Sn reaction
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Transcript of Sn reaction
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ReactionsReactions
R X R+ + X-
Ionic reactions:Ionic reactions:
Bond breaking and bond Bond breaking and bond making take place in a making take place in a heterolytic fashionheterolytic fashion
R X R. + X.
Bond breaking and bond Bond breaking and bond making take place in amaking take place in ahomolytic fashionhomolytic fashion
Radical reactions:Radical reactions:
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Ionic Reactions :
1) substitution :
3) addition :
Y R X R Y X+ +
R = aliphatic as well as aromatic
X
R2
R1
Y
X
R2 Y
R1H XH
R2 Y
R1
+
X = O , NH
2) elimination :
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Nucleophilic Nucleophilic SubstitutionSubstitution
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NucleophilicNucleophilic SubstitutionSubstitution
Y R X R Y X+ +
R = aliphatic as well as aromatic
Nucleophile + Substrate Product + Leaving group
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NucleophilicNucleophilic SubstitutionSubstitution
SN1 SN2
S: Substitution N: Nucleophilic1: unimolecular
S: SubstitutionN: Nucleophilic2: Bimolecular
Y C X C Y X++ C+
leaving group goes first and nucleophile comes later
Y C X C Y X++
nucleophile attacks and leaving group goes simultaneously
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SN2
reaction and mechanism kinetics stereochemistry substrate structure nucleophiles leaving groups solvents
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SN2 reaction and mechanism
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9
Synthetic Utility of the SN2 ReactionA variety of functional groups can be prepared employing a good
nucleophile and an electrophile with a good leaving group:
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SN2 reaction : Kinetics
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12
The Rate Law of an SN2 Reaction
Obtained experimentally:
Rate law includes both the alkyl halide and the nucleophile, a second-order process
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SN2 reaction : Stereochemistry
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SSNN2 MECHANISM2 MECHANISM
O
C
R
HCH3
H O:....attacks
back lobe
nucleophilic attack
(R)-config
(S)-config
C
R
HCH3
Br: :....H :..
..
INVERSION
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Stereochemistry of SStereochemistry of SNN22Walden inversionWalden inversion
=>
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C
R
HCH3
Br:C
R
HCH3
HO :
C
HCH3
R
BrHO
H O:....
activated complexis trigonal planar (sp2 )
(R)-configuration(S)-configuration
configurationis inverted
Ea
HO C B
partial bonding
2pTHE INVERSIONTHE INVERSIONPROCESSPROCESS
sp3
sp3
sp2
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SN2 reaction : substrate substrate structurestructure
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Less bulky
Should stabilize the transition state
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SN2 Reaction: substrate structure
KI in Acetone at 25°krel
150
1
0.008
unreactive!
CH3 Br
CH3 CH2 Br
CH3 CH Br
CH3
CH3 C Br
CH3
CH3
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Relative rates of SN2 reactions of alkyl chlorides with the iodide ion
Alkyl chloride relative rate
Me Cl 200
Cl0.02
MeO Cl
920
Cl
O
1,00,000
The rates are given with respect to n-BuCl
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RHal
ONu-
RNu
O
R = alkyl, aryl, OR'
Me ClCl
O
H
H
Cl* of the C=O
* of of the C-Cl
Nu
only SN2, no SN1
Relative rates of SN2 reactions with iodide ion
1:500
Nu
Cl
H
H
++* of the C=O
C=O group stabilizes the T.S. by Overlap of its * orbital with full P-orbital of the C-atom under attack
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SN2 reaction : Effect of Nucleophile Effect of Nucleophile
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The nucleophilicity may be correlated with the availability of the electron pairs and the ease with which it is donated
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Trends in Nuc. Trends in Nuc. StrengthStrength
Increases down Periodic Table, as Increases down Periodic Table, as size and polarizability increase: size and polarizability increase: II-- > Br > Br-- > Cl > Cl-- >F >F- -
Of a conjugate acid-base pair, the Of a conjugate acid-base pair, the base is stronger: base is stronger: OHOH-- > H > H22O, NHO, NH22
-- > NH > NH33
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Polarizability and nucleophilicity - increased polarizability makes for a better nucleophile
I- > Br- > Cl- >F-I- > Br- > Cl- >F-
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Effect of Nucleophile :
The nucleophilicity may be correlated to its basicity as both involve the availability of the electron pairs and the ease with which it is donated
CH3O H3C I
CH3OH H3C I
CH3OCH3
CH3OCH3 HI
I+ +rapid
very
slow++
Nucleophilicity of CH3O CH3OH> A negatively charged nucleophile is always stronger than its conjugate acid.
