unimolecular nucleophilic substitution s n 1
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Unimolecular Nucleophilic Substitution S N 1. A question. Tertiary alkyl halides are very unreactive in substitutions that proceed by the S N 2 mechanism. Do they undergo nucleophilic substitution at all?. - PowerPoint PPT PresentationTRANSCRIPT
Unimolecular Nucleophilic SubstitutionUnimolecular Nucleophilic SubstitutionSSNN11
Tertiary alkyl halides are very unreactive in Tertiary alkyl halides are very unreactive in substitutions that proceed by the Ssubstitutions that proceed by the SNN2 mechanism.2 mechanism.Do they undergo nucleophilic substitution at all?Do they undergo nucleophilic substitution at all?
Yes. But by a mechanism different from SYes. But by a mechanism different from SNN2. 2. The most common examples are seen in The most common examples are seen in solvolysis reactions.solvolysis reactions.
A question...A question...
The Sn1 ReactionThe Sn1 Reaction
++
++
HH BrBr........ ::
OO:: ::
HH
HH
CCCHCH33
CHCH33
CHCH33
BrBr
CC OHOH........
....
.... ::
CHCH33
CHCH33
CHCH33
Example of a solvolysis. Hydrolysis of Example of a solvolysis. Hydrolysis of terttert-butyl bromide.-butyl bromide.
rate = rate = k k [alkyl halide][alkyl halide]First-order kinetics implies a First-order kinetics implies a unimolecularunimolecular
rate-determining step.rate-determining step.
Mechanism is called SMechanism is called SNN1, which stands for1, which stands forsubstitution nucleophilic unimolecular (1)substitution nucleophilic unimolecular (1)
Kinetics and MechanismKinetics and Mechanism
++ BrBr––
....:: ::
....
unimolecular unimolecular slowslow
CCCHCH33
CHCH33
CHCH33
BrBr........ ::
CC
HH33CC CHCH33
CHCH33
++
MechanismMechanism
bimolecular bimolecular fastfast
CC
HH33CC CHCH33
CHCH33
++
OO:: ::
HH
HH
CC++OO ::
HH
HHCHCH33
CHCH33
CHCH33
MechanismMechanism
proton transfer
ROHROH22
++
carbocation formation
RR++
ROHROH
carbocation capture
RXRX
• First order kinetics: rate = k [RX]First order kinetics: rate = k [RX]unimolecular rate-determining stepunimolecular rate-determining step
• Carbocation intermediateCarbocation intermediaterate follows carbocation stabilityrate follows carbocation stabilityrearrangements are observedrearrangements are observed
• Reaction is not stereospecific:Reaction is not stereospecific:racemization in reactions of racemization in reactions of optically active alkyl halidesoptically active alkyl halides
Characteristics of the SCharacteristics of the SNN1 mechanism1 mechanism
Reaction Coordinate Diagram for an SN1 Reaction
Carbocation Stability and SCarbocation Stability and SNN1 Reaction 1 Reaction RatesRates
The rate of nucleophilic substitutionThe rate of nucleophilic substitutionby the Sby the SNN1 mechanism is governed1 mechanism is governed
by electronic effects.by electronic effects.
Carbocation formation is rate-determining.Carbocation formation is rate-determining.The more stable the carbocation, the fasterThe more stable the carbocation, the faster
its rate of formation, and the greater the its rate of formation, and the greater the rate of unimolecular nucleophilic substitution.rate of unimolecular nucleophilic substitution.
