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ELIMINATION

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occur in a different fashion? Three types of elimination reactions are conceivable ... to high concentration, or the substrate. is a primary halide, the E2 ... – PowerPoint PPT presentation

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Title: ELIMINATION


1
ELIMINATION OTHER POSSIBLE
MECHANISMS
Do some elimination reactions occur in a
different fashion?
2
Three types of elimination reactions are
conceivable
concerted
E2
carbocation
E1
halogen first
proton second
carbanion
E1cb
proton first
halogen second
3
E1
ALKYL HALIDES WEAK BASE
(SOLVOLYSIS)
The removal of a b-hydrogen becomes difficult
without a strong base and a different mechanism
(ionization) begins to take place
….. if the substrate is capable.
4
The E1 Elimination Reaction (two steps)
weak base
carbocation
slow
X

step one
3o gt 2o gt 1o
also favored if a resonance stabilized
carbocation is formed
step two
unimolecular
fast
rate k RX
Works best in a polar solvent.
IONS FORMED
5
ENERGY PROFILE
two step reaction
carbocation
intermediate
TS1
TS2
E N E R G Y
Ea2
Ea1
starting material
step 1
step 2
DH
slow
product
6
STEREOSPECIFICITY
7
THE E1 REACTION IS NOT STEREOSPECIFIC
( THE OPEN CARBOCATION IS PLANAR AND CAN ROTATE)
H
Carbocation is sp2 hybridized ( planar
) and can react from either side.

H
anti
C
C
C
C
rotation
X
These two carbocations are equivalent by rotation
and by symmetry.
syn
C
C
C
C
H
X
H

rate of C-C rotation 1010 to 1012 / sec
Elimination can be either syn or anti.
8
REGIOSELECTIVITY
9
E1 REACTION IS REGIOSELECTIVE
THE ZAITSEV RULE IS FOLLOWED
(stereochemistry is not a problem as in E2)
very dilute base
0.001 M
KOH / EtOH
major
minor
tertiary
trisubstituted
disubstituted
Zaitsev
10
DIFFERENCES BETWEEN E1 AND E2
11
BEHAVIOR OF THE RATE WITH INCREASING BASE
CONCENTRATION
E2
rate k2 RX B
second order
E1 dominates at low base concentration
E2 dominates at higher base concentration
rate k1 RX
first order
Rate
E1
RX constant, B increasing
12
EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS
k2 RX B
E2
k1 RX
tertiary RX, k
1
E1
secondary RX, k
Rate
1
primary RX, k
1
RX constant, Base increasing
For E1 elimination k (tertiary) gt k
(secondary) gt k (primary).
13
EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS
tertiary
k
2
secondary
k
2
E2
k
k2 RX B
2
primary
k1 RX
tertiary RX, k
1
E1
secondary RX, k
Rate
1
primary RX, k
RX constant, B
For E2 elimination line slopes k2 differ for
1o,2o,3o . Different substrates react at
different rates,
14
STRUCTURE OF SUBSTRATE
R
H
H
R-C-X
R-C-X
R-C-X
R
R
H
primary
secondary
tertiary
Obviously for E1 which forms a carbocation
intermediate
rate tertiary gt secondary gt primary gt
methyl
But this same order holds for E2 also.
tertiary has more b -hydrogens
more opportunites for reaction
EtO-
15
WHEN THE E1 MECHANISM OCCURS
E1 occurs only
1) at zero or low base concentration
2) with solvolysis (the solvent is the base)
3) with tertiary and resonance capable
substrates (alkyl halides)
If a strong base is present in moderate to high
concentration, or the substrate is a primary
halide, the E2 reaction dominates.
16
ALKYL HALIDE BASE
strong base high base conc.
weak base low base conc.
or
solvolysis
(solvent is base)
E2 mechanism
E1 mechanism
must be able to make good carbocation
anti-coplanar requirement
stereospecific
not stereospecific
regioselective
regioselective
17
EXAMPLES
rate k RBr OEt
E2
6M
N
a
O
E
t
E
t
O
H
E1
0.01 M
K
O
H
E
t
O
H
rate k RBr
18
SOLVOLYSIS
The solvent is the thing !
19
SOLVOLYSIS
MANY E1 REACTIONS ARE SOLVOLYSIS REACTIONS
SOLVOLYSIS
THE SOLVENT IS THE REAGENT (BASE)
competing product
E1
EtOH adds to the carbocation
EtOH solvent acts as base -
no other base is present
20
SOMETIMES E1 AND E2 RESULTS DIFFER
21
A COMPARISON OF E1 AND E2
major product
E2
syn
Anti-Zaitsev
H
NaOEt
H
stereospecific anti
Br
H
EtOH / D
H
CH3
H
CH3
EtOH / D
E1
anti
Zaitsev
not stereospecific
CH3
E1 doesnt require anti-coplanarity
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