Elimination Reactions

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Part 4 Elimination Reactions
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Learning Objectives Part 4 Elimination
Reactions
CHM1C3 Introduction to Chemical Reactivity of
Organic Compounds

• 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

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Elimination Reactions
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
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The E2 Reaction Mechanism
Compare to SN2
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The E1 Reaction Mechanism
Compare to SN1
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Reactive Intermediate
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Stereochemistry Compared
E1
E2
H, C, C and Cl are antiperiplanar
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Alkene Stability
Stability Increases
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Constitutionally Different Eliminations
Base
Constitutional Isomers
Statistically favoured!
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High Energy Transition State
Conformational Equilibria
Diastereoisomers
Conformers
Low Energy Transition State
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High Energy Transition State
Low Energy Transition State
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Cyclohexane Rings E2
Two C-H bonds are antiperiplanar to the C-Cl bond
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Cyclohexane Rings E1
No C-H bonds are antiperiplanar to the C-Cl bond
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Summary Sheet Part 4 Elimination Reactions
CHM1C3 Introduction to Chemical Reactivity of
Organic Compounds
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.
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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.
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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.
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.
SN2
E2
E2
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Exercise 2 Elimination Reactions
Rationalise the following
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Answer2 Elimination Reactions
Rationalise the following
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.