Chapter 10
Nucleophilic
Substitution:
The SN1 and SN2
Mechanisms
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Assignment for Chapter 10
• We will cover all the sections in this
chapter, except Sections 10.12 and
10.13
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Problem Assignment for
Chapter 10
In-Text Problems
1 - 15
17, 18 19 (SN2 react)
20 (SN1 reaction), 21, 22, 24, 25, 26,
27, 28
End-of-Chapter Problems
30 - 37
39 - 42 44 – 49
51 - 55
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Sect. 10.1: Nomenclature of
alkyl halides -- common
names
methylene chloride
chloroform
carbon tetrachloride
CH2Cl2
CHCl3
CCl4
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More common and IUPAC names
isopropyl chloride
sec-butyl chloride
isobutyl chloride
tert-butyl chloride
allyl chloride
vinyl chloride
(2-chloropropane)
(2-chlorobutane)
(1-chloro-2-methylpropane)
(2-chloro-2-methylpropane)
(3-chloro-1-propene)
(chloroethene)
benzyl chloride
phenyl chloride
(chloromethylbenzene)
(chlorobenzene)
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Sect. 10.2: Overview of
nucleophilic substitution
•
•
•
•
•
The substitution reaction: SN1 and SN2
Primary halides = SN2
Secondary halides = both mechanisms!
Tertiary halides = SN1
Leaving groups: halogens most
common
• There are a number of different
nucleophiles!!
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Nucleophilic Substitution (SN2)
Oxygen Nucleophiles (SN2)
_
R-CH2-X
substrate
+
Nu
nucleophile
_
R-CH2-Nu
product
+
X
leaving group
_
R-CH2-X +
O-H
hydroxide
_
R-CH2-X + O-R
alkoxide
R-CH2-X +
_
O-C-R
O
carboxylate
R-CH2-O-H +
alcohol
R-CH2-O-R +
ether
R-CH2-O-C-R +
X
_
X
_
X
_
O
ester
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Nitrogen as a nucleophile (SN2)
_
R-CH2-X
+
substrate
nucleophile
product
NH3
ammonia
X
+
R-CH2-Nu
leaving group
_
+
R-CH2-X +
+
Nu
_
R-CH2-NH3
X
_
R-CH2-NH3 X
R-CH2-NH2
primary
amine
+
H-X
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Carbon as a nucleophile (SN2)
_
R-CH2-X
substrate
+
Nu
nucleophile
_
C N
R-CH2-X +
cyanide
_
R-CH2-X + C C-H
_
R-CH2-X + CH2-C-R
O
_
R-CH2-Nu
product
+
X
leaving group
R-CH2 C N +
nitrile
_
X
_
R-CH2- C C-H +
alkyne
X
_
R-CH2 CH2-C-R + X
ketone
O
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H
R
d-
O
C
H
H
Br
d-
energy
H
.. __
O:
..
R
H
C
H
Br
OH
R C H
H
Br
Reaction coordinate
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The SN1 Mechanism
1)
CH3
CH3 C
CH3
CH3
slow
CH3 C
+
: Br:
..
CH3
.. _
: Br :
..
+
carbocation
2)
CH3
CH3 C
+
CH3
CH3
+ : Nu
_
fast
CH3 C
CH3
Nu
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d+R
R C R
d+
R
Br
d-
C R
R
Nu
d+R
R C
R
energy
intermediate
R
R
R C R
Br
R
C R
Nu
Reaction coordinate
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Sect. 10.3: SN2 Mechanism
•
•
•
•
•
•
•
reaction and mechanism
kinetics
stereochemistry
substrate structure
nucleophiles
leaving groups
solvents
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The SN2 Reaction
Sterically accessible compounds react
by this mechanism!!
CH3 Br
_ ..
+ :O
..
H
..
CH3 OH
..
+
.. _
: Br :
..
