcenterexamplesofcompleteracemizationhavebeenobserved,butpartialracemizationwithaslightexcessofinversionismorecommonStereochemistryForSN1reactioStereochemistryForSN2reactionsatachiralcenter,thereisinversionofconfigurationatthechiralcenterExperimentofHughesandIngoldStereochemistryForSN2reactioHughes-IngoldExptthereactionis2ndorder,therefore,SN2therateofracemizationofenantiomericallypure2-iodooctaneistwicetherateofincorporationofI-131Hughes-IngoldExptthereactionStructureofRXSN1reactions:governedbyelectronicfactorstherelativestabilitiesofcarbocationintermediatesSN2reactions:governedbystericfactorstherelativeeaseofapproachofanucleophiletothereactionsiteGovernedbyelectronicfactorsGovernedbystericfactorsSN1SN2R3CXR2CHXRCH2XCH3XAccesstothesiteofreaction(3°)(methyl)(2°)(1°)CarbocationstabilityStructureofRXSN1reactions:Effectof-Branching1.2x10-51.2x10-3RelativeRateAlkylBromideb-Branches01231.04.1x10-1BrBrBrBrbbbbEffectof-Branching1.2x10-Effectof-BranchingBromoethane(Ethylbromide)1-Bromo-2,2-dimethylpropane(Neopentylbromide)Effectof-BranchingBromoethaAllylicHalidesAllyliccationsarestabilizedbyresonancedelocalizationofthepositivechargea1°allyliccationisaboutasstableasa2°alkylcation++Allylcation(ahybridoftwoequivalentcontributingstructures)CH2=CH-CH2CH2-CH=CH2AllylicHalidesAllyliccationsAllylicCations2°&3°allyliccationsareevenmorestableasalsoarebenzyliccationsaddingthesecarbocationstothosefromSection6.3AllylicCations2°&3°allylicTheLeavingGroupThemorestabletheanion,thebettertheleavingabilitythemoststableanionsaretheconjugatebasesofstrongacidsTheLeavingGroupThemorestabTheSolvent-SN2ThemostcommontypeofSN2reactioninvolvesanegativeNuandanegativeleavinggrouptheweakerthesolvationofNu,thelesstheenergyrequiredtoremoveitfromitssolvationshellandthegreatertherateofSN2TheSolvent-SN2ThemostcommTheSolvent-SN2BrN3-CH3CNCH3OHH2O(CH3)2S=O(CH3)2NCHON3Br-SolventTypepolaraproticpolarprotic50002800130071k(methanol)k(solvent)Solvent+solventSN2+TheSolvent-SN2BrN3-CH3CNCH3TheSolvent-SN1SN1reactionsinvolvecreationandseparationofunlikechargeinthetransitionstateoftherate-determiningstepRatedependsontheabilityofthesolventtokeepthesechargesseparatedandtosolvateboththeanionandthecationPolarproticsolvents(formicacid,water,methanol)arethemosteffectivesolventsforSN1reactionsTheSolvent-SN1SN1reactionsTheSolvent-SN1TheSolvent-SN1RearrangementsinSN1RearrangementsarecommoninSN1reactionsiftheinitialcarbocationcanrearrangetoamorestableoneRearrangementsinSN1RearrangeRearrangementsinSN1MechanismofacarbocationrearrangementRearrangementsinSN1MechanismSummaryofSN1&SN2SummaryofSN1&SN2SN1/SN2ProblemsProblem1:predictthemechanismforthisreaction,andthestereochemistryofeachproductProblem2:predictthemechanismofthisreactionSN1/SN2ProblemsProblem1:preSN1/SN2ProblemsProblem3:predictthemechanismofthisreactionandtheconfigurationofproductProblem4:predictthemechanismofthisreactionandtheconfigurationoftheproductSN1/SN2ProblemsProblem3:preSN1/SN2ProblemsProblem5:predictthemechanismofthisreactionSN1/SN2ProblemsProblem5:pre-Elimination-Elimination:

