alkyl halides and elimination reactions
DESCRIPTION
ChemistryTRANSCRIPT
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Organic Chemistry
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Organic Chemistry
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At the end of this lecture, students
should be able to:
Specify types of reactions for alkyl
halides
Able to identify the mechanism involved
for a given reaction (unimolecular or
bimolecular)
Draw/write the reaction mechanism
Draw the energy diagram
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Elimination reactions involve the loss of elements from the starting material to form a new pi bond in the product.
General Features of Elimination
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Equations [1] and [2] illustrate examples of elimination reactions. In both reactions a base removes the elements of an acid, HX, from the organic starting material.
General Features of Elimination
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Removal of the elements HX is called dehydrohalogenation.
Dehydrohalogenation is an example of elimination. The curved arrow formalism shown below illustrates
how four bonds are broken or formed in the process.
General Features of Elimination
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The most common bases used in elimination reactions are negatively charged oxygen compounds, such as HO and its alkyl derivatives, RO, called alkoxides.
General Features of Elimination
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To draw any product of dehydrohalogenation
Find the carbon. Identify ALL carbons with H atoms. Remove the elements of H and X form the and carbons and form a pi bond.
General Features of Elimination
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9
Recall that the double bond of an alkene consists of a bond and a pi bond.
Alkene: The Products of Elimination
Alkene: The Products of Elimination
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Alkenes are classified according to the number of carbon atoms bonded to the carbons of the
double bond.
Classification of Alkenes
Fig. Classifying alkenes by the number
of R groups bonded to the double bond
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Recall that rotation about double bonds is restricted.
Alkene: The Products of Elimination
Figure 8.2 Rotation around C-C and C=C compared
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Because of restricted rotation, two stereoisomers of 2-butene are possible. cis-2-Butene and trans-2-butene are diastereomers, because they are stereoisomers that are not mirror images of each other.
Alkene: The Products of Elimination
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Whenever the two groups on each end of a carbon-carbon double bond are different from each other, two diastereomers are possible.
Alkene: The Products of Elimination
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In general, trans alkenes are more stable than cis alkenes because the groups bonded to the double bond carbons are further apart, reducing steric interactions.
Alkene: The Products of Elimination
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The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases.
The higher the percent s-character, the more readily
an atom accepts electron density. Thus, sp2 carbons
are more able to accept electron density and sp3
carbons are more able to donate electron density.
Consequently, increasing the number of electron
donating groups on a carbon atom able to accept
electron density makes the alkene more stable.
Alkene: The Products of Elimination
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trans-2-Butene (a disubstituted alkene) is more stable than cis-2-butene (another disubstituted alkene), but both are more stable than 1-butene (a monosubstituted alkene).
Alkene: The Products of Elimination
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An elimination involves the
loss of two atoms or groups
from the substrate, usually
with formation of a pi bond.
Depending on the reagents and conditions
involved, an elimination might be a first order
(E1) or second-order (E2) process.
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There are TWO mechanisms of
eliminationE2 and E1, just as there are two mechanisms of substitution, S
N2 and S
N1.
E2 mechanismbimolecular elimination E1 mechanismunimolecular elimination
The E2 and E1 mechanisms differ in the
timing of bond cleavage and bond
formation, analogous to the SN2 and S
N1
mechanisms.
E2 and SN2 reactions have some features
in common, as do E1 and SN1 reactions.
Mechanisms of Elimination
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The most common mechanism for dehydrohalogenation is the E2 mechanism.
It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation i.e.
rate = k[(CH3)3CBr][OH]
The reaction is concertedall bonds are broken and formed in a single step.
Mechanisms of EliminationE2
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Mechanisms of EliminationE2
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Mechanisms of EliminationE2
An energy diagram for the E2 reaction
(CH3)3CBr + OH (CH3)2C=CH2 + H2O + Br
In the transition state, the CH and CBr bonds are partially broken, The OH and pi bonds are partially formed, and both the base and the departing leaving group bear a partial negative charge.
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There are close parallels between E2 and SN2 mechanisms in how the identity of the base, the leaving group and the solvent affect the rate.
The base appears in the rate equation, so the rate of the E2 reaction increases as the strength
of the base increases. E2 reactions are generally run with strong,
negatively charged bases like OH and OR.
