3 Eliminasi Student Edition PDF

8/12/2019 3 Eliminasi Student Edition PDF

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MEKANISME KIMIA ORGANIK

Elimination Reactions

Elimination reactions involve the loss of elements fromthe starting material to form a new bond in the product.

General Features of Elimination

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Equations [1] and [2] illustrate examples of eliminationreactions. In both reactions a base removes theelements of an acid, HX, from the organic startingmaterial.



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Removal of the elements HX is calleddehydrohalogenation.

Dehydrohalogenation is an example of elimination.

The curved arrow formalism shown below illustrateshow four bonds are broken or formed in the process.


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The most common bases used in elimination reactionsare negatively charged oxygen compounds, such as HOand its alkyl derivatives, RO, called alkoxides.



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To draw any product of dehydrohalogenationFind thecarbon. Identify all carbons with H atoms. Remove

the elements of H and X from the and carbons andform a bond.


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Alkenes are classi fied accord ing to the number ofcarbon atoms bonded to the carbons of the double bond.

AlkenesThe Products of Elimination

Classifying alkenes by the

number of R groups bonded

to the double bond


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Whenever the two groups on each end of a carbon-carbon double bond are different from each other, twodiastereomers are possible.


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In general, trans alkenes are more stable than cisalkenes because the groups bonded to the double bondcarbons are further apart, reducing steric interactions.



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The stability of an alkene increases as the number of Rgroups bonded to the double bond carbons increases.


The higher the percent s-character, the more readily an atomaccepts electron density. Thus, sp2 carbons are more able toaccept 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 densitymakes the alkene more stable.

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trans-2-Butene (a disubstituted alkene) is more stablethan cis-2-butene (another disubstituted alkene), butboth are more stable than 1-butene (a monosubstitutedalkene).



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There are two mechanisms of eliminationE2 and E1,just as there are two mechanisms of substi tution, SN2and SN1.

E2 mechanismbimolecular elimination

E1 mechanismunimolecular elimination

The E2 and E1 mechanisms differ in the timing of bondcleavage and bond formation, analogous to the SN2 andSN1 mechanisms.

E2 and SN2 reactions have some features in common, asdo E1 and SN1 reactions.

Mechanisms of Elimination

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The most common mechanism for dehydrohalogenationis the E2 mechanism.

It exhibits second-order kinetics, and both the alkylhalide and the base appear in the rate equation, i.e.,

Mechanisms of EliminationE2

rate = k[(CH3)3CBr][OH]

The reaction is concertedall bonds are broken andformed in a sing le step.


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An energy diagram

for an E2 reaction:


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There are close parallels between E2 and SN2 mechanisms inhow the identity of the base, the leaving group and thesolvent affect the rate.

The base appears in the rate equation, so the rate of the E2reaction increases as the strength o f the base increases.

E2 reactions are generally run with strong, negatively chargedbases likeOH and OR. Two strong sterically hinderednitrogen bases called DBN and DBU are also sometimesused.


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The SN2 and E2 mechanisms differ in how the R groupaffects the reaction rate.


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The increase in E2 reaction rate with increasing alkylsubstitution can be rationalized in terms of transition statestability.

In the transition state, the double bond is partially formed.Thus, increasing the stability of the double bond with alkylsubstituents stabilizes the transition state (i.e., lowers Ea,which increases the rate of the reaction.



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Increasing the number of R groups on the carbon with theleaving group forms more highly substituted, more stablealkenes in E2 reactions.

In the reactions below, since the disubstituted alkene is morestable, the 3 alkyl halide reacts faster than the 10 alkyl halide.


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Table 8.2 summarizes the characteristics of the E2 mechanism.



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Recall that when alkyl halides have two or more differentcarbons, more than one alkene product is fo rmed.

When this happens, one of the products usuallypredominates.

The major product is the more stable productthe one withthe more substituted double bond.

This phenomenon is called the Zaitsev rule.

The Zaitsev (Saytzeff) Rule

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The Zaitsev rule: the major product in elimination has the moresubstituted double bond.

A reaction is regioselective when it yields predominantly orexclusively one constitutional isomer when more than one ispossible. Thus, the E2 reaction is regioselective.