areactioninwhichamolecule,suchasHCl,HBr,HI,orHOH,issplitoutoreliminatedfromadjacentcarbons-Elimination-Elimination:a-EliminationZaitsevrule:themajorproductofa-eliminationisthemorestable(themorehighlysubstituted)alkene2-Methyl-2-butene(majorproduct)CH3CH2O-Na+CH3CH2OH2-Bromo-2-methylbutane2-Methyl-1-buteneBr++1-Methyl-cyclopentene(majorproduct)CH3O-Na+CH3OH1-Bromo-1-methyl-cyclopentaneBrMethylene-cyclopentane-EliminationZaitsevrule:the-EliminationTherearetwolimitingmechanismsfor-eliminationreactionsE1mechanism:

atoneextreme,breakingoftheR-LvbondtogiveacarbocationiscompletebeforereactionwithbasetobreaktheC-HbondonlyR-Lvisinvolvedintherate-determiningstepE2mechanism:

attheotherextreme,breakingoftheR-LvandC-HbondsisconcertedbothR-Lvandbaseareinvolvedintherate-determiningstep-EliminationTherearetwolimE1MechanismionizationofC-Lvgivesacarbocationintermediateprotontransferfromthecarbocationintermediatetothebase(inthiscase,thesolvent)givesthealkeneE1MechanismionizationofC-LvE1MechanismE1MechanismE2MechanismE2MechanismKineticsofE1andE2E1mechanismreactionoccursintwostepstherate-determiningstepiscarbocationformationthereactionis1storderinRLvandzeroorderisbaseE2mechanismreactionoccursinonestepreactionis2ndorder;firstorderinRLvand1storderinbaseKineticsofE1andE2E1mechanRegioselectivityofE1/E2E1:majorproductisthemorestablealkeneE2:withstrongbase,themajorproductisthemorestable(moresubstituted)alkenedoublebondcharacterishighlydevelopedinthetransitionstatethus,thetransitionstateoflowestenergyisthatleadingtothemoststable(themosthighlysubstituted)alkeneE2:withastrong,stericallyhinderedbasesuchastert-butoxide,themajorproductisoftenthelessstable(lesssubstituted)alkeneRegioselectivityofE1/E2E1:mStereoselectivityofE2E2ismostfavorable(lowestactivationenergy)whenHandLvareorientedantiandcoplanarStereoselectivityofE2E2ismStereochemistryofE2ConsiderE2ofthesestereoisomersStereochemistryofE2ConsiderStereochemistryofE2inthemorestablechairofthecisisomer,thelargerisopropylisequatorialandchlorineisaxialStereochemistryofE2inthemoStereochemistryofE2inthemorestablechairofthetransisomer,thereisnoHantiandcoplanarwithLv,butthereisoneinthelessstablechairStereochemistryofE2inthemoStereochemistryofE2itisonlythelessstablechairconformationofthisisomerthatcanundergoanE2reactionStereochemistryofE2itisonlStereochemistryofE2

Problem:

accountforthefactthatE2reactionofthemeso-dibromidegivesonlytheEalkeneStereochemistryofE2 Problem:SummaryofE2vsE1SummaryofE2vsE1SNvsEManynucleophilesarealsostrongbases(OH-andRO-)andSNandEreactionsoftencompeteTheratioofSN/EproductsdependsontherelativeratesofthetworeactionsSNvsEManynucleophilesareaSNvsESNvsESNvsE(cont’d)The

