Tutorial 4 - Elimination Reactions
Relevant Lectures:
Michaelmas Term week 8 – Dr Jenkinson
Make sure you can explain the following elimination mechanisms: E1, E2, E1CB and Ei, including why
they operate in particular cases.
Make sure you understand the following:
•
•
•
•
syn and anti elimination and their stereochemical consequences
Regioselectivity in E2 – Hoffmann and Saytzev elimination
Competition between elimination and substitution – elimination is promoted when stable double
bonds are formed, with bulky bases/nucleophiles and when heating is applied
Cycloeliminations – 5-ring transition states eg. Cope elimination; and 6-ring transition states eg.
ester pyrolysis
Questions
1. Explain the following observations and predict the major product where appropriate:
i)
Br
NaOEt
EtOH
Minor
ii)
Br
tBuOK
tBuOH
?
iii)
Br
NaOEt
EtOH
Major
iv)
Br
NaSEt
EtSH
?
2. Predict the products of the following elimination reactions, giving mechanisms and reasons for your
answers:
a)
Br
Br
b)
Br
c) Br
tBuOK
Heat
tBuOK
Heat
tBuOK
Ph
DMSO
C4H8
and
NMe3 OH
N
C7H12
Heat
Br
NaOEt
EtOH
SMe2
NaOEt
EtOH
Ph
Br
D
e)
H
H
tBuOK
DMSO
f)
C 4H 8
O
and
d)
Heat
C7H12
3. Rationalise the following observations:
OH
OH
Me
Me
NMe3
C4H8 + Me3N
Heat
tBu
tBu
But
+ HOD
D
NMe3
C10H20 + Me2NCH2D
Heat
+ H 2O
D
4. Explain the following observations, giving mechanisms:
tBuOK
a)
Br
FAST
Cl
No Rxn
Br
NaOEt
Heat
b)
tBuOK
but
c)
OMs
Ph
tBuOK
Ph
OMs
tBuOK
Ph
0oC
Ph
Ph
Ph
FAST
tBuOK
Ph
SLOW
Ph
e)
NO2
NO2
tBuOK
FAST
Cl
d)
tBuOK
whereas
SLOW
Cl
tBuOK
tBu
tBuOK
Br
tBu
FAST
SLOW
tBuOK,
Racemic
NaI
FAST
tBu
Br