Stronger base weaker basebetter nucleophile poorer nucleophile
HO > H2O
CH3O > CH3OH
H2N > NH3
The direct relationship between basicity and nucleophilicity is maintained if the reaction occurs in the gas phase
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SSNN2: Nucleophilic Strength2: Nucleophilic Strength Stronger nucleophiles react faster.Stronger nucleophiles react faster. Strong bases are strong nucleophiles, Strong bases are strong nucleophiles,
but not all strong nucleophiles are but not all strong nucleophiles are basic.basic.
=>
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SN2 reaction : Effect of leaving Effect of leaving groupgroup
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31
•A good leaving group needs to be a Stable anions that are weak bases which can delocalize charge
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The Leaving Group
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Sulfonate Leaving Groups
S CH3OR
O
O
S BrOR
O
O
para-Toluenesulfonate Tosylate
para-Bromobenzenesulfonate Brosylate
R OTs
R OBs
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SN2 reaction : Effect of Solvent Effect of Solvent
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Solvent
Polar Solvent nonpolar Solvent
Polar protic Solvent Polar aprotic Solvent
SN2 reaction prefers polar aprotic solvent
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Solvent Effects (1)Solvent Effects (1)Polar protic solvents (O-H or N-H) reduce Polar protic solvents (O-H or N-H) reduce
the strength of the nucleophile. the strength of the nucleophile. Hydrogen bonds must be broken before Hydrogen bonds must be broken before nucleophile can attack the carbon.nucleophile can attack the carbon.
=>
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Solvent Effects (2)Solvent Effects (2) Polar aprotic solvents (no O-H or N-H) do Polar aprotic solvents (no O-H or N-H) do
not form hydrogen bonds with nucleophilenot form hydrogen bonds with nucleophile Examples: Examples:
CH3 C Nacetonitrile C
O
H3C CH3
acetone =>
dimethylformamide (DMF)
CH
O
NCH3
CH3
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Me I + Me N3N3-Na+ + NaI
solvent
Rate in MeOH ( 33) 1 DMF ( 37) 4.5X104 DMF: HCONMe2
Marked effect on the rate of SN2 reaction, when that transferred from polar protic solvent to polar aprotic solvent.
• In MeOH both Na+ and N3- are solvated.
• In DMF only Na+ is solvated, but not N3-.
• So, unsolvated N3- is a much more powerful
nucleophile
DMSO ( 46) 1X109 DMSO: Me2SO
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SN1
reaction and mechanism kinetics stereochemistry substrate structure nucleophiles leaving groups solvents
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SN1: reaction and mechanism
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Solvolysis of tert-Butyl Bromide
+ H2O +
+ other products
acetoneCH3 C CH3
CH3
Br
CH3 C CH3
CH3
OH
H Br
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SN1: Mechanism
1935: Hughes & Ingold
carbocation
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SN1 reaction : Kinetics
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SN1 Reaction: kinetics
The reactions follows first order (unimolecular) kinetics
Rate = k [R-Br]1
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R LG R R NuNu
LGslow
+fast
Rate = K[R-LG]
Reaction profile for SN1 reaction
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SN1 reaction : Stereochemistry
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SN1: Hydrolysis of t-butyl chloride by base proceed according to
Rate = k1[t-BuCl] or independent of [OH-]
1. Halide undergoes slow ionization to yield the ion pair R+ and Cl- followed by first attack by –OH or solvent or nuleophile.
2. The energy necessary to effect the initial ionization is largely recovered from the energy evolved through solvation of the resultant ion-pair.
ClMe
MeMe
Me
MeMe+
slow
sp3 sp2
fast
fast
OHMe
MeMe
HOMe
MeMe
-OH
-OH
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SN1 reaction : substrate substrate structurestructure
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Stability of carbocation
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C
tertiary
C
tertiary secondary
>
primary
+
carbocation (very stable)
secondarycarbocation
+
CH3
>Br
CH3
CH3
CH3 CH
CH3
Br
CH3 CH
CH3
CH3CH3-CH2-Br
CH3
+
primarycarboc
CH3
carbocation(unstable)
CH3
+
CH3-Br>
very unstable carbocation
three methyl groups
two methyl groups
one methyl group
no methyl groups
CH3 CH2
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SN1 Reaction: substrate structure
krel
no reaction
1.00
11.6
61.2 x 10
CH3 Br
CH3 CH2 Br
CH3 CH Br
CH3
CH3 C Br
CH3
CH3
Solvolysis in water at 50°C
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SN1 reaction : Effect of Nucleophile Effect of Nucleophile
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Nucleophiles in SN1
Since nucleophilic addition occurs after formation of carbocation, reaction rate is not normally affected by nature or concentration of nucleophile (weak nucleophile)