main

reaction

with

bases/nucleophiles

where

theR3CXpKa

of

the

conjugate

acid

is

11

or

less,

as

for

exampleI-

and

CH3COO-.R2CHXMain

reaction

with

strong

bases

such

as

HO-

and

RO-.Main

reactions

with

poor

nucleophiles/weak

bases.Themainreactionwithbases/nucleophileswhereE2SN2E2SN2reactionsoftertiaryhalidesareneverobservedSN1/E1SecondaryTertiarybecauseoftheextremecrowdingaroundthe3°carbon.SN1/E1Commoninreactionswithweaknucleophilesinpolarproticsolvents,suchaswater,methanol,andethanol.pKaoftheconjugateacidis11orgreater,asforexampleOH-andCH3CH2O-.SNvsE(cont’d)ThemainreactNeighboringGroupsInanSN2reaction,departureoftheleavinggroupisassistedbyNu;inanSN1reaction,itisnotThesetwotypesofreactionsaredistinguishedbytheirorderofreaction;SN2reactionsare2ndorder,andSN1reactionsare1storderButsomesubstitutionreactionsare1storderandyetinvolvetwosuccessiveSN2reactionsNeighboringGroupsInanSN2reMustardGasesMustardgasescontaineitherS-C-C-XorN-C-C-Xwhatisunusualaboutthemustardgasesisthattheyundergohydrolysissorapidlyinwater,averypoornucleophileBis(2-chloroethyl)sulfide(asulfurmustardgas)Bis(2-chloroethyl)methylamine(anitrogenmustardgas)ClSClClNClMustardGasesMustardgasesBisMustardGasesthereasonisneighboringgroupparticipationbytheadjacentheteroatomprotontransfertosolventcompletesthereactionMustardGasesthereasonisneiPhase-TransferCatalysisAsubstancethattransfersionsfromanaqueousphasetoanorganicphaseAneffectivephase-transfercatalystmusthavesufficienthydrophiliccharactertodissolveinwaterandformanionpairwiththeiontobetransportedhydrophobiccharactertodissolveintheorganicphaseandtransporttheionintoitThefollowingsaltisaneffectivephase-transfercatalystsforthetransportofanions(CH3CH2CH2CH2)4N+Cl-Tetrabutylammoniumchloride(Bu4N+Cl-)Phase-TransferCatalysisAsubsPhase-TransferCatalysisPhase-TransferCatalysis

NucleophilicSubstitutionand

-EliminationEndChapter9

NucleophilicSubstitutionandOrganicChemistryWilliamH.BrownChristopherS.FooteBrentL.IversonOrganicChemistryWilliamH.BrNucleophilicSubstitutionand

-Elimination

Chapter8Chapter9NucleophilicSubstitutionandNucleophilicSubstitutionNucleophilicsubstitution:

anyreactioninwhichonenucleophilesubstitutesforanotheratatetravalentcarbonNucleophile:amoleculeorionthatdonatesapairofelectronstoanothermoleculeoriontoformanewcovalentbond;aLewisbaseNucleophilicSubstitutionNucleophilicSubstitutionSomenucleophilicsubstitutionreactionsNucleophilicSubstitutionSomeSolventsProticsolvent:asolventthatisahydrogenbonddonorthemostcommonproticsolventscontain-OHgroupsAproticsolvent:asolventthatcannotserveasahydrogenbonddonornowhereinthemoleculeisthereahydrogenbondedtoanatomofhighelectronegativitySolventsProticsolvent:asolvDielectricConstantSolventsareclassifiedaspolarandnonpolarthemostcommonmeasureofsolventpolarityisdielectricconstantDielectricconstant:ameasureofasolvent’sabilitytoinsulateoppositechargesfromoneanotherthegreaterthevalueofthedielectricconstantofasolvent,thesmallertheinteractionbetweenionsofoppositechargedissolvedinthatsolventpolarsolvent:dielectricconstant>15nonpolarsolvent:dielectricconstant<15DielectricConstantSolventsarProticSolventsProticSolventsAproticSolventsAproticSolventsMechanismsChemistsproposetwolimitingmechanismsfornucleophilicsubstitutionafundamentaldifferencebetweenthemisthetimingofbond-breakingandbond-formingstepsAtoneextreme,thetwoprocessestakeplacesimultaneously;designatedSN2S=substitutionN=nucleophilic2=bimolecular(twospeciesareinvolvedintherate-determiningstep)MechanismsChemistsproposetwoMechanism-SN2bothreactantsareinvolvedinthetransitionstateoftherate-determiningstepMechanism-SN2bothreactantsMechanism-SN2Mechanism-SN2Mechanism-SN1BondbreakingbetweencarbonandtheleavinggroupisentirelycompletedbeforebondformingwiththenucleophilebeginsThismechanismisdesignatedSN1whereS=substitutionN=nucleophilic1=unimolecular(onlyonespeciesisinvolvedintherate-determiningstep)Mechanism-SN1BondbreakingbMechanism-SN1Step1:ionizationoftheC-XbondgivesacarbocationintermediateMechanism-SN1Step1:ionizatMechanism-SN1Step2:reactionofthecarbocation(anelectrophile)withmethanol(anucleophile)givesanoxoniumionStep3:protontransfercompletesthereactionMechanism-SN1Step2:reactioMechanism-SN1Mechanism-SN1EvidenceofSNreactions1.WhatisrelationshipbetweentherateofanSNreactionand:thestructureofNu?thestructureofRLv?thestructureoftheleavinggroup?thesolvent?2.Whatisthestereochemicaloutcomeiftheleavinggroupisdisplacedfromachiralcenter?3.Underwhatconditionsareskeletalrearrangementsobserved?EvidenceofSNreactions1.WhaKineticsForanSN1reactionreactionoccursintwostepsthereactionleadingtoformationtransitionstateforthecarbocationintermediateinvolvesonlythehaloalkaneandnotthenucleophiletheresultisafirst-orderreactionKineticsForanSN1reactionKineticsForanSN2reaction,reactionoccursinonestepthereactionleadingtothetransitionstateinvolvesthehaloalkaneandthenucleophiletheresultisasecond-orderreaction;firstorderinhaloalkaneandfirstorderinnucleophileKineticsForanSN2reaction,NucleophilicityNucleophilicity:akineticpropertymeasuredbytherateatwhichaNucausesanucleophilicsubstitutionunderastandardizedsetofexperimentalconditionsBasicity:aequilibriumpropertymeasuredbythepositionofequilibriuminanacid-basereactionBecauseallnucleophilesarealsobases,westudycorrelationsbetweennucleophilicityandbasicityNucleophilicityNucleophilicityNucleophilicityNucleophilicityNucleophilicityRelativenucleophilicitiesofhalideionsinpolaraproticsolventsarequitedifferentfromthoseinpolarproticsolventsHowdoweaccountforthesedifferences?IncreasingNucleophilicitySolventPolaraproticPolarproticF-

