Chapter 8 Alkene Elimination Reactions '13 BW Edit

8/13/2019 Chapter 8 Alkene Elimination Reactions '13 BW Edit

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Elimination Reactions

Elimination reactions often compete with substitution reactions.

What are the two main ingredients for a substitution?

A nucleophile and an electrophile with a leaving group

What are the two main ingredients for an elimination?

A base and an electrophile with a leaving group

How is a base both similar and different from a nucleophile?

Ethylene and Propylene

Ethylene and Propylene Ethylene and Propylene



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8.2 Alkenes: Nomenclature by IUPAC

Recall:

1. Leaving groups: Identify and name the parent chain.

2. Identify the name of the substituents (side groups).

3. Assign a locant (number) to each substituent.

4. Assemble the name alphabetically.

Alkenes: Nomenclature by IUPAC

Recall:






Recall:






Recall:



3. Assign a locant (number) to each substituent: Give the C=C double bondthe lowest number possible.



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Recall:



3. Assign a locant (number) to each substituent: Give the C=C double bondthe lowest number possible.


Alkenes: Common Names

Alkenes: Common Names The Degree of Substitution

Stability of Alkenes Stereoisomerism in Alkenes: Not Exhibiting Free Rotation


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Stereoisomerism in Alkenes Stereoisomerism in Alkenes

Bredts rule:

A bridgehead carbon of a bicyclic system cannot possesses a C=C double bond if it

involves a trans pi-bond being incorporated in a small ring.

Stereoisomerism in Alkenes: Not Exhibiting Free Rotation E and Z Designations

E and Z Designations

1. Prioritize the groups attached to the C=C double bond based on atomic number.

2. If the top priority groups are on the same side of the C=C double bond, it is Z

(for zusammen, which means together).

3. If the top priority groups are on opposite sides of the C=C double bond, it is E

(for entgegen, which means opposite).

E and Z Designations

1. Prioritize the groups attached to the C=C double bond based on atomic number.

2. If the top priority groups are on the same side of the C=C double bond, it is Z

(for zusammen, which means together).

3. If the top priority groups are on opposite sides of the C=C double bond, it is E

(for entgegen, which means opposite).


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8.5 Alkene Stability

Because of steric strain, cis isomers are generally less stable than trans .

Stability Measurement: Heat of Combustion

Relative Stability: Isomeric Alkenes Stability: Electron Donation of Alkyl Hyperconjugation

The delocalization of electron

density by hyperconjucation

is stabilizing effect!

8.6 Elimination Reactions Recall Four Patterns


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Two Patterns in Elimination Reactions

E2: A concerted mechanism

E1: A stepwise mechanism







8.7 E2


E2: Example


E2: Example



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E2: Kinetics


The kinetics of E2 and S N2 are quite similar!

E2


E2: Effect of Substrate

Tertiary substrates are unreactive toward

S N2 while they react readily by E2.

E2: Effect of Substrate

! ! !

E2: Effect of Substrate E2: Energy Diagram

Look at TS-->


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E2: Transition State

! The 3 substrate should proceed through a more stable transition state(kinetically favored) and a more stable product (thermodynamically favored).

Stability: Electron Donation of Alkyl Hyperconjugation

The delocalization of electron

density by hyperconjucation

is stabilizing effect!





H H

H H

H H

H3C H3C

H

H H

H3C (1) (2)

(3)



H

H

H H

H H

H3C

H3C

H

H H

H3C (1) (2)

(3)



H

H

H H

H H

H3C

H3C

H

H H

H3C (1) (2)

(3)


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H

H

H H

H H

H3C

H3C

H

H H

H3C (1) (2)

(3)

E2: Not Very Sensitive to Steric Hindrance #

E2: Regioselectivity E2: Regioselectivity

E2: Regioselectivity Sterically Less Hindered Bases

! Zaitsev product: the more substituted alkene

! Hofmann product: the less substituted alkene

E2: Regioselectivity Sterically Hindered Bases

The regiochemistry of the E2 reaction can be controlled by choosing the base.


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E2: Regioselectivity Sterically Hindered Bases

Examples of bulky bases

E2: Stereoselectivity E / Z Selection

E2: Stereoselectivity E / Z Selection Recall Hammond Postulate E2: Stereospecicity

Only product No other possible stereoisomers

E2: Stereospecicity

! To rationalize the stereospecicity of the reaction, consider the alignment ofthe orbitals in the transition state for the reaction.

! In the transition state, the CH and CBr bonds that are breaking must berotated into the same plane as the pi bond that is forming.

E2: Stereospecicity


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E2: Stereospecicity E2: Stereospecicity

E2: Stereospecicity

! Evidence suggests that a strict 180 angle is not necessary for E2mechanisms.