Methyl group is small
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SN2 Mechanism: kinetics
• The reactions follows second
order (bimolecular) kinetics
• Rate = k
1
[R-Br]
1
[OH ]
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H
R
d-
O
C
H
H
Br
d-
energy
H
.. __
O:
..
R
H
C
H
Br
OH
R C H
H
Br
Reaction coordinate
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SN2 Reaction: stereochemistry
H
.. _
O:
..
CH3
CH3
C
H
Br
H
dO
Et
dBr
C
H
Et
(R)- enantiomer
Inversion of configuration
H
..
O
..
CH3
C
_
+
H
Et
Br
(S) enantiomer
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For an SN2 Reaction:
EVERY REACTION EVENT
ALWAYS LEADS TO
INVERSION OF CONFIGURATION
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SN2 Reaction: substrate
structure (Table 10-5)
krel
CH3 Br
150
CH3 CH2 Br
1
CH3 CH Br
0.008
CH3
CH3
CH3 C
Br
unreactive! KI in Acetone at 25°
CH3
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Chloromethane + Iodide as
the Nucleophile
Fast
I-
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tert-Butyl Chloride + Iodide
as the Nucleophile
No
reaction
I-
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SN2 Reaction: substrate
structure
CH3
CH3-Br > CH3-CH2-Br >
CH3
CH
Br >
CH3
primary
secondary
CH3
C
Br
CH3
tertiary
Reactivity order---- fastest to slowest!
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SN2 Reaction:
nucleophilicity
Basicity
Nucleophilicity
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Predict which is more
nucleophilic
-
-
CH3-O or CH3-S
-
CH3-S
is more nucleophilic!
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Relative Nucleophilicity
CH3OH
H2O
_
OH
O
CH3 C
_
O
_
O
_
OCH3
_
I
_
SH
_
N
C
Increasing Nucleophilicty
1) In general, stronger bases are better nucleophiles
2) However, iodide doesn’t fit that pattern (weak base, but
great nucleophile!)
3) Cyanide is an excellent nucleophile because of its linear
structure
4) Sulfur is better than oxygen as a nucleophile
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SN2 Reaction: Leaving
Groups
• Best leaving groups leave to form weak Lewis
bases.
• Good leaving groups:
– Br, I, Cl, OTs, OH2+
• “Lousy” leaving groups:
– OH, OR, NH2,, F
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Sulfonate Leaving Groups
O
R
O
S
CH3
R
OTs
O
para-Toluenesulfonate
Tosylate
O
R
O
S
Br
R
OBs
O
para-Bromobenzenesulfonate
Brosylate
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Tosylate leaving group
CH3
H
C
OH
CH3
CH2
CH3
O
H
S
Cl
O
[Ts-Cl]
C
O
Ts
CH2
+ H-Cl
Retention of configuration
(S)-(+)-1-Phenyl-2-propanol
(S)-(+)-1-Methyl-2-phenylethyl
tosylate
CH3
H
C
O
Ts
CH2
C2H5O
_
CH3
CH2 CH
O CH2 CH3
Retention or inversion?
2-Ethoxy-1-phenylpropane
Is this ether (R) or (S)?
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Inversion of Configuration
CH3 CH2
H
O_
O
CH3
C
CH2
O
S
CH3
O
(S)
CH3 CH2 O
CH3
H
C
CH2
O
+
(R)
CH3
S
O
_
O
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SN2 Reaction: solvents
SN2 reactions are accelerated in polar,
aprotic solvents. Consider Na+ -OEt as an
example of a nucleophile.
Why are reactions accelerated? The Na+
cation is complexed by the negative part of
the aprotic solvent molecule pulling it
away from –OEt.
Now that the sodium ion is complexed, the
oxygen in the nucleophile –OEt is more
available for attack.
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Aprotic solvents
• These solvents do not have OH bonds in them.