<

Cl-

<

Br-

<

I-I-

<

Br-

<

Cl-

<

F-NucleophilicityRelativenucleoNucleophilicityAguidingprincipleisthefreerthenucleophile,thegreateritsnucleophilicityPolaraproticsolvents(e.g.,DMSO,acetone,acetonitrile,DMF)areveryeffectiveinsolvatingcations,butnotnearlysoeffectiveinsolvatinganions.becauseanionsareonlypoorlysolvated,theyparticipatereadilyinSNreactions,andnucleophilicityparallelsbasicity:F->Cl->Br->I-NucleophilicityAguidingprincNucleophilicityPolarproticsolvents(e.g.,water,methanol)anionsarehighlysolvatedbyhydrogenbondingwiththesolventthemoreconcentratedthenegativechargeoftheanion,themoretightlyitisheldinasolventshellthenucleophilemustbeatleastpartiallyremovedfromitssolventshelltoparticipateinSNreactionsbecauseF-ismosttightlysolvatedandI-theleast,nucleophilicityisI->Br->Cl->F-NucleophilicityPolarproticsoNucleophilicityGeneralizationwithinarowofthePeriodicTable,nucleophilicityincreasesfromlefttoright;thatis,itincreaseswithbasicityIncreasingNucleophilicityPeriodPeriod

2Period

3F-

<

OH-

<

NH2-

<

CH3-

Cl-

<

SH-

<

PH2-

NucleophilicityGeneralizationINucleophilicityGeneralizationinaseriesofreagentswiththesamenucleophilicatom,anionicreagentsarestrongernucleophilesthanneutralreagents;thistrendparallelsthebasicityofthenucleophileIncreasingNucleophilicityROH

<

RO-H2O

<

OH-NH3

<

NH2-RSH

<

RS-NucleophilicityGeneralizationINucleophilicityGeneralizationwhencomparinggroupsofreagentsinwhichthenucleophilicatomisthesame,thestrongerthebase,thegreaterthenucleophilicity

