ch13 - 호환성 모드 - Konkukhome.konkuk.ac.kr/~parkyong/Classes/ch13.pdf · 7 O _ O _ _ O _ O _ E N E R G Y MAJOR CONTRIBUTORS MINOR CONTRIBUTORS RESONANCE HYBRID UNEQUAL CONTRIBUTIONS

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Created byProfessor William Tam & Dr. Phillis Chang

Chapter 13

Conjugated UnsaturatedSystems

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.

© 2014 by John Wiley & Sons, Inc. All rights reserved.

1. Introduction A conjugated system involves at least

one atom with a p orbital adjacent to at least one bond● e.g.

O

conjugateddiene

allylicradical

allylic cation

allylicanion

enone enyne


2

2A. Molecular Orbital Description of the Allyl Radical

2. The Stability of the Allyl Radical

© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.


2B. Resonance Description of the Allyl Radical

12

3 12

3

1

23

1

2

3


3

RHINOCEROS

DRAGON UNICORN

real

mythical

WHELAND’S ANALOGY FROM HIS 1955 BOOK“Resonance in Organic Chemistry”

© 2014 Pearson Education, Inc.

Chapter 6

• Move a single nonbonding electron toward a (forming) bond

Relative order of carbocation stability3. The Allyl Cation


In writing resonance structures, we are only allowed to move electrons

HH

resonance structures

not resonance structures© 2014 by John Wiley & Sons, Inc. All rights reserved.

4. Resonance Theory Revisited

4

All of the structures must be proper Lewis structures

O O: : 10 electrons!Xnot a proper

Lewis structure


All resonance structures must have the same number of unpaired electrons

X


All atoms that are part of the delocalized -electron system must lie in a plane or be nearly planar

no delocalizationof -electrons

delocalizationof -electrons


5

The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid

(3o allylic cation)greater contribution

(2o allylic cation)


4B. How to Estimate the Relative Stability of Contributing Resonance Structures

The more covalent bonds a structure has, the more stable it is

(more stable) (less stable)

O O

(more stable) (less stable)© 2014 by John Wiley & Sons, Inc. All rights reserved.

Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid

O O

this carbon has6 electrons this carbon has

8 electrons© 2014 by John Wiley & Sons, Inc. All rights reserved.

Charge separation decreases stability

(more stable) (less stable)

OMe OMe


6

K is More Stable Than J



The Number of Relatively Stable Resonance Contributors is What is Important

Little delocalization energy

Significant delocalization energy

RESONANCE STRUCTURES

REALMOLECULE

ENERGY

the real moleculehas lower energythan any contributingstructure wouldsuggest

RESONANCE LOWERS THE ENERGY OF A MOLECULE OR ION

CO

OO C

O

OO C

O

OO

-

- --

--

7

O_

O

_

O_

O_

O_

ENERGY

MAJOR CONTRIBUTORS

MINORCONTRIBUTORS

RESONANCEHYBRID

UNEQUAL CONTRIBUTIONS OF RESONANCE STRUCTURES

charge oncarbon

charge on oxygen

1,3-Butadiene

(2E,4E)-2,4-Hexadiene

1,3-Cyclohexadiene

12

3

4

12

3

4

5

6

1

2 3

4

56

Alkadienes (“Dienes”)

5. Alkadienes & Polyunsaturated Hydrocarbons



+ _ +_

ENERGY

real molecule

resonancestructures

(resonance hybrid)

Usually the resonance structure having the most covalent bonds and the fewest charges will bethe one that best reflects the nature of the actual molecule.

Unequalcontributors :

is more

important.


Resonance Contributors of 1,3-Butadiene

8

Alkatrienes (“Trienes”)

1

2

3

4

5

6

7

8

(2E,4E,6E)-Octa-2,4,6-triene


Alkadiynes (“Diynes”)1 2 3 4 5 6

2,4-Hexadiyne

1

23

456 1

2

3

4

5 6 7 8

Hex-1-en-5-yne (2E)-Oct-2-en-6-yne

Alkenynes (“Enynes”)


Conjugation


Cumulated Double Bonds

9

Cumulenes

(Allene)(a 1,2-diene)

C C CH

HH

HC C C

H

HH

H


Is a chiral carbon needed? No!

Reflection (in this plane) yields.

Different, not superimposable, enantiomers.

The (distorted) tetrahedral array of the substitutents suffices to allow for enantiomers.