Mechanisms of EliminationE2
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Mechanisms of EliminationE2
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The SN2 and E2 mechanisms differ in how the R group affects the reaction rate.
Mechanisms of EliminationE2
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Increasing the number of R groups on the carbon with the leaving group forms more highly substituted, more stable alkenes in E2 reactions.
In the reactions below, since the disubstituted alkene is more stable, the 3 alkyl halide reacts faster than the 1 alkyl halide.
Mechanisms of EliminationE2
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Recall that when alkyl halides have two or more different carbons, more than one alkene product is formed.
When this happens, one of the products usually predominates.
The major product is the more stable productthe one with the more substituted double bond.
This phenomenon is called the Zaitsev rule.
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The Zaitsev rule: the major product in elimination has the more substituted double bond.
A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one
is possible. Thus, the E2 reaction is regioselective.
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When a mixture of stereoisomers is possible from a dehydrohalogenation, the major product is the more stable stereoisomer.
A reaction is stereoselective when it forms predominantly or exclusively one stereoisomer
when two or more are possible. The E2 reaction is stereoselective because one stereoisomer is formed preferentially.
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The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)2C=CH2 can be used to illustrate the second general mechanism of elimination, the E1 mechanism.
An E1 reaction exhibits first-order kinetics:
rate = k[(CH3)3CI]
The E1 reaction proceed via a two-step mechanism: the bond to the leaving group breaks first
before the pi bond is formed. The slow step is unimolecular, involving only
the alkyl halide.
Mechanisms of EliminationE1
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The E1 and E2 mechanisms both involve
the same number of bonds broken and
formed.
The only difference is TIMING. In an E1, the leaving group comes off before
the proton is removed, and the reaction occurs in TWO steps.
In an E2 reaction, the leaving group comes off as the proton is removed, and the reaction occurs in ONE step.
Mechanisms of EliminationE1
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Mechanisms of EliminationE1
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Mechanisms of EliminationE1
An energy diagram for the E1 reaction
(CH3)3CI + OH (CH3)2C=CH2 + H2O + I
Since the E1 mechanism has two steps, there are two energy barriers. Step (1) is rate-determining; Ea[1]>Ea[2]
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The rate of an E1 reaction increases as the number of R
groups on the carbon with the leaving group increases.
The strength of the base usually determines whether a
reaction follows the E1 or E2 mechanism. Strong bases
like OH and OR favor E2 reactions, whereas weaker
bases like H2O and ROH favor E1 reactions.
Mechanisms of EliminationE1
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E1 reactions are regioselective, favoring formation of the more substituted, more stable alkene.
Zaitsevs rule applies to E1 reactions also.
Mechanisms of EliminationE1
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SN1 and E1 reactions have exactly the same first stepformation of a carbocation. They differ in what happens to the carbocation.
Because E1 reactions often occur with a competing S
N1 reaction, E1 reactions of alkyl
halides are much less useful than E2 reactions.
SN1 and E1 Reactions
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The strength of the base is the most important factor in determining the mechanism for elimination.
Strong bases favor the E2 mechanism. Weak bases favor the E1 mechanism.
When is the Mechanism E1 or E2?
Summary
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A single elimination reaction produces a pi bond of an alkene.
Two consecutive elimination reactions produce two pi bonds of an alkyne.
E2 Reactions and Alkyne Synthesis
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Two elimination reactions are needed to remove two moles of HX from a dihalide substrate.
Two different starting materials can be useda vicinal
dihalide or a geminal dihalide.
E2 Reactions and Alkyne Synthesis
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Stronger bases are needed to synthesize alkynes by dehydrohalogenation than are needed to synthesize alkenes.
The typical base used is NH2 (amide), used as the sodium
salt of NaNH2. KOC(CH3)3 can also be used with DMSO as solvent.
E2 Reactions and Alkyne Synthesis
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E2 Reactions and Alkyne Synthesis
Example of dehydrohalogenation of dihalides to afford alkynes
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Elimination Reactions
Promoting Factors E1 E2
Base Weak base works Strong base requires
Solvent Good ionising Wide variety
Substrate 3>2 3>2>1
Leaving group Good one required
Characteristics
Kinetics 1st order 2nd order
Orientation Most highly substituted alkene
Stereochemistry No special geometry
Rearrangement common impossible