1. Part 4
Elimination Reactions
Cl
H R2
R1
R3
R4
R2
R1
R4
R3
R2
R1
R4
R3
Loss of
Stereochemistry
R3
R1
R4
R2
Retension
of
Steroechemistry
Rate =k[R-Hal][Nu] Rate =k[R-Hal]
B
2. – Learning Objectives Part 4 –
Elimination Reactions
After completing PART 4 of this course you should have an understanding of, and be able to demonstrate, the following
terms, ideas and methods.
(i) Understand E2 and E1 reaction mechanisms
(ii) Understand how experimental evidence from rate equations and stereochemical outcomes in the product
lead to the proposal of reaction mechanisms
(iii) Understand the experimental factors which favour E2 or E1 reaction mechanisms
(vi) Understand the term antiperiplanar in the context of E2 reaction mechanisms
(v) Understand that in assessing the reaction outcome in an elimination reaction, the stereoelectronic of the
alkylhalide needs to be considered carefully, ie. the constitution and conformation
CHM1C3
– Introduction to Chemical Reactivity of Organic
Compounds–
3. Elimination Reactions
Descriptor Rate Equation Stereochemical
Outcome
E2 rate = k[R-Hal][Nu] Retension
E1 rate = k[R-Hal] Loss of
Stereochemistry
Clearly, two different reaction mechanisms must be in operation.
It is the job of the chemists to fit the experimental data to any proposed
mechanism
C
Cl
C Cl
R4
R3
H
R1
R2
C
C
R4
R3
R1
R2
NuH
B BH
4.
5. The E2 Reaction Mechanism
Cl
H R2
R1
R3
R4
B
Rate = k[R-Hal][B]
Bimolecular
Process
H and Cl must be
antiperiplanar
Transition State –
Energy Maxima
Cl
H R2
R1
R3
R4
B
H
B
R2
R1
R4
R3
Cl
Retension
of
Steroechemistry
Compare to SN2
7. The E1 Reaction Mechanism
Cl
H R2
R1
R3
R4
Unimolecular
Process
Rate = k[R-Hal]
B
Rotation about
C-C Bond
H
R2
R1 R3
R4
Cl
H
R3
R1 R2
R4
Cl
Reactive Intermediate – Energy Minima
R2
R1
R4
R3
R3
R1
R4
R2
Loss of
Stereochemistry
Compare to SN1
12. Constitutionally Different Eliminations
Br
Minor Alkene
Product
Major Alkene
Product
Br
H 2 Equivalent
Hydrogen atoms
Br
H
6 Equivalent
Hydrogen atoms
Statistically favoured!
Base
Constitutional
Isomers
14. Cl
Ph D
H
H
Ph
B
Steric clash of the
two phenyl groups
raises the energy of
the transition state,
as the two carbon
centres become sp2
hybridised
Cl
Ph
H
D
H
Ph
B
High Energy
Transition State
Low Energy
Transition State
15. Cl
CH3
H3C CH3
NaOEt
EtOH
Rate = k[R-Cl][NaOEt] CH3
H3C CH3
CH3
H3C CH3
25% 75%
Cl
H H
Cl
H H
EtO EtO
Cyclohexane Rings – E2
Cl
H H
Two C-H bonds are
antiperiplanar to the
C-Cl bond
16. Cyclohexane Rings – E1
No C-H bonds are
antiperiplanar to the
C-Cl bond
Cl
H H
EtO
H H
H
H H
EtO
H
Cl
CH3
H3C CH3
CH3
H3C CH3
CH3
H3C CH3
H2O
EtOH
Rate = k[R-Cl]
32% 68%
Polar Solvent
Supports Carbocation
Formation
Cl
H H
17. – Summary Sheet Part 4 –
Elimination Reactions
The difference in electronegativity between the carbon and chlorine atoms in the C-Cl sigma () bond result in a polarised bond,
such that there is a partial positive charge (+) on the -carbon atom and a slight negative charge (-) on the halogen atom, which
in turn is transmitted to the -carbon atom and the protons associated with it. Thus, the hydrogen atoms on the -carbon atom are
slightly acidic. Thus, if we react haloalkanes with bases (chemical species which react with acids), the base will abstract the
proton atom, leading to carbon-carbon double bond being formed with cleavage of the C-Cl bond.
The mechanism of this -elimination (or 1,2 elimination) can take two limiting forms described as Bimolecular Elimination (E2)
and Unimolecular Elimination (E1).
The E2 mechanism fits with a rate equation which is dependent on both the base and haloalkane, and that the product retains
the stereochemical information about the C-C bond. This retension of stereochemical integrity requires an antiperiplanar
relationship of the eliminated atoms.
In contrast, The E1 mechanism fits with a rate equation which is dependent on only the haloalkane, and that the product
undergoes a loss of the stereochemical information about the C-C bond. Thus, with appropriately substituted haloalkane a
pair of diastereomeric alkenes are formed, as result of rotation around the C-C bond upon formation of the carbocationic
intermediate.
CHM1C3
– Introduction to Chemical Reactivity of Organic
Compounds–
18.
19. Exercise 1: Substitution/Elimination Reactions
Rationalise the experimental results that when 1 is reacted with NaOEt in EtOH, two alkenes are formed,
whereas 2 under the same conditions affords an inverted substitution product.
H
H
Cl
1
NaOEt
EtOH
H
H
H
H
H
H
Cl
2
H
H
OEt
NaOEt
EtOH
20. Answer 1: Substitution/Elimination Reactions
Rationalise the experimental results that when 1 is reacted with NaOEt in EtOH, two alkenes are formed,
whereas 2 under the same conditions affords an inverted substitution product.
H
H
Cl
1
NaOEt
EtOH
H
H
H
H
H
H
Cl
2
H
H
OEt
NaOEt
EtOH
As 2 undergoes an inversion of stereochemistry one must assume SN2 mechanism.
As 1 is subject to the same reaction conditions as 2 one must assume that elimination of HCl does
not involve the formation of a carbocation, and thus E2 mechanism must operate.
Cl
H
H
H
EtO
H
H
H
Cl
H
H
H
EtO
H
H
Cl
H
H
EtO
OEt
H
H
Antiperiplanar Conformational Relationships
E2 E2
SN2
21. Cl Cl
NaOEt
EtOH
MODERATE
Temperature
rate = k[R-Cl][NaOEt]
NaOEt
EtOH
HIGH
Temperature
rate = k[R-Cl][NaOEt]
Exercise 2: Elimination Reactions
Rationalise the following
Cl Cl
NaOEt
EtOH
MODERATE
Temperature
rate = k[R-Cl][NaOEt]
NaOEt
EtOH
HIGH
Temperature
rate = k[R-Cl][NaOEt]
22. Cl Cl
NaOEt
EtOH
MODERATE
Temperature
rate = k[R-Cl][NaOEt]
NaOEt
EtOH
HIGH
Temperature
rate = k[R-Cl][NaOEt]
Answer2: Elimination Reactions
Rationalise the following
Cl Cl
NaOEt
EtOH
MODERATE
Temperature
rate = k[R-Cl][NaOEt]
NaOEt
EtOH
HIGH
Temperature
rate = k[R-Cl][NaOEt]
H
Cl
Cl
H
Cl
H
H
Cl
H
Cl
H
Cl
H
H
EtO
EtO
TS2
TS1
OEt
OEt
The energy to attain this transition
state TS2 geometry is much higher
that TS1, because the largest
substituent (t-Bu) and the Cl are
both in the axial positions, which
leads to large steric clashes. Thus,
more energy, i.e. higher reaction
temperatures, are required to attain
TS2 relative to TS1.