Hanyang University
Chapter 9
ELIMINATION REACTIONS
: REACTIONS OF ALKYL HALIDES, ALCOHOLS,
AND RELATED COMPOUNDS
Information and Communication Materials Lab.
Contents
9.1 The General Reaction
9.2 Bimolecular Elimination
9.3 Stereochemistry of the E2 Reaction
9.4 Direction of Elimination
9.5 Unimolecular Elimination
9.6 Regiochemistry and Stereochemistry of the E1 Reaction
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9.7 The Competition between Elimination and Substitution
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Organic Chemistry Ⅰ
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MASTERING ORGANIC CHEMISTRY
▶ Understanding the E2 Mechanism for Elimination,
Including Its Stereochemistry and When It Is Favored
▶ Understanding the E1 mechanism for Elimination,
Including Its stereochemistry and When It Is Favored
▶ Using Zaitsev’s Rule and Hofmann’s Rule to
Predict Elimination Products
▶ Predicting the Products of Elimination Reactions
▶ Predicting the Major Reaction Pathway of
Substitution and Elimination Reactions
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9.1 The general reaction
- An elimination reaction occurs when a proton and the leaving group are
lost from adjacent carbons, resulting in the formation of a double bond
- The reaction is termed a 1,2-elimination or β-elimination.
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9.2 Bimolecular elimination (E2 Reaction)
-The rate law is consistent with a concerted or one-step mechanism.
- Two species, ethoxide ion and tert-butyl bromide, react n this step, the
reaction is described as a bimolecular elimination or an E2 reaction.
This reaction follows the second-order rate law:
rate = k[EtO-][t-BuBr]
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The overall process and the transition state can be represented as follows:
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As an example, comparison of the experimental rate of this elimination
reaction to the rate of the reaction of the deuterated analog
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9.3 Stereochemistry of the E2 reaction
-The E2 reaction the carbon sp3 orbital of the carbon-hydrogen
sigma bond and the carbon sp3 orbital of the carbon-leaving group
sigma bond must begin to overlap to form the pi bond.
- This requires that these two sigma bonds lie in the same plane;
they must be coplanar.
- The two bonds may be on the same side of the C-C bond (syncoplanar) or on opposite sides of the C-C bond (anti-coplanar).
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FIGURE 9.1: MECHANISM OF E2 ELIMINATION FROM (A) SYN-PERIPLANAR AND
(B) ANTI-PERIPLANAR CONFORMATIONS.
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- Closer examination reveals that the syn-periplanar conformation has all
the bonds elcipsed, whereas the anti-periplanar conformation has these
bonds staggered, as shown in the following Newman projections:
The anti-periplanar conformation is more stable because it is staggered rather
than eclipsed.
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ⓐ
ⓑ
ACTIVE FIGURE 9.2: MECHANISM AND STEREOCHEMISTRY OF THE E2 ELIMINATION
REACTIONS OF THE DIASTEREOMERS OF 1-BROMO-1,2-DIPHENYL-PROPANE TO
PRODUCE (A) THE (Z) STEREOISOMER AND (B) THE (E) STEREOISOMER OF 1,2-DIPHENYL1-PROPENE.
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-Syn elimination is less favorable than anti elimination
-However, syn elimination does occur in rigid molecules where the
leaving group and the hydrogen are held in an eclipsed conformation.
FIGURE 9.3: SYN ELIMINATION IN A RIGID MOLECULE.
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For anti elimination to occur in a cyclohexane ring, the leaving group
and the hydrogen must be trans. Furthermore, they will be antiperiplanar only in the conformation where both are axial.
FIGURE 9.4: MECHANISM AND STEROCHEMISTRY OF THE E2 ELIMINATION REACTION OF
MENTHYL CHLORIDE.
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FIGURE 9.5: MECHANISM AND STEROCHEMISTRY OF THE E2 ELIMINATION
REACTION OF NEOMENTHYL CHLORIDE.
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9.4 Direction of elimination
- This section discusses the regiochemistry of the elimination reaction-that is,
what happens when a reaction can produce two or more structural isomers.
▶ Zaitsev’s Rule
The major alkene product is the one with more alkyl groups on the carbons
of the double bond (the more highly substituted product).