NucleophilicityGeneralizationStereochemistryForanSN1reactionatachiralcenter,theRandSenantiomersareformedinequalamounts,andtheproductisaracemicmixtureCHClCl-Cl-C+HClCH3OH-H+CH3OCHClClCOCH3HREnantiomerSEnantiomer+REnantiomerAracemicmixturePlanarcarbocation(achiral)StereochemistryForanSN1reacStereochemistryForSN1reactionsatachiralcenterexamplesofcompleteracemizationhavebeenobserved,butpartialracemizationwithaslightexcessofinversionismorecommonStereochemistryForSN1reactioStereochemistryForSN2reactionsatachiralcenter,thereisinversionofconfigurationatthechiralcenterExperimentofHughesandIngoldStereochemistryForSN2reactioHughes-IngoldExptthereactionis2ndorder,therefore,SN2therateofracemizationofenantiomericallypure2-iodooctaneistwicetherateofincorporationofI-131Hughes-IngoldExptthereactionStructureofRXSN1reactions:governedbyelectronicfactorstherelativestabilitiesofcarbocationintermediatesSN2reactions:governedbystericfactorstherelativeeaseofapproachofanucleophiletothereactionsiteGovernedbyelectronicfactorsGovernedbystericfactorsSN1SN2R3CXR2CHXRCH2XCH3XAccesstothesiteofreaction(3°)(methyl)(2°)(1°)CarbocationstabilityStructureofRXSN1reactions:Effectof-Branching1.2x10-51.2x10-3RelativeRateAlkylBromideb-Branches01231.04.1x10-1BrBrBrBrbbbbEffectof-Branching1.2x10-Effectof-BranchingBromoethane(Ethylbromide)1-Bromo-2,2-dimethylpropane(Neopentylbromide)Effectof-BranchingBromoethaAllylicHalidesAllyliccationsarestabilizedbyresonancedelocalizationofthepositivechargea1°allyliccationisaboutasstableasa2°alkylcation++Allylcation(ahybridoftwoequivalentcontributingstructures)CH2=CH-CH2CH2-CH=CH2AllylicHalidesAllyliccationsAllylicCations2°&3°allyliccationsareevenmorestableasalsoarebenzyliccationsaddingthesecarbocationstothosefromSection6.3AllylicCations2°&3°allylicTheLeavingGroupThemorestabletheanion,thebettertheleavingabilitythemoststableanionsaretheconjugatebasesofstrongacidsTheLeavingGroupThemorestabTheSolvent-SN2ThemostcommontypeofSN2reactioninvolvesanegativeNuandanegativeleavinggrouptheweakerthesolvationofNu,thelesstheenergyrequiredtoremoveitfromitssolvationshellandthegreatertherateofSN2TheSolvent-SN2ThemostcommTheSolvent-SN2BrN3-CH3CNCH3OHH2O(CH3)2S=O(CH3)2NCHON3Br-SolventTypepolaraproticpolarprotic50002800130071k(methanol)k(solvent)Solvent+solventSN2+TheSolvent-SN2BrN3-CH3CNCH3TheSolvent-SN1SN1reactionsinvolvecreationandseparationofunlikechargeinthetransitionstateoftherate-determiningstepRatedependsontheabilityofthesolventtokeepthesechargesseparatedandtosolvateboththeanionandthecationPolarproticsolvents(formicacid,water,methanol)arethemosteffectivesolventsforSN1reactionsTheSolvent-SN1SN1reactionsTheSolvent-SN1TheSolvent-SN1RearrangementsinSN1RearrangementsarecommoninSN1reactionsiftheinitialcarbocationcanrearrangetoamorestableoneRearrangementsinSN1RearrangeRearrangementsinSN1MechanismofacarbocationrearrangementRearrangementsinSN1MechanismSummaryofSN1&SN2SummaryofSN1&SN2SN1/SN2ProblemsProblem1:predictthemechanismforthisreaction,andthestereochemistryofeachproductProblem2:predictthemechanismofthisreactionSN1/SN2ProblemsProblem1:preSN1/SN2ProblemsProblem3:predictthemechanismofthisreactionandtheconfigurationofproductProblem4:predictthemechanismofthisreactionandtheconfigurationoftheproductSN1/SN2ProblemsProblem3:preSN1/SN2ProblemsProblem5:predictthemechanismofthisreactionSN1/SN2ProblemsProblem5:pre-Elimination-Elimination:

areactioninwhichamolecule,suchasHCl,HBr,HI,orHOH,issplitoutoreliminatedfromadjacentcarbons-Elimination-Elimination:a-EliminationZaitsevrule:themajorproductofa-eliminationisthemorestable(themorehighlysubstituted)alkene2-Methyl-2-butene(majorproduct)CH3CH2O-Na+CH3CH2OH2-Bromo-2-methylbutane2-Methyl-1-buteneBr++1-Methyl