They complex the cation through the lone pairs on
oxygen or nitrogen:
O
Acetone
H3 C
CH3
Dimethyl sulfoxide (DMSO)
H3 C
O
S
CH3
O
Dimethylf ormamide (DMF)
Acetonitrile
H
N
CH 3
CH 3
H3 C C N
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How cations are complexed
with aprotic solvents
H3C S CH3
O
Na
H3C
O
S
CH3
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Now that the Na+ is complexed,
the –OEt can react more easily
Et O
H3C Br
Et O CH3
Br
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SN2 Reaction: solvents
SN2 reactions are retarded (slowed) in polar,
protic solvents. Protic solvents have O-H
groups.
Why are reactions retarded? Nucleophile is
hydrogen bonded to solvent!
Et O
H O
Et
The nucleophile is
hydrogen bonded
to ethanol - reduces
nucleophilicity
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Protic solvents
Typical protic solvents:
Water
H
O
Methanol
H
Ethanol
H
Acetic acid
Formic acid
H
H
H
O
CH3
HOMe
O
CH2 CH3
HOEt
O
C CH3
O
O
abbreviations
HOAc
C H
O
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Sect. 10.4: SN1 Mechanism
• reaction and mechanism
• kinetics
• stereochemistry
• substrate structure
• nucleophiles
• leaving groups
• solvents
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Solvolysis of tert-Butyl
Bromide
Acetone is used to dissolve everything! Water is the
solvent and nucleophile (solvolysis).
CH3
CH3
CH3 C
Br
CH3
+
H2O
acetone
CH3 C
CH3
+
H
Br
OH
+ other products
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The SN1 Mechanism
1)
CH3
CH3 C
CH3
CH3
slow
CH3 C
+
: Br:
..
CH3
.. _
: Br :
..
+
carbocation
2)
CH3
CH3 C
+
H
CH3
+ :O:
CH3
fast
CH3 C
H
:O
CH3
+
H
H
3)
CH3
CH3
CH3 C
:O
H
CH3
+
H
fast
CH3 C
:O
..
CH3
+
H
+
H
1935: Hughes & Ingold
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d+R
R C R
Br
d-
+ R
R
R C R
O
H intermediate
H
R C
R
energy
intermediate
R
R
R C R
Br
R
C R
OH
Reaction coordinate
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SN1 Reaction: kinetics
• The reactions follows first
order (unimolecular) kinetics
• Rate = k
1
[R-Br]
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SN1 Reaction:
stereochemistry
With chiral R-X compounds, the
product will be racemic (50% of each
enantiomer).
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Stereochemistry in SN1
reactions – racemic product
Slow
Pr
C
CH3-O-H
Pr
C Br
polar
H3C
protic
Et
o
3 substrate solvent!
(S) enantiomer
H3C Et
planar carbocation
CH3-O-H
front side
attack
Pr
C
H3C Et
CH3-O-H
Fast
back side
attack
Pr
H3C
Et
H3C
H
C O
CH3
H
Pr
H
fast
Pr
H
O
CH3
Et
fast
H
Et
H3C
C O
CH3
50% (S)
Pr
O
CH3
Et
50% (R)
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d+R
R C R
Br
d-
+ R
R
R C R
O
H intermediate
H3C
R C
R
energy
intermediate
R
R
C R
O-CH3
R
R C R
Br
Reaction coordinate
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SN1 Reaction: substrate
structure
krel
CH3 Br
no reaction
CH3 CH2 Br
1.00
CH3 CH Br
11.6
CH3
CH3
CH3 C
Br
6
1.2 x 10
CH3
Solvolysis in water at 50°C
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SN1 Reaction: substrate
structure
tertiary>secondary>primary > methyl
Primary and methyl halides are
very unreactive! They don’t go by
SN1 reactions.