! Similar angles (175-179) are sufcient. ! The term, anti-periplanar is generally used instead of anti-coplanar to account

for slight deviations from coplanarity.

Anti-Periplanar

The stereoisomeric product of an E2 process depends on the congurationof the starting alkyl halide.

E2: Stereospecicity

! Evidence suggests that a strict 180 angle is not necessary for E2mechanisms.

! Similar angles (175-179) are sufcient. ! The term, anti-periplanar is generally used instead of anti-coplanar to account

for slight deviations from coplanarity.

Anti-Periplanar

The stereoisomeric product of an E2 process depends on the congurationof the starting alkyl halide.

E2: Stereospecicity Anti -Periplanar E2: Stereospecicity Anti -Periplanar


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Cl

Base

+ HBase2.

The rate of E2 reaction is greatly affected by theamount of time that the leaving group spends inan axial position. Which is more reactive?

3.

Me Me Me

ClMe ClMe

vs.

E2: Stereospecicity on Substituted Cyclohexanes (Take-Home Quiz)

Anti-Periplanar

+

Cl

Base

+ HBase2.


3.

Me Me Me

ClMe ClMe

vs.


Anti-Periplanar

+


3.

ClMe ClMe

vs.

Neomenthyl chloride Menthyl chloride


Anti-Periplanar


3.

ClMe ClMe

vs.



Anti-Periplanar

200 more reactive


3.

ClMe ClMe

vs.



Anti-Periplanar

to the board

200 more reactive

Due by next Tuesday

8.8 E2: Acyclic Substrate


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8.8 E2: Acyclic Substrate with the Less Sterically Hindered Base 8.8 E2: Acyclic Substrate Newman Projection Is Helpful!

8.8 E2: Acyclic Substrate Newman Projection Is Helpful! 8.8 E2: Acyclic Substrate Newman Projection Is Helpful!

Zaitsev Hofmann

8.9 E1

cf. slide 32 (E2)

8.9 E1


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E1 E1

E1 E1

E1: 1. Effect of Substrate Similar with S N1 E1: 1. Effect of Substrate

Because E1 and S N1 proceed by the same rst step, their competition willgenerally result in a mixture of products


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E1: 1. Effect of Substrate

Because E1 and S N1 proceed by the same rst step, their competition willgenerally result in a mixture of products

E1: 2. Regioselectivity Compare to E2

The regiochemical outcome of an E2 reaction can be controlled by the base (sterically hindered or not sterically hindered)

E1. Regioselectivity Generally the Zaitsev Product

Generally E1 gives the more substituted alkene (Zaitsev product)

E1 reactions are not stereospecic that is, they are not required anti-periplanarity in order for the reaction to occur.

Nevertheless, E1 reactions are stereoselective (E vs Z ).









E1. 3. Drawing the Mechanism


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S N1 process

E1 process


E1. 3. Drawing the Mechanism E1. 3. Drawing the Mechanism



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In fact,


In fact,

8.11 E2: Drawing the Mechanism


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8.12 Substitution vs. Elimination #

Substitution and elimination are always in competition.

Sometimes, products are only observed from either substitution orelimination.

Sometimes a mixture of products is observed.

Substitution vs. Elimination #








8.12 Substitution vs. Elimination #

To predict whether substitution or elimination will predominate, consider thefactors below:

1. Determine the function of the reagent. Is it more likely to act as abase, a nucleophile, or both?

a. Kinetics control nucleophilicity. (the rate, good at reacting)b. Thermodynamics control basicity. (the position of equilibrium)

2. Analyze the substrate and predict the expected mechanism (S N1,S N2, E1, or E2).

3. Consider relevant regiochemical and stereochemical requirements.

Substitution vs. Elimination: Reagent #



a. Kinetics control nucleophilicity. (the rate, good at reacting)

b. Thermodynamics control basicity. (the position of equilibrium)











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3. Consider relevant regiochemical and stereochemical requirements. Determine the function of the reagent: assess the strength of a nucleophile:

! The greater the negative charge, the more nucleophilic it is likely to be.

! The more polarizable it is, the more nucleophilic it should be.

! The less sterically hindered it is, the more nucleophilic it should be.







