ELIMINATION REACTIONS
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Elimination Reactions
Elimination reactions involve the loss of fragments or
groups from a molecule to generate multiple bonds.
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Types of Elimination Reaction
α-elimination: two atoms or groups are removed from the
same atom. It is also known as 1,1-elimination.
β-elimination(1,2 elimination): loss of atoms or groups on
adjacent atoms
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Types of Elimination Reaction
γ elimination: loss of atoms or groups from the 1st and 3rd
positions - result in cyclic compounds.
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Dehydrohalogenation of alkyl
halides
CH3CH2CH2Cl
KOH
C2H5OH
CH3CH2CH2CH2Cl
CH3CH=CH2
KOH
C2H5OH
KOH
CH3CH2CHCH3
C2H5OH
Cl
CH3CH2CH=CH2
- no
rearrangement
CH3CH=CHCH3 +
80%
CH3CH2CH=CH2
20%
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Mechanism
two mechanisms for this elimination: E1 and
E2.
with weak bases at low concentrations and
as we move from a primary halide to a
secondary and a tertiary, the reaction
becomes first order.
a strong base, the reaction follows second
order kinetics
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E2 mechanism
X
C C
α
β
H
:B
X
C C
H
B
-
+ HB + X-
RI > RBr > RCl > RF
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Free Energy Diagram of E2 Reaction
Rate = k[CH3CHBrCH3][EtO⊖]
 Second-order (bimolecular)
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E1 mechanism
X
αC C
β
H
+
C C
H :B
+
C C
H
+ HB
slow
-
+ X
fast
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E1 mechanism
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E1 mechanism
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Orientation and reactivity
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Orientation and reactivity
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The Hammond–Leffler
Postulate
Formation of the carbocation, (+ve Δ G° and Δ H°) therefore, this
step is endothermic.
According to the Hammond–Leffler postulate, the transitionstate structure for a step that is uphill in energy should
show a strong resemblance to the structure of the product
of that step
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Temperature
• Increasing the reaction temperature favors elimination
(E1 and E2) over substitution.
 elimination are entropically favored over substitution
REASON
1. As ΔG° =ΔH° - T Δ S°, an increase in temperature
further enhances the entropy effect.
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Temperature
2.The products of an elimination reaction are greater in
number than the reactants
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Substitution vs elimination

All nucleophiles are potential bases and
all bases are potential nucleophiles

Substitution reactions are always in
competition with elimination reactions

Different factors can affect which type of
reaction is favoured
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Substitution vs elimination
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Primary Substrate
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Secondary Substrate
 An SN2 reaction occurs if a good nucleophile that is a weak bases is
used in a polar aprotic solvent.(I–, Br–, SCN–, N3–, CN–, Amines, etc)
 An SN1 reaction along with an E1 reaction occurs if a poor
nucleophile that is a weak bases is used in a protic solvent.
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Tertiary Substrate
with weak base, SN1 is preferred over E1 mechanism.
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Base : Small vs. Bulky
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Base : Small vs. Bulky
Hindered ‘bulky ‘ base
 when the base used for E2 elimination is a bulky base,
exceptions to the zaitsev rule occur
 a high proportion of the less substituted alkene
(Hofmann Elimination).
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Base : Small vs. Bulky
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Basicity vs. Polarizability
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Strong Bases/Strong Nucleophiles
• A good base is usually a good nucleophile.
•
Strong bases — substances with negatively
charged O, N, and C atoms — are strong
nucleophiles.
• Participate in SN2-type substitutions
• Examples are: RO⁻, OH⁻, RLi, RC≡C:⁻, and
NH₂⁻.
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Weak Nucleophiles & Bases
• Typically neutral molecules
• Participate in SN1-type
concurrently with E1
• Examples: H2O, ROH, H2S, RSH
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Strong Bases /Poor Nucleophiles
Some strong bases are poor nucleophiles
because of steric hindrance.
• Participate in E2 ONLY
Examples are t-BuO⁻, t-BuLi, and
LiN[CH(CH₃)₂]
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Weak Bases/Good Nucleophiles
• anions with a full negative charge
it is a good nucleophile
 Weak Base,as the large electron cloud is
highly polarizable.
 Participate in SN2-type substitutions
 any NaOR, any RLi, CN-, NaCCR (acetylide anion),
NaNH2, NaNHR, NaNR2, NaI, LiBr, KI, NaN3
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Solvent Effects
 solvated species are more stable and less
reactive than the unsolvated "naked" anions
 Polar, protic solvents such as water and alcohols
solvate anions by hydrogen bonding
 Polar, aprotic solvents such as DMSO ,
DMF,acetonitrile etc do not solvate anions but
provide good solvation of the accompanying
cations
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?
Rank the halide anions wrt nucleophilicity in polar protic and
aprotic solvent
In polar protic solvents, the order of nucleophilicity is
I⁻ > Br⁻ > Cl⁻ > F⁻
F⁻ is a small ion with a high charge density. It is tightly solvated.
I⁻ is a large ion with a low charge density. It is loosely solvated.
There are only a few solvent molecules to push out of the way
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?
In polar aprotic solvents, the order of nucleophilicity is
reversed:
F⁻ > Cl⁻ > Br⁻ > I⁻
the negative ends of the dipoles point away from the
molecule.
It is easy for them to solvate cations.
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?
Arrange the following wrt nucleophilicity
CH3CO2– : Br – : CH3O– : CH3S– : I– : Cl–
Nucleophilicity: CH3CO2(–) < Cl(–) < Br(–) < CH3O(–)< I(–) <
CH3S(–)
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Summary
(i) For a given element, negatively charged species are
more nucleophilic (and basic) than are equivalent neutral
species.
(ii) For a given period of the periodic table, nucleophilicity
(and basicity) decreases on moving from left to right.
(iii) For a given group of the periodic table, nucleophilicity
increases from top to bottom (i.e. with increasing size),
although there is a solvent dependence due to hydrogen
bonding. Basicity varies in the opposite manner.
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