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A protic solvent acts as both a solvent and nucleophile in SN1
reactions - solvolysis:Water
HO
H
Methanol HO
CH3
HO
CH2CH3
HOEtEthanol
HO
C
O
CH3Acetic acid HOAc
HO
C
O
HFormic acid
HOMe
abbreviations
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SN1 reaction : Effect of leaving Effect of leaving groupgroup
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Leaving groups
Leaving groups are the same as in SN2 reactions:
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SN1 reaction : Effect of Solvent Effect of Solvent
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Dielectric constant (, at 25 C): H2O 79
EtOH 25
Solvent effect
Increase in dielectric constant and/or ion-solvating ability result in a marked increase in reaction rate
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Polar protic solvents favoring the SN1 reaction since it stabilizes carbocation of the transition state
Protic solvents disfavor the SN2 reaction by stabilizing the ground state
Transfer from polar, protic to polar, aprotic solvents can change the reaction mode from SN1 SN2
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Things to remember
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SSNN22 or or SSNN1?1? Primary or methylPrimary or methyl Strong Strong
nucleophilenucleophile Polar aprotic Polar aprotic
solventsolvent Rate = Rate = k k [halide] [halide]
[Nuc][Nuc] Inversion Inversion No No
rearrangementsrearrangements
TertiaryTertiary Weak nucleophile Weak nucleophile
(may also be (may also be solvent)solvent)
Polar protic solvent, Polar protic solvent, silver saltssilver salts
Rate = Rate = k k [halide][halide] Racemization Racemization Rearranged products Rearranged products
=>=>
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Competition reaction in SN1
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RearrangementsRearrangements
Carbocations can rearrange to form a Carbocations can rearrange to form a more stable carbocation.more stable carbocation.
Hydride shift: HHydride shift: H-- on adjacent carbon on adjacent carbon bonds with Cbonds with C++..
Methyl shift: CHMethyl shift: CH33-- moves from adjacent moves from adjacent
carbon carbon ifif no H’s are available. no H’s are available.
=> =>
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Hydride ShiftHydride Shift
CH3 C
Br
H
C
H
CH3
CH3CH3 C
H
C
H
CH3
CH3
CH3 C
H
C
H
CH3
CH3CH3 C
H
C
CH3
CH3
H
CH3 C
H
C
CH3
CH3
HNuc
CH3 C
H
C
CH3
CH3
H Nuc
=>
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Methyl ShiftMethyl Shift
CH3 C
Br
H
C
CH3
CH3
CH3CH3 C
H
C
CH3
CH3
CH3
CH3 C
H
C
CH3
CH3
CH3CH3 C
H
C
CH3
CH3
CH3
CH3 C
H
C
CH3
CH3
CH3
NucCH3 C
H
C
CH3
CH3
CH3 Nuc
=>
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Important substrates……..
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Allylic and Benzylic compounds
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Allylic and Benzylic compounds
Allylic and benzylic compounds are especially reactive in SN1 reactions.Even though they are primary substrates, they are more reactive most other halides! They form resonance stabilized carbocations.
CH2-Br CH2=CH-CH2-Br
benzyl bromide allyl bromide
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CH2 CH2 etc+
+
++
etcCH2 CH2
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Allylic and Benzylic compounds
Allylic and benzylic compounds are especially reactive in SN2 reactions.They are more reactive than typical primary compounds!
CH2-Br CH2=CH-CH2-Br
benzyl bromide allyl bromide
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For SN2: stabilisation of TS by conjugation with allylic -bond
Br
Nu
H
Nu
Br
H
(-)
(-)
Nu
++
T.S.
Nu
Br
H
H
++
T.S.
(-)
(-)
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BENZYL ( GOOD FOR SBENZYL ( GOOD FOR SNN1 ) 1 ) IS ALSO A GOOD SIS ALSO A GOOD SNN2 SUBSTRATE2 SUBSTRATE
CH2 Br + NaI CH2 I + NaBr
primary, but faster than other primary
overlap inthe activatedcomplexlowers theactivationenergy
I
Br
H
H
criticaloverlap
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Vinyl and aryl halides
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Vinyl and aryl halides do not undergo SN1 because:
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Vinyl and aryl halides do not undergo SN2 because:
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Cyclic systems
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-- You cannot form a carbocation at a bridgehead position.
You can’t have p orbitals on a bridgehead position
rigid bicyclic molecule.
+
+
cannot becomeplanar
X
“steric rigidity”
Br +
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Problems :
BrBr Br
1 10-6 10-14
Explain?
2) Rate of solvolysis in EtOH :
CH3CH2 Br Me2HCCH2 Br Me3CCH2 BrCH3CH2CH2 Br
relative rate 1 2.8X10-1 3.0X10-2 24.2X10-6
1) SN2 reaction by EtO- in EtOH:
Explain ?
Br Br
1 10-23
Explain ?
1-bromotriptycene
Rigid structure, cation empty p-orbitals are at right angles to orbitals of Ph
cc at bridge head, less stable, difficult to attain planarity due to rigidity
A)
A)B)