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3/14/18
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https://labs.chem.ucsb.edu/zakarian/armen/courses.html
CHEM 109A Organic Chemistry
Chapter 9
Substitution Reactions: SN2 and SN1
Elimination Reactions: E2 and E1
Final exam is cumulative
Chapter 1: 1.1 – 1.16 Chapter 2: 2.1 – 2.10, 2.12 (Lewis acids) Chapter 3: 3.1 – 3.15
Chapter 4: 4.1 – 4.15 Chapter 5: 5.1 – 5.12 (5.13, 5.14) Chapter 6: 6.1 – 6.13, 6.16 (6.14, 6.15) Chapter 7: 7.1 – 7.12 Chapter 8: 8.1 – 8.7, 8.9, 8.11, 8.16 – 8.18 (resonance effect, on pKa,
carbocations, intro to aromaticity) Chapter 9: 9.1 –
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Midterm 3 information
Average: 59.5 Standard deviation: 16.1 Max 95 Min 18.5
The Families of Group II
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1. Boththealkylhalideandthenucleophileareinthetransitionstateoftherate-limitingstep.
2. Therelativerate:primaryalkylhalide>secondaryalkylhalide>tertiaryalkylhalide
3. Theconfigurationoftheproductisinvertedcomparedtotheconfigurationofthereactingchiralalkylhalide.
Summary of the Experimental Evidence for the Mechanism of an SN2 Reaction
back-sideattack
The Mechanism
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Why Bimolecular?
Why Do Methyl Halides React the Fastest and Tertiary the Slowest?
sterichindrance
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Why the Configuration of the Product is Inverted
SN2 reaction: Nucleophile, The Carbon Center, and the Leaving Group
nucleophile strong or weak? carbon center
(electrophilic) hindered or not?
leaving group good or bad?
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TheRateofanSN2ReactionisAffectedbytheLeavingGroup
Leaving Group Effect: TheWeakestBaseistheBest
Nucleophile effect: Bases and Nucleophiles
Abasesharesitslonepairwithaproton.
Anucleophilesharesitslonepairwithanatomotherthanaproton.
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1. Nucleophile Strength: stronger base, better Nu
Anegativelychargedatomisastrongerbaseandabetternucleophilethanthesameatomthatisneutral.
Anegativelychargedatomisastrongerbaseandabetternucleophilethanthesameatomthatisneutral.
1. Nucleophile Strength: stronger base, better Nu
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Anegativelychargedatomisastrongerbaseandabetternucleophilethanthesameatomthatisneutral.
1. Nucleophile Strength: stronger base, better Nu
Anegativelychargedatomisastrongerbaseandabetternucleophilethanthesameatomthatisneutral.
1. Nucleophile Strength: stronger base, better Nu
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2. Nucleophile Strength: same row elements
Ifatomsareinthesamerow,thestrongestbaseisthebestnucleophile.
nucleophilicity goes up
same for NH3 > H2O
3. Nucleophile Strength: Polarizability
Thelargertheatom,themorepolarizableitis(canmovemorefreelytowardapositivecharge).
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3. Nucleophile Strength: Polarizability
Thelargertheatom,themorepolarizableitis(canmovemorefreelytowardapositivecharge).
polar protic
solvents
CH3OH
3. Nucleophile Strength: Polarizability
Thelargertheatom,themorepolarizableitis(canmovemorefreelytowardapositivecharge).
polar protic
solvents
polar aprotic solvents
DMF, DMSO
CH3OH
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Solvents
1.nonpolarsolvents:hexane,benzene–normallynotusedinsubstitutionreactions(Negativelychargedspeciescannotdissolveinnonpolarsolvents).2.proticpolarsolvents:haveanOHgroup(H2O,CH3OH)3.aproticpolarsolvents:polar,butnoOHgroup
H N
O
DMF
SO
DMSO
Why Do Protic Polar Solvents Make the Strongest Bases the Poorest Nucleophiles?