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CH3
CH3
C
Br
>
CH3
secondary
CH3
CH3
C+
Br >
CH3-CH2-Br
> CH3-Br
CH3
CH3
tertiary
CH
primary
+
CH3
CH
+
CH3
CH 2
+
CH3
CH3
CH3
tertiary
carbocation
(very stable)
three methyl
groups
secondary
carbocation
two methyl
groups
primary
carboc
carbocation
(unstable)
one methyl
group
very unstable
carbocation
no methyl
groups
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Nucleophiles
• Usually SN1 reactions are run in polar
protic solvents; compounds with O-H
groups.
• The polar protic solvent acts as BOTH
nucleophile as well as the solvent.
• Common solvent/nucleophiles include:
water, ethanol, methanol, acetic acid,
and formic acid.
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A protic solvent acts as both a solvent and
nucleophile in SN1 reactions - solvolysis:
Water
H
O
Methanol
H
Ethanol
H
Acetic acid
Formic acid
abbreviations
H
H
H
O
CH3
HOMe
O
CH2 CH3
HOEt
O
C CH3
O
O
HOAc
C H
O
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Typical solvolysis reaction
Slow
Pr
C
CH3-O-H
Pr
C Br
polar
H3C
protic
Et
3o substrate solvent!
(S) enantiomer
Solvent is the
nucleophileCH -O-H
3
Pr
C
CH3-O-H
Fast
H3C Et
planar carbocation
Pr
front side
attack
H3C Et
Polar solvent stabilizes
the carbocation!
back side
attack
H3C
Et
H3C
H
C O
CH3
H
Pr
H
fast
Pr
H
O
CH3
Et
fast
H
Et
H3C
C O
CH3
50% (S)
Pr
O
CH3
Et
50% (R)
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Leaving groups
• Leaving groups are the same as in SN2
reactions:
• Cl, Br, I, OTs are the usual ones.
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SN1 Reaction: solvent
polarity
• SN1 solvolysis reactions go much
faster in trifluoroacetic acid and
water (high ionizing power).
• SN1 solvolysis reactions go slower
in ethanol and acetic acid (lower
ionizing power).
• See table 10-9.
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SN2 versus SN1 Reactions
• A primary alkyl halide or a methyl halide
should react by an SN2 process. Look for a
good nucleophile, such as hydroxide,
methoxide, etc. in an polar aprotic solvent.
• A tertiary alkyl halide should react by an SN1
mechanism. Make sure to run the reaction
under solvolysis (polar protic solvent)
conditions! Don’t use strong base conditions
-- it will give you nothing but E2 elimination!
• A secondary alkyl halide can go by either
mechanism. Look at the solvent/nucleophile
conditions!!
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SN2 versus SN1 Reactions
(continued)
• If the reaction medium is KI or NaI in acetone,
this demands an SN2 mechanism.
• If the reaction medium is AgNO3 in ethanol,
this demands an SN1 mechanism.
• If the medium is basic, look for SN2.
• If the medium is acidic or neutral, expect SN1.
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Comparison of SN1 and SN2
Reactions
• See Table 10-10 on page 936.
Great table!!
• Section 10-5: Solvent effects; been
there done that!!
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Sect. 10.6: classification
tests
• Sodium iodide and potassium
iodide in acetone are typical SN2
reagents!!
• Silver nitrate in ethanol is a typical
SN1 reagent!!
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Sect. 10.7: Special Cases
Neopentyl compounds are very
unreactive in SN2 reactions.
