The document discusses elimination reactions, focusing on carbocation rearrangements and the differences between E1 and E2 mechanisms. It highlights Saytzeff's rule for predicting major products and factors influencing the rates of E1 and E2 reactions, such as substrate structure, base strength, and solvent effects. Additionally, the document notes the competitive nature of elimination and substitution reactions, with conditions favoring one over the other depending on various factors.

1. ELIMINATION REACTIONS
Dr. Priyanka
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2. CONTENTS
Elimination Reactions
• Rearrangements
Carbocations may undergo rearrangements to yield
their more stable counterparts, and this phenomenon is
typically labeled as carbocationic rearrangement. In this
section, we will discuss the carbocation rearrangement
for classical and non-classical carbocations.
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3. Rearrangement Reactions in Classical Carbocation:
The 1-2 methyl shit or 1-2 hydride shifts are very common in
carbocation chemistry to attain a more stable counterpart. For
instance, a primary carbocation will prefer to rearrange itself into a
more stable tertiary carbocation.
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4. Dr.Priyanka
Some typical examples of methyl or hydride shift are given below.
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5. Dr.Priyanka
Saytzeff’s Rule
.
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Elimination reactions of some alkyl
halides and alcohol will result in different
alkenes, and Saytzeff’s rule is used to
predict the major product. The major and
minor products are predicted based on
the number of alkyl groups attached to
the alkene.

6. Dr.Priyanka
Important Concept Behind Saytzeff’s Rule
•If more than one elimination product is possible, the most substituted alkene
is the most stable product (major product).
•CH2 = CHR < RCH = CHR < R2C = CHR < R2 C = CR2
•Mono < di < Tri < Tetra
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7. General Mechanism for E1 Elimination
• E1 describes an elimination reaction (E) in which the rate-
determining step is unimolecular (1) and does not involve the base.
The leaving group leaves in this step, and the proton is removed in
a separate second step.
• Tertiary-butyl bromide undergoes elimination reaction in presence
of ethanol, a weak base to give isobutene. Mechanism of E1
reaction is shown in Figure 3.
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8. E1 verses E2 reactions
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Characteristics of E1 reaction Characteristics of E2 reaction
1. Unimolecular reaction
2. Two step reaction
3. Carbocation intermediate formed.
4. Reactivity order of RX is 3°>2°>1°
5. No stereospecific.
6. Follow ziatsev rule.
7. Polar protic solvent good because
stabilized ionic intermediate.
8. Rate of reaction increases when
concentration of substrate increases.
9. Rearrangement may take place.
1.Biomolecular reaction.
2. Single step reaction.
3. Hydrogen removes from beta carbon.
4. Trans elimination because low energy
consumption.
5. Anti periplanar attack.
6. Polar aprotic solvent best.
7. Phenyl group influence elimination
because product alkene further stabilised
by resonance.
8. Reactivity order 3°>2°>1°. No steric
effect.
9. Strong nucleophile influence
elimination.
10. No intermediate formed.

9. Factors affecting E1 and E2 reactions
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Effects of R-
Reactivity order: (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-
More highly substituted systems undergo E2 eliminations more rapidly.
This is the same reactivity trend as seen in E1 reactions
As the number of R groups on the carbon with the leaving group increases,
the rate of the E2 reaction increases.

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(ii) Leaving Group (LG) Rate of reaction follows the order Rate of reaction follows the
order, R−I > R−Br > R−Cl > R−F
(iii) Base (B) The strength of the base usually determines whether a reaction follows
the E1 or E2 mechanism. Strong bases like OH− and OR− g favor E2
reactions, whereas weaker bases like H2O and ROH favor E1 reactions.
RCH2 X R2CH X R3C X
Increasing rate of E1 reaction

11. Kinetics – First order
Mechanism – Two steps
Identity of R group – More substituted halides react faster
Rate: R 3CX > R 2CHX > RCH 2 X
Strength of the base – Favored by weaker bases such as
H2O and ROH
Leaving group – Better leaving group leads to faster
reaction rates. Just as in S N 1 reactions reactions, the rate
determining determining step involves the C—X bond
cleavage
Type of solvent – Favored by polar protic solvents, which
can stabilize the ionic intermediates
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12. Elimination v/s substitution
Elimination and substitution reactions are closely related
and competitive but form different products.
However, substitution becomes favorable as it involves
less bond reorganization and energetically being more
favorable.
1)Basicity and nucleophilicity:For elimination reaction
basicity of nucleophile or base is important while for
substitution it is nucleophilicity that matters more.
Therefore, strongly basic conditions favor elimination
reaction and use of strong bases is advisable. If some
compounds which are not strong bases but are good
nucleophiles are used, then substitution predominates.
Use of strong and slightly polarizable base such as amide
or alkoxide favours elimination over substitution.
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13. 2)Substrate structure : Crowded reactant favors
elimination reaction over substitution due to
hindrance to approach of nucleophile. In primary
alkyl halides, easy approach of nucleophile is
possible hence substitution is favored over
elimination. In secondary alkyl halides, substitution
is difficult due to steric hindrance and elimination is
favored. Whereas ,in case of tertiary halides SN2
reaction is not possible, elimination is favored
particularly at elevated temperature. If substitution
occurs, then it is by SN1 pathway.
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More examples: nature of the base Structure of
base has similar effect on the ratio of
substitution or elimination reaction. A more
crowded base of similar strength favors
elimination over a less crowded base.
Therefore, tert-butoxide gives more of
elimination product than substitution whereas
ethoxide shows opposite effect altogether.
3)Solvent : In E2reaction, five atoms are
involved, thus, charge is more dispersed due to
which less polar solvents favor E2 elimination
over SN2 reaction. (Similar is the case with
E1reaction).

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More examples: role of temperature
4)Temperature : Elimination reactions are
favored at higher temperature than
substitution reaction because elimination
reaction has high free energies of activation
than substitution due to higher degree of
bond reorganization (or change in the
bonding pattern)i.e., more bonds are broken
and formed.