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When a mixture of stereoisomers is possible from adehydrohalogenation, the major product is the more stablestereoisomer.

A reac tion is stereoselective when it forms predominantly orexclusively one stereoisomer when two or more are possible.

The E2 reaction is stereoselective because one stereoisomeris formed preferentially.


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The dehydrohalogenation of (CH3)3CCI with H2O to form(CH3)2C=CH2 can be used to illustrate the second generalmechanism of elimination, the E1 mechanism.

An E1 react ion exhibi ts fi rs t-order kinet ics:


rate = k[(CH3)3CCI]

The E1 reaction proceeds via a two-step mechanism: thebond to the leaving group breaks first before the bond isformed. The slow step is unimolecular, involving only thealkyl halide.

The E1 and E2 mechanisms both involve the same number ofbonds broken and formed. The only dif ference is timing. In anE1, the leaving group comes off before the proton isremoved, and the reaction occurs in two steps. In an E2reaction, the leaving group comes off as the proton isremoved, and the reaction occurs in one step.


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Energy diagram

for an E1 reaction:


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The rate of an E1 reaction increases as the number of R groupson the carbon with the leaving group increases.


The strength of the base usually determines whether a reactionfollows the E1 or E2 mechanism. Strong bases like OH and ORfavor E2 reactions, whereas weaker bases like H2O and ROHfavor E1 reactions.

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E1 reactions are regioselective, favoring formation of themore substituted, more stable alkene.

Zaitsevs rule applies to E1 reactions also.



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Table 8.3 summarizes the characteristics of the E1 mechanism.


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SN1 and E1 reactions have exactly the same first stepformationof a carbocation. They differ in what happens to the carbocation.

SN1 and E1 Reactions

Because E1 reactions often occur with a competing SN1 reaction,E1 reactions of alkyl halides are much less useful than E2reactions.


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The transition state of an E2 reaction consists of four atoms

from an alkyl halideone hydrogen atom, two carbon atoms,and the leaving group (X)all aligned in a plane. There are twoways for the CH and CX bonds to be coplanar.

Stereochemistry of the E2 Reaction

E2 elimination occurs most often in the anti periplanar geometry.This arrangement allows the molecule to react in the lowerenergy staggered conformation, and allows the incoming baseand leaving group to be further away from each other.

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Two possible geometriesfor the E2 reaction


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The stereochemical requirement of an anti periplanar geometry

in an E2 reaction has important consequences for compoundscontaining six-membered rings.

Consider chlorocyclohexane which exists as two chairconformations. Conformation A is preferred since the bulkier Clgroup is in the equatorial position.


For E2 elimination, the C-Cl bond must be anti periplanar to theCH bond on a carbon, and this occurs only when the H andCl atoms are both in the axial position. The requirement for transdiaxial geometry means that elimination must occur from theless stable conformer, B.

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The trans diaxial geometry

for the E2 elimination in

chlorocyclohexane


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Now consider the E2 dehydrohalogenation of cis- and trans-1-

chloro-2-methylcyclohexane.


This cis isomer exists as two conformations, A and B, each ofwhich as one group axial and one group equatorial. E2 reactionmust occur from conformation B, which contains an axial Clatom.

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Because conformation B has two different axialhydrogens, labeled Ha and Hb, E2 reaction occurs in twodifferent directions to afford two alkenes.

The major product contains the more stabletrisubstituted double bond, as predicted by the Zaitsevrule.



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The trans isomer of 1-chloro-2-methylcyclohexane existsas two conformers: C, having two equatorial substituents,and D, having two axial substituents.


E2 reaction must occur f rom D, since it contains an axial Clatom.

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Because conformer D has only one axial H, the E2reaction occurs only in one direction to afford a singleproduct . This is not predicted by the Zaitsev rule.



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The strength of the base is the most important factor indetermining the mechanism for elimination. Strongbases favor the E2 mechanism. Weak bases favor the E1mechanism.

When is the Mechanism E1 or E2?

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Zaitsev's rule

Alexander Mikhaylovich Zaytsev

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