C

Recall allene:

10

Conjugated dienes

Non-conjugated dienes (isolated alkenes)


1

2

3

4

6A. Bond Lengths of 1,3-Butadiene

1.34 Å

1.47 Å

1.54 Å 1.50 Å 1.46 Å

sp3 sp3spsp3sp2

6. 1,3-Butadiene: Electron Delocalization


6B. Conformations of 1,3-Butadiene

(s-cis) (s-trans)

H H

(less stable)

cis

transsinglebond

singlebond


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6C. Molecular Orbitals of 1,3-Butadiene


Conjugated alkadienes are thermo-dynamically more stable than isomeric isolated alkadienes

2 + 2 H2 2 2 x (-127)=-254

H o (kJmol-1)

=-239

Difference 15

+ 2 H2

7. The Stability of Conjugated Dienes


12

The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals

8. Ultraviolet–Visible Spectroscopy


UV and Visible Spectroscopy

8A. The Electromagnetic Spectrum


T

13


* transition

Beer’s lawA = absorbance = molar absorptivityc = concentrationℓ = path length

A = x c x ℓA

c x ℓor =

● e.g. 2,5-Dimethyl-2,4-hexadienemax(methanol) 242.5 nm( = 13,100)


8C. Absorption Maxima for Noncon-jugated and Conjugated Dienes


14


Particle-in-a-box model

Conjugation and max

-carotene - carrots, apricots, sweet potatoes -orange

Lycopene - tomatoes, watermelon, pink grapefruit

15

O OAcetone

Ground state

n

max = 280 nmmax = 15

* Excited state

O

n

max = 324 nm,max = 24

max = 219 nm,max = 3600


Conjugation and max

8D. Analytical Uses of UV–Vis Spectroscopy

UV–Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample

A more widespread use of UV–Vis spectroscopy, however, has to do with determining the concentration of an unknown sample


16

ClH

ClH

1

2

3

4 H Cl25oC

+

(78%)(1,2-Addition)

(22%)(1,4-Addition)

9. Electrophilic Attack on Conju-gated Dienes: 1,4-Addition


(a)

ClH

Mechanism

Cl H + H(a)

H(b)

HX

H+ +

Cl

(b)

ClH

(a)

(b)



Mechanism for the Reaction of a Conjugated Diene

9A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction

+HBr

Br

Br+

(80%)

-80oC

(20%)

(80%)40oC

Br

Br+

(20%)© 2014 by John Wiley & Sons, Inc. All rights reserved.

17


If the Reaction is Irreversible, the Kinetic Product Predominates


If the Reaction is Reversible, the Thermodynamic Product Predominates

Br

Br

40oC, HBr

1,2-Additionproduct

1,4-Additionproduct


Kinetic Versus Thermodynamic Products• Recall that the rate of a reaction is determined by its energy of activation (Ea),

whereas the amount of product present at equilibrium is determined by its stability.


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[4+2]+

(diene) (dienophile) (adduct)

10. The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes



The Mechanism

a pericyclic reaction takes place by a cyclic shift of electrons

a [4+2] cycloaddition reaction

O

CH3CH3

CH3

CH3

CH3

O

CH3

Diene Dienophile A Cyclohexene

Works best if the dienophilehas electron-withdrawinggroups, and the diene haselectron-donating groups.

The HOMO of the dienedonates PUSHES electrons into the LUMO of the dieno-phile PULLS.

EXAMPLE - WITH ELECTRONIC FACTORS

push

pull

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O

O

O

O

O

O

1,3-Butadiene(diene)

Maleicanhydride

(dienophile)

Adduct(100%)

+benzene

100oC

e.g.


10A. Factors Favoring the Diels–AlderReaction

EDGEWG

EDGEWG

+

Type A

● Type A and Type B are normal Diels-Alder reactions

+Type B

EDG

EWG EWG

EDG



The Reactants Can Be Aligned in Two Ways


Which Alignment Gives The Major Product?

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The Answer


Unsymmetrical Reagents: 1,4-Product

EWGEDG

EWGEDG

+

Type C

● Type C and Type D are Inverse Demand Diels-Alder reactions

+Type D

EWG

EDG EDG

EWG


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Relative rate

Diene D.A. cycloadduct+ 30oCO

O

OOMe

> >Diene

t1/2 20 min. 70 min. 4 h.© 2014 by John Wiley & Sons, Inc. All rights reserved.

Relative rate

Dienophile D.A. cycloadduct+ 20oC

> >Dienophile

t1/2 0.002 sec. 20 min. 28 h.

NC CN

NC CN

CN

CN

CN


Steric effects

> >Dienophile:

Relative rate: 1 0.14 0.007

COOEt COOEt COOEt


10B. Stereochemistry of the Diels–Alder Reaction

1. The Diels–Alder reaction is stereospecific: the reaction is a syn addition, and the configuration of the dienophile is retained in the product


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2. The diene, of necessity, reacts in the s-cis rather than in the s-transconformation

s-cis Configuration s-trans Configuration

R

O

+

O

R

Highly strained

X


e.g.

COOMe+ No Reaction

(diene lockedin s-trans

conformation)

heat


Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction

Relative rates:

Diene D.A. cycloadduct+ 30oCO

O

O

> >Diene

t1/2 11 sec. 130 sec. 4 h.© 2014 by John Wiley & Sons, Inc. All rights reserved.

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3. The Diels–Alder reaction occurs primarily in an endo, rather than an exo, fashion when the reaction is kinetically controlled

H H

H H

R

H

H

Rlongest bridge R is exo

R is endo© 2014 by John Wiley & Sons, Inc. All rights reserved.

Alder-Endo Rule● If a dienophile contains activating

groups (group X) with bonds they will prefer an ENDO orientation in the transition state

X

XX

X

HH


e.g.OO O

O

O

O

HