elimination reaction...4 elimination reactions •elimination reactions involve the loss of elements...

Elimination Reaction

4

Elimination Reactions

• Elimination reactions involve the loss of elements from

the starting material to form a new bond in the product.

General Features of Elimination

5

• 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.

Example of Elimination reaction

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• Removal of the elements HX is called

dehydrohalogenation.

• Dehydrohalogenation is an example of elimination.

• The curved arrow shown below illustrates how four

bonds are broken or formed in the process.

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 from

the and carbons and form a bond.

Example 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.

Common bases in elimination reaction

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• The double bond of an alkene consists of a bond and a

bond.

Alkenes—The Products of Elimination

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Rotation about double bonds is restricted.

The rotation of bonds in the

Elimination Reaction

Figure 8.2

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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.

The Products of Elimination Reaction (Cis , Trans)

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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

Figure 8.1Classifying alkenes by the

number of R groups bonded

to the double bond

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• The stability of an alkene increases as the number of R

groups bonded to the double bond carbons increases.

• 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.

The stability of different type of alkenes

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• trans-2-Butene (a disubstituted alkene) is more stable

than cis-2-butene, but both are more stable than 1-

butene (a monosubstituted alkene).

The stability of isomers of alkenes

Mechanisms of Elimination

• There are two mechanisms

of elimination—

E2 and E1, just as there are

two mechanisms of

substitution, SN2 and SN1.

• E2 mechanism—

bimolecular elimination

• E1 mechanism—

unimolecular elimination

• The E2 and E1 mechanisms

differ in the timing of bond

cleavage and bond

formation, analogous to the

SN2 and SN1 mechanisms.

• E2 and SN2 reactions have

some features in common,

as do E1 and SN1 reactions.

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E1 and E2 mechanism

• The E1 and E2 mechanisms both involve

the same number of bonds broken and

formed. The only difference is timing.

• In an E1 mechanism, the leaving group

comes off before the proton is removed,

and the reaction occurs in two steps.

• In an E2 mechanism , the leaving group

comes off as the proton is removed, and

the reaction occurs in one step.

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E2 mechanism

Bimolecular Elimination

• The most common

mechanism for

dehydrohalogenation

is the E2 mechanism.

• The reaction , all

bonds are broken and

formed in a single

step.

• 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]

17

Summarizes the characteristics of

the E2 mechanism.

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E2 mechanism


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E2 mechanism


• 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.

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E2 mechanism


21

E2 mechanism


• The SN2 and E2

mechanisms

differ in how

the R group

affects the

reaction rate

22

23

Figure 8.3An energy diagram

for an E2 reaction:

E2 mechanism


E2 mechanism


• The increase in E2 reaction

rate with increasing alkyl

substitution can be

rationalized in terms of

transition state stability.

• In the transition state, the

double bond is partially

formed. Thus, increasing

the stability of the double

bond with alkyl

substituents stabilizes the

transition state (i.e., lowers

Ea, which increases the rate

of the reaction. 24

E2 mechanism

Bimolecular Elimination• 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 10

alkyl halide25

E1 mechanism

Unimolecular Elimination

• The dehydrohalogenation of (CH3)3CCI 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:

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rate = k[(CH3)3CCI]

E1 mechanism


•The E1 reaction proceeds via a two-

step mechanism: the bond to the

leaving group breaks first before

the bond is formed. The slow step

is unimolecular, involving only the

alkyl halide.

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E1 mechanism


28

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Figure 8.6Energy diagram

for an E1 reaction:

E1 mechanism


E1 mechanism


• 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.30

Summarizes the characteristics

of the E1 mechanism

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SN1 and E1 Reactions

• SN1 and E1 reactions have exactly

the same first step—formation of a

carbocation. They differ in what

happens to the carbocation.

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Comparison between the SN1 and

E1 mechanism

• Because E1

reactions often

occur with a

competing SN1

reaction, E1

reactions of alkyl

halides are much

less useful than E2

reactions.33

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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?

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• A single elimination reaction produces a bond of an

alkene. Two elimination reactions produce two 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 used—a vicinal

dihalide or a geminal dihalide.


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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.


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In the aldol reaction condensation, two

molecules of an aldehyde or ketone react

with each other in the presence of a base to

form a -hydroxy carbonyl compound

The Aldol Condinsation

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The Aldol Reaction• These examples illustrate the general features of the

aldol reaction. The carbon of one carbonyl component

becomes bonded to the carbonyl carbon of the other

component.

Carbonyl Condensation Reactions

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The Aldol Reaction• The mechanism of the aldol reaction occurs in three

steps.


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The Aldol Reaction

• The characteristic reaction of aldehydes and ketones is

nucleophilic addition. An aldol reaction is a nucleophilic

addition in which an enolate is the nucleophile.


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The Aldol Reaction• A second example of an aldol reaction is shown with

propanal as the starting material. The two molecules of

the aldehyde that participate in the aldol reaction react

in opposite ways.


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The Aldol Reaction—Dehydration of the Aldol Product

• Under the basic reaction conditions, the initial aldol

product is often not isolated. Instead, it loses the

elements of H2O from the and carbons to form an

,-unsaturated carbonyl compound.


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The Aldol Reaction• The mechanism of dehydration consists of two steps:

deprotonation followed by loss of ¯OH.


46

Crossed Aldol Reactions• Sometimes it is possible to carry out an aldol reaction between

two different carbonyl compounds. Such reactions are called

crossed or mixed aldol reactions.


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The Claisen Reaction• In the Claisen reaction, two molecules of an ester react

with each other in the presence of an alkoxide base to

form a -keto ester.

• Unlike the aldol reaction which is base-catalyzed, base

is needed to deprotonate the -keto ester formed in

Step [3] of the Claisen reaction.

• Note that because esters have a leaving group on the

carbonyl carbon, loss of the leaving group occurs to

form the product of substitution, not addition.


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The Claisen Reaction• Keep in mind that the characteristic reaction of esters is

nucleophilic substitution. A Claisen reaction is a

nucleophilic substitution in which an enolate is the

nucleophile.


Figure 24.6The Claisen reaction—An

example of nucleophilic

substitution

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The Claisen Reaction


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The Crossed Claisen :

• Like the aldol reaction, it is sometimes possible to carry out

a Claisen reaction with two different carbonyl components

as starting materials.

• A Claisen reaction between two different carbonyl

compounds is called a crossed Claisen reaction.

• A crossed Claisen is synthetically useful in two different

instances:

[1] Between two different esters when only one has

hydrogens, one product is usually formed.


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The Crossed Claisen

[2] Between a ketone and an ester—the enolate is always

formed from the ketone component, and the reaction

works best when the ester has no hydrogens.

The product of this crossed Claisen reaction is a -

dicarbonyl compound, not a -keto ester.


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The Crossed Claisen

Reaction [2] is noteworthy because it provides easy

access to -ketoesters, which are useful starting

materials in the acetoacetic ester synthesis.

In this reaction, Cl¯ is eliminated rather than ¯OEt in Step

[3] because Cl¯ is a better leaving group, as shown in the

following steps.


elimination reaction...4 elimination reactions •elimination reactions involve the loss of elements...

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