Electronic Effects Govern SElectronic Effects Govern SNN1 Rates1 Rates
RBr solvolysis in aqueous formic acidRBr solvolysis in aqueous formic acid
Alkyl bromideAlkyl bromide ClassClass Relative rateRelative rate
CHCH33BrBr MethylMethyl 11
CHCH33CHCH22BrBr PrimaryPrimary 22
(CH(CH33))22CHBrCHBr SecondarySecondary 4343
(CH(CH33))33CBrCBr TertiaryTertiary 100,000,000100,000,000
Reactivity toward substitution by the SReactivity toward substitution by the SNN1 1 mechanismmechanism
CHCH33BrBr
CHCH33CHCH22BrBr
(CH(CH33))22CHBrCHBr
(CH(CH33))33CBrCBr
Decreasing SDecreasing SNN1 Reactivity1 Reactivity
Stereochemistry of SStereochemistry of SNN1 Reactions1 Reactions
Nucleophilic substitutions that exhibitNucleophilic substitutions that exhibitfirst-order kinetic behavior are first-order kinetic behavior are
not stereospecific.not stereospecific.
GeneralizationGeneralization
RR-(–)-2-Bromooctane-(–)-2-Bromooctane
HH
CC
CHCH33
BrBr
CHCH33(CH(CH22))55
((RR)-(–)-2-Octanol (17%))-(–)-2-Octanol (17%)
HH
CC
CHCH33
OHOH
CHCH33(CH(CH22))55
CC
HH CHCH33
HOHO
(CH(CH22))55CHCH33
((SS)-(+)-2-Octanol (83%))-(+)-2-Octanol (83%)
HH22OO
Stereochemistry of an SStereochemistry of an SNN1 Reaction1 Reaction
The carbocation reaction intermediate leads to the formation of two stereoisomeric products
Ionization stepIonization stepgives carbocation; threegives carbocation; threebonds to stereogenicbonds to stereogeniccenter become coplanarcenter become coplanar
++
Step 1Step 1
Leaving group shieldsLeaving group shieldsone face of carbocation;one face of carbocation;nucleophile attacks nucleophile attacks faster at opposite face.faster at opposite face.
++
Step 2Step 2
More than 50%More than 50% Less than 50%Less than 50%
++
Carbocation RearrangementsCarbocation Rearrangementsin Sin SNN1 Reactions1 Reactions
carbocations are intermediatescarbocations are intermediatesin Sin SNN1 reactions, rearrangements1 reactions, rearrangements
are possible.are possible.
Because...Because...
CarbocationsCarbocations
Carbocations rearrange to Carbocations rearrange to the more stable form(s)the more stable form(s)
Carbocation Rearrangement Carbocation Rearrangement MechanismMechanism
What is the starting carbocation: 1What is the starting carbocation: 1oo, 2, 2oo or 3 or 3oo??What is the rearranged carbocation: 1What is the rearranged carbocation: 1oo, 2, 2oo or 3 or 3oo??
Carbocation RearrangementCarbocation Rearrangement
CHCH33 CC
HH
CHCHCHCH33
BrBr
CHCH33 HH22OOCHCH33 CC
OHOH
CHCH22CHCH33
CHCH33
(93%)(93%)
ExampleExample
CHCH33 CC
HH
CHCHCHCH33
CHCH33
CHCH33 CC CHCHCHCH33
CHCH33
CHCH33 CC
HH
CHCHCHCH33
BrBr
CHCH33
HH22OO
CHCH33 CC
OHOH
CHCH22CHCH33
CHCH33
(93%)(93%)
++HH
++
ExampleExample
Mechanism SummaryMechanism SummarySSNN1 and S1 and SNN22
When...When...
primaryprimary alkyl halides undergo nucleophilic alkyl halides undergo nucleophilic substitution, they substitution, they alwaysalways react by the react by the SSNN2 2 mechanismmechanism
tertiarytertiary alkyl halides undergo nucleophilic alkyl halides undergo nucleophilic substitution, they substitution, they alwaysalways react by the react by the SSNN1 1 mechanismmechanism
secondarysecondary alkyl halides undergo nucleophilic alkyl halides undergo nucleophilic substitution, they react by thesubstitution, they react by the
SSNN11 mechanism in the presence of a weak mechanism in the presence of a weak nucleophile (solvolysis)nucleophile (solvolysis)SSNN22 mechanism in the presence of a good mechanism in the presence of a good nucleophilenucleophile
Putting things togetherPutting things together