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-The formation of the more highly substituted product in elimination reactions
results from the lower energy of the product being reflected in a somewhat
more stable transition state.
- This is termed product development control.
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FIGURE 9.6: FREE ENERGY VERSUS REACTION PROGRESS DIAGRAM FOR
THE ELIMINATION REACTION OF 2-BROMOBUTANE.
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Although most E2 reactions follow Zaitsev’s rule, there are some
exceptions.
One major exception is a reaction known as the Hofmann elimination.
An example is shown in the following equation:
▶ Hofmann’s Rule
The major alkene product has fewer alkyl groups bonded to the carbons of
the double bond (the less highly substituted product).
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For example, the elimination reaction of 2-bromobutane using tert-butoxide
ion as the base. as shown in the following equation:
For example, the following elimination produces 98% of the conjugated
product, even though the other possible product is more highly substituted:
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In summary, the regiochemistry of E2 reactions can be predicted by using
the following rules :
1. Most E2 reactions follow Zaitsev’s rule; the more highly substituted alkene
is the major product
2. Hofmann eliminations follow Hofmann’s rule; the less highly substituted
product is the major product.
3. A conjugated alkene is always preferred to an unconjugated alkene.
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9.5 Unimolecular elimination (E1 반응)
-E2 elimination is greatly slowed, and both substitution and elimination
products are formed.
- The first-order rate law:
rate = k[t-BuBr]
-The elimination reaction is described as a unimolecular elimination or
an E1 reaction.
- E1 reactions almost always accompany SN1 reactions.
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Figure 9.7: MECHANISM OF THE SN1 SUBSTITUTION AND E1 ELIMINATION
REACTIONS OF TERT-BUTYL BROMIDE (2-BROMO-2-METHYLPROPANE).
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Figure 9.7: MECHANISM OF THE SN1 SUBSTITUTION AND E1 ELIMINATION
REACTIONS OF TERT-BUTYL BROMIDE (2-BROMO-2-METHYLPROPANE).
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9.6 Regiochemistry and stereochemistry
of the E1 reaction
-When regioisomers are possible in an E1 reaction, the product distribution
is found to follow Zaitsev’s rule.
- Unlike the E2 reaction, the relative stereochemistry of the leaving group an
the hydrogen is not important in the E1 elimination reaction.
- In the first step of the reaction the leaving group departs, producing a
planar carbocation.
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For example, the reaction of menthyl chloride under SN1/E1 conditions
gives the product distribution shown in the following equation:
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9.7 The competition between elimination
and substitution
SN2 and E2
These two pathways require the reaction of the carbon substrate with
a nucleophile or a base in the rate-determining step of the reaction.
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For example, the reaction of 2-bromopropane with ethoxide ion, a strong
base and a strong nucleophile.
In contrast, the reaction of 2-bromobutane with acetate ion a weaker base
and a weaker nucleophile.
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As the temperature is increased, the entropy term in the equation
△G = △H - T△S becomes more important and the amount of elimination
increases. This effect is illustrated in the following equations:
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SN1 and E1
These mechanisms involve rate-determining formation of a carbocation.
So they most commonly occur with tertiary (best) or secondary substrates
In polar solvents and in the absence of strong bases and strong nucleophiles.
Primary Substrates: RCH2L
- Primary substrates do follow the SN2 reaction.
-Follow a predominantly E2 pathway by the use of a strong base that is not a
very good nucleophile because of its steric bulk.
Most commonly, potassium tert-butoxide is used in this role, as shown in the
following equation:
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Secondary Substrates: R2CHL
Secondary substrates can potentially react by any of the four mechanisms.
- Do follow the SN2 reaction when treated with nucleophiles
that are not too basic (CH3COO-, RCOO-, CN-, RS-).
- Do follow the E2 reaction when treated with strong bases such as
OH- or OR-.
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Tertiary Substrates: R3CL
-Tertiary substrates do not give SN2 reactions because they are too hindered.
- Therefore if a substitution reaction is desired, it must be done under SN1
condition (polar solvent, absence of base).
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Visual Summary of Key Reactions
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