I−isthebestnucleophileinaproticpolar
solvent.
polar protic
solvents
CH3OH
excellent solvation poor solvation
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Aprotic Polar Solvents
Theycansolvateacation(+)betterthantheycansolvateananion(−).
polar aprotic solvents
DMF, DMSO
Aprotic Polar Solvents
Rank the following from best to poorest nucleophile in an aqueous solution:
O HO CH3OHO
OCH3S
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Aprotic Polar Solvents
Rank the following from best to poorest nucleophile in an aqueous solution:
O HO CH3OHO
OCH3S
how about in DMF?
4.StericHindranceDecreasesNucleophilicity
mostly when comparing same-class nucleophiles – here, alkoxides
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SN2ReactionsCanBeUsedtoMakeaVarietyofCompounds
Thereactionsareirreversiblebecauseaweakbasecannotdisplaceastrongbase.
SynthesizinganAmine
K2CO3makesthesolutionbasicsothattheaminewillexistinitsbasicform.
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AnotherSubstitutionReaction:SN1
Thereactionissurprisinglyfast,soitmustbetakingplacebyadifferentmechanism.
Thereactionissurprisinglyfast,soitmustbetakingplacebyadifferentmechanism.
an SN1 reaction: only the alkyl halide is in the transition state of the rate-limiting step.
AnotherSubstitutionReaction:SN1
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Solvolysis Reaction
AlotofSN1reactionsaresolvolysisreactions;thesolventisthenucleophile.
The Mechanism
Theleavinggroupdepartsbeforethenucleophileapproaches.Theslowstepiscarbocationformation.
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The Reaction Coordinate Diagram
TheProductisaPairofEnantiomers
Ifthehalogenisbondedtoanasymmetriccenter,theproductwillbeapairofenantiomers.
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Why a Pair of Enantiomers?
MostSN1ReactionsLeadtoPartialRacemization
Generallymoreinvertedproductisformed,becausethefrontsideispartiallyblocked.
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LeavingGroupEffectonSN1reactions:TheWeakestBaseistheBestLeavingGroup
Comparing SN2 and SN1 Reactions
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SN1 and SN2 reactions: brief summary
primaryalkylhalidesandmethylhalidesonlySN2Theycannotformcarbocations.
secondaryalkylhalidesonlySN2Carbocationformationistooslowtomakeupforthelargeconcentrationofthenucleophileinasolvolysisreaction.
tertiaryalkylhalidesonlySN1Theycannotundergoback-sideattack.
AlkylHalidesUndergoSubstitutionandEliminationReactions
Inaneliminationreaction,ahalogenisremovedfromonecarbonandahydrogenisremovedfromanadjacentcarbon.
Adoublebondisformedbetweenthetwocarbons
fromwhichtheatomswereremoved.
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An E2 Reaction
Thealkylhalideandthebaseareinthetransitionstateoftherate-limitingstep.
The Mechanism for an E2 Reaction
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TheHalogenComesofftheAlphaCarbon;theHydrogenComesofftheBetaCarbon
dehydrohalogenation
Alkene-LikeTransitionState
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AnE2ReactionisRegioselective
Themajorproductisthemoststablealkene.Themoststablealkeneisgenerallyobtainedbyremovingahydrogenfromthebetacarbonthatisbondedtothefewesthydrogens.
Zaitsev rule: more substituted alkene is the major product
TheMoreStableAlkeneHastheMoreStableTransitionState
Themajorproductisthemorestablealkene.
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MoreE2Reactions
Zaitsev’sruleleadstothemoresubstitutedalkene.Ifthereisconjugation,themoresubstitutedalkenemightnotbethemorestablealkene.
LimitationsofZaitsev’sRule
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Ifthealkylhalideandthebasearebothbulky,thebasewillremovethemoreaccessiblehydrogen,
whichwillleadtoformationofthelessstableproduct.
LimitationsofZaitsev’sRule
The Steric Properties of the Base Affect the Product Distribution
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The Steric Properties of the Base Affect the Product Distribution
The Steric Properties of the Base Affect the Product Distribution