11 Elimination Reactions To Form Alkenes ANSWERS

©Kevin Burgess, December 4, 2016
EliminationReactionsToFormAlkenes
from chapter(s) _____________ in the recommended text
A. Introduction
.
1
©Kevin Burgess, December 4, 2016
2
B. E1 Mechanisms
OH
OH2
H+
fast
-H2O
-H+
slow
fast
protonated intermediate
carbocation intermediate
alkene product
OH2
energy
OH
reaction progress
Kinetics
rate is proportional to
[ tBuOH]
Carbocation Stability
Rates of E1 reactions tend to increase
rate =
k [ tBuOH]
©Kevin Burgess, December 4, 2016
3
H
H
most stable
least stable
I
Br
Br
I
fastest
sp3 so it has ideal dihedral angles of about 109
the same
more
is not
+
are not favorable.
Br
slowest
Bredt’s Rule
adamantane
H
adamantane
cation
adamantene
©Kevin Burgess, December 4, 2016
4
C. E2 Mechanisms
δ-
Br
D
HO -
Ph
Br
Ph
D
H
H
Ph
D
H
δ+
HO -
transition state
product
(show stereochemistry)
δ-
Br
Ph
D
H
δ+
HO -
energy
Br
D
Ph
H
Ph
D
H
reaction progress
Kinetics
rate is proportional to
doubles
Stereoselectivity
different
perpendicular.
[DCHBrCH 2Ph] [OH -]
rate =
k [DCHBrCH 2Ph] [OH -]
©Kevin Burgess, December 4, 2016
Br
H
H
Br
H
H
H
Br
Br
Br
H
H
Br
Cl
syn-periplanar
EtCl
H
Br
H
I
Et
Et
Me
Br
H
H
Et
anti-periplanar
1-iodopropane
anti-periplanar
EtBrHCCHMe 2
H
Me
anti-periplanar
EtMeHCCBrMeEt
I
anti-periplanar
ethyl iodide
Br
Ph
Br
H
..... can be
represented as
Ph
CH 3
H
H
base
Ph
H
complete Newman
projection
show alkene product
5
©Kevin Burgess, December 4, 2016
OMs
OMs
..... can be
represented as
Ph
6
Ph
CH 3
H
CH 3
H
base
Ph
H
complete Newman
projection
OTs
OTs
Ph
..... can be
represented as
Ph
show alkene product
CH 3
H 3C
H
base
Ph
H
H
complete Newman
projection
show alkene product
Show structures for:
O
O
S
O
O S
O
O
mesyl group, -OMs
Cl
Br
tBu
H
H
tosyl group, -OTs
Br
tBu
tBu
H
H
Cl
H
tBu
H
Cl
tBu
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
Cl
Cl
Cl
Cl
H
tBu
Cl
tBu
H
H
H
tBu
©Kevin Burgess, December 4, 2016
anti-periplanar.
H
H
Br
H
H
H
H
Ph
H
Ph
H
Ph
Br
Br
Br
Ph
H
Ph
Ph
Ph
Ph
H
H
H
H
tBu
Br
tBu
H
tBu
tBu
H
Br
Br
Br
tBu
tBu
tBu
tBu
tBu
tBu
Br
H
H
Br
H
Br
H
H
Br
H
H
H
Br
Br
H
Br
H
H
Br
H
7
©Kevin Burgess, December 4, 2016
8
D. Factors That Favor E1, E2, SN1, or SN2
BasicityvsNucleophilicity
(i)
(ii)
(iii)
(iv)
E2 relative to E1 reactions and it will tend to favor E2
E2 relative to E1 favor E1
will not will not
SN2 relative to SN1 SN2
increases
Nucleophilicity
(i)
(ii)
SN1 relative to SN2 reactions; and,
E1 over E2
(i)
(ii)
SN2 relative to SN1 reactions;
E2 over E1 reactions.
CN-
N 3-
I-
Cl-
MeO -
NH 3
most nucleophilc
OH-
most basic
H 2O
least nucleophilic
Cl-
PhO -
NH 2-
NH 3
H 2O
least basic
©Kevin Burgess, December 4, 2016
9
Temperature(andEntropy)
E2 and SN2 over E1 and SN1.
ΔG # =
ΔH #
-
TΔS #
E2 and SN2 over E1 and SN1 reactions.
E1 and SN1 over E2 and SN2 reactions.
E. E1cB
FMOC
carbamates.
NH
H
H
N
O
O
H
N
O
CO2Me
O
Ph
CO2Me
electron flow
Ph
anionic
intermediate
O
+
+
H 2N
CO2Me
O
Ph
product
(methyl phenylalanine)
14 pe,
aromatic.
draw arrows to depict
©Kevin Burgess, December 4, 2016
F. Eliminations To Give Allenes, Alkynes, Ketenes And Sulfenes
O
Ph
NEt 3
H
O
S O
Cl
O
Ph
Cl
NEt 3
H
H
ketene
Cl
base
sulfene
base
Me
O
O
Br
H
ketene
alkyne
Br
base
Ph
O
S
O
Br
Ph
alkyne
Ph
Br
excess
base
Ph
alkyne
10

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