Substitution vs. Elimination: Reagent






Determine the function of the reagent: assess the strength of a base:

! Quantitatively, using the p K a of its conjugate acid ! Qualitatively, using ARIO (atom, resonance, induction, orbital)

Classication of Common Reagents Used for Substition/Elimination


Highly polarizable,

weak bases (very strongconjugate acids)


Highly polarizable,


Very low polarizability,sterically hindered


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Classication of Common Reagents Used for Substition/Elimination Classication of Common Reagents Used for Substition/Elimination

DBN


Highly polarizable,



Stron ger reagents,

promote S N2 or E2


Highly polarizable,



Stron ger reagents,

promote S N2 or E2


Highly polarizable,



Stronger reagents,

promote S N2 or E2

Weaker reagents,

promote S N1 or E1









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Identifying the Mechanism(s): 1. Reagent and 2. Substrate Identifying the Mechanism(s): 1. Reagents: Nucleophiles (only)

Identifying the Mechanism(s): 1. Reagents: Nucleophiles (only) Identifying the Mechanism(s): 1. Reagents: Nucleophiles (only)

Identifying the Mechanism(s): 1. Reagents: Nucleophiles (only)

" The rate enhance-ment by using polaraprotic solvent

Identifying the Mechanism(s): 1. Reagents: Bases (only)

" E2 mechanism is notsensitive to steric hindrance.


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Identifying the Mechanism(s): 1. Reagents: Bases (only)


1. Reagents: Strong Nucleophile and Strong Bases Competition


1. Reagents: Strong Nucleophile and Strong Bases Competition 1. Reagents: Strong Nucleophile and Strong Bases Competition

1. Reagents: Strong Nucleophile and Strong Bases Competition 1. Reagents: Strong Nucleophile and Strong Bases Competition


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1. Reagents: Weak Nucleophile and Strong Bases Competition

at high temperature, E1

Well..

Not to just memorize, but understand/digest







Regio- and Stereochemical Outcome of Substitution and Elimination Reactions

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-12 34562781962 C286:52= >1262:D93E EC748 B9>1 C:52M9N:@73G


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. / 0 -12 34562781962 :;:512 ?87=9@73A B12C2 >12 62:D93EEC748 9= 573325>2FG

-12 34562781962 C286:52= >1262:D93E EC748 B9>1 93D2C=973 7H573IE4C:@73G

. / J -12 34562781962 :;:512

5:CK75:@73A B1951 9= B12C2 >1262:D93E EC748 B:= 7C9E93:66L573325>2FA 4362== : 5:CK75:@73C2:CC:3E2M23> >77< 86:52G

-12 34562781962 C286:52= >12

62:D93E EC748 B9>1 C:52M9N:@73G


!"#$%&'"($&)* %,-&%(" .-"!"%&'"($&)* %,-&%("

"0 -12 O:9>=2D 8C7F45> 9= E232C:66LH:D7C2F 7D2C >12 '7HM:33 8C7F45>A4362== : =>2C95:66L 193F2C2F K:=2 9=4=2FA 93 B1951 5:=2 >12 '7HM:338C7F45> B966 K2 H:D7C2F

-19= 8C752== 9= K7>1 =>2C27=2625@D2:3F =>2C27=8259I5GP123 :88695:K62A : !"#$% F9=4K=@>4>2F:6G

-12 8C752== 9= =>2C27=2625@D2G P123:88695:K62A : !"#$% F9=4K=@>4>2F :6


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"0 -12 O:9>=2D 8C7F45> 9= E232C:66LH:D7C2F 7D2C >12 '7HM:33 8C7F45>A4362== : =>2C95:66L 193F2C2F K:=2 9=

4=2FA 93 B1951 5:=2 >12 '7HM:338C7F45> B966 K2 H:D7C2F

-19= 8C752== 9= K7>1 =>2C27=2625@D2:3F =>2C27=8259I5GP123 :88695:K62A : !"#$% F9=4K=@>4>2F

:6G

-12 8C752== 9= =>2C27=2625@D2G P123:88695:K62A : !"#$% F9=4K=@>4>2F :6A4362== : =>2C95:66L 193F2C2F K:=2 9=

4=2FA 93 B1951 5:=2 >12 '7HM:338C7F45> B966 K2 H:D7C2F

-19= 8C752== 9= K7>1 =>2C27=2625@D2:3F =>2C27=8259I5GP123 :88695:K62A : !"#$% F9=4K=@>4>2F

:6G

-12 8C752== 9= =>2C27=2625@D2G P123:88695:K62A : !"#$% F9=4K=@>4>2F :6A4362== : =>2C95:66L 193F2C2F K:=2 9=4=2FA 93 B1951 5:=2 >12 '7HM:338C7F45> B966 K2 H:D7C2F


-12 8C752== 9= =>2C27=2625@D2G P123:88695:K62A : !"#$% F9=4K=@>4>2F :6A4362== : =>2C95:66L 193F2C2F K:=2 9=4=2FA 93 B1951 5:=2 >12 '7HM:338C7F45> B966 K2 H:D7C2F


-12 8C752== 9= =>2C27=2625@D2G P123:88695:K62A : !"#$% F9=4K=@>4>2F :6


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Lets Solve this Problem Together! Lets Solve this Problem Together!

Lets Solve this Problem Together! Lets Solve this Problem Together!

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Chapter 8 Alkene Elimination Reactions '13 BW Edit

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