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Effect of b-substitution
on SN2 reactivity (Table 10-11)
krel
b
H CH2 CH2 Br
1.0
b
CH3 CH2 CH2 Br
0.65
CH3
CH3 CH CH2 Br
b
0.15
CH3
CH3 C
b
CH3
CH2 Br
Neopentyl bromide
0.000026
KI
in Acetone
at 25°
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-- Chemistry
Neopentyl Transition State
Y
Y
R1
R1
H
C
C
R2
H
R3
Nu
Nu
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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
benzyl bromide
CH2=CH-CH2-Br
allyl bromide
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Solvolysis Rates: SN1
Table 10-13
krel
Ethyl chloride
Isopropyl chloride
Allyl chloride
Benzyl chloride
tert-Butyl chloride
very small
1
74
140
12,000
80% Ethanol-water at 50°
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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
benzyl bromide
CH2=CH-CH2-Br
allyl bromide
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Reaction with KI in Acetone:
SN2
Table 10-14
krel
Ethyl chloride
Allyl chloride
Methyl chloride
Benzyl chloride
1
33
93
93
60° C
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Vinyl and Phenyl
Compounds
Vinyl and Phenyl compounds are completely inert in both
SN1 and SN2 reactions!!
Cl
H
CH2
C
Cl
vinyl
phenyl
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Reactivity order for SN1
CH2
R
C
R R
3o
Br
Br
>
>
H
H C C
CH2 Br
H
Benzyl
Allyl
R
CH Br
>>
R CH2
Br
>> CH3-Br >>
Br
H
C
Br
R
o
2
o
1
methyl
R
R
phenyl
vinyl
Inert!!
No reaction
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Reactivity order for SN2
About same reactivity
CH2
Br
H
H C C
CH2 Br
H
Benzyl
Allyl
=
CH3-Br
methyl
>
R CH2
Br
R
>
CH Br
R
o
1
2o
>>>
R
C
R R
Br
3o
Can not
undergo
SN2
>>>
Br
H
C
Br
R
R
phenyl
vinyl
Inert!!
No reaction!!
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Sect. 10.8: Cyclic Systems
• Cyclopropyl and cyclobutyl halides
are very unreactive in both SN1 and
SN2 reactions
• Cyclopentyl halides are more
reactive than cyclohexyl halides in
SN1 and SN2 reactions.
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Bicyclic systems: Bredt’s Rule
You can’t have p orbitals on a bridgehead
position in a rigid bicyclic molecule.
-- You cannot form a carbocation
at a bridgehead position.
bridgehead
+
bridgehead
--You cannot have a double bond
at a bridgehead position.
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AgNO 3
No reaction!
Ethanol
Cl
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Sect. 10.9: Carbocation
Rearrangement
1)
CH3
CH3 C
CH3
slow
CH CH3
CH3 Br
CH3 C
CH3
CH CH3
+
_
+ Br
a carbocation
2)
CH3 C
CH3
3)
CH3
CH3
CH3 C
+
CH CH3
+
CH3
CH3
CH3
CH3 C
+
CH CH3
CH CH3
CH3
+
ROH
CH3 C
CH CH3
+
+
H
OR CH3
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A Closer Look...
CH3
CH3 C
CH3
CH CH3
+
CH3 C
+
CH3
CH CH3
CH3
CH3
CH3
C
+
CH CH3
CH3
transition state
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Carbocation Rearrangement
CH3
CH3 C
CH
CH3
CH3
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Sir Christopher Ingold
Source: Michigan State University, Department of Chemistry
http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml
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Saul Winstein
Source: Michigan State University, Department of Chemistry
http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml
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Sect. 10.10 Competing
Reactions: Elimination -Table 10-16
• Lower temperatures favor substitution; higher
temperatures give more elimination.
• Highly branched compounds (secondary and tertiary
compounds) give mostly elimination with strong
bases. Weaker bases give more substitution. A
basic medium favors E2; a more nucleophilic medium
favors SN2.
• Primary compounds give mostly substitution with
non-bulky nucleophiles. A bulky base (tert-butoxide)
gives elimination.
• Tertiary compounds should be reacted under
solvolysis conditions to give substitution!!!
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Sect. 10.11: Neighboring
group participation
O
O
CH3 CH C
_
O
Br
_
+ CH3O
CH3OH
inversion
> 0.5 M
(R)-(+)
+ Br
OCH3
O
O
Br
_
(S)-(-)
(R)-(+)
CH3 CH C
CH3 CH C
_
O
_
O
_
+ CH3O
< 0.1 M
CH3OH
retention
CH3 CH C
_
O
_
+ Br
OCH3
(R)-(+)
!!!
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Under SN2 Conditions
_
..
CH3 O :
..
CH3
C
H
Br
C
_O
CH3
dCH3 O
C
H
O
dBr
_O
C
O
(R)
CH3
Inversion of configuration
CH3 O
C
H
+
_
Br
C
_O
O
(S)
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Internal SN2 reaction
followed by an external SN2
reaction
H
CH3
_
..
:O
..
C
H
Br
slow
C
O
C
C
O
O
(R)
CH3
:O:
CH3
H
CH3
_
+ Br
C
H
..
O
..
_ .. C
:O
.. O
CH3
+
+
H
(R)
Retention of Configuration
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Neighboring Group
Participation
1)
G
G:
C
C
slow
C
C
+
.. _
:X:
..
X
2)
C
G:
G:
G
C
fast
C
C
Nu
Nu :
X
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Neighboring group
participation: Summary
• Retention of configuration
• Enhanced rate of reaction
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Mustard gas
•
Mustard gas is a substance that causes tissue blistering (a vesicant). It
is highly reactive compound that combines with proteins and DNA and
results in cellular changes immediately after exposure. Mustard gas
was used as a chemical warfare agent in World War I by both sides.
Cl
Cl
S
Mustard gas
Cl
Cl
S
Cl
Cl
S
Neighboring
group participation
Internal SN2
O-Enzyme
External SN2
O-Enzyme
Cl
Cl
S
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Sect. 10.13: Ion-pair
mechanisms (skip!!)
• SN1 reactions are “expected” to give a 50-50
(racemic) mixture of the two enantiomers!!
• But, if the leaving group doesn’t get out of the
way, you will get more inversion than
retention, which makes it “look like” SN2.
• In the extreme, you could have a carbocation
give only inversion of configuration by an SN1
mechanism!!
WWU -- Chemistry
In-Class Problem
For the following reaction,
H2O
CH3 CH
CH
CH2 OTs
acetone
A)
Identify the mechanism of this reaction.
B)
Predict the product(s) of this reaction, and identify
them as major or minor, if appropriate.
WWU -- Chemistry
The following table may be
helpful as a review
WWU -- Chemistry
Substitution versus Elimination
SN1
SN2
E1
E2
Substrate
Strong effect; reaction
favored by tertiary
halide
Strong effect; reaction
favored by methyl or
primary halide
Strong effect; reaction
favored by tertiary
halide
Strong effect; reaction
favored by tertiary
halide
Reactivity – primary
Does not occur
Highly favored
Does not occur
Occurs with strong
base!
Reactivity – tertiary
Favored when
nucleophile is the
solvent – solvolysis
Does not occur
Occurs under
solvolysis conditions
or with strong acids
Highly favored when
strong bases (OH-,
OR-) are used
Reactivity –
secondary
Can occur in polar,
protic solvents
Favored by good
nucleophile in polar,
aprotic solvents
Can occur in polar,
protic solvents
Favored when strong
bases are used
Solvent
Very strong effect;
reaction favored by
polar, protic solvents
Strong effect;
reaction favored by
polar, aprotic solvents
Very strong effect;
reaction favored by
polar, protic solvents
Strong effect; reaction
favored by polar,
aprotic solvent
Nucleophile/Base
Weak effect; reaction
favored by good
nucleophile/weak
base
Strong effect; reaction
favored by good
nucleophile/weak
base
Weak effect; reaction
favored by weak base
Strong effect; reaction
favored by strong
base
Leaving Group
Strong effect; reaction
favored by good
leaving group
Strong effect; reaction
favored by good
leaving group
Strong effect; reaction
favored by good
leaving group
Strong effect; reaction
favored by good
leaving group
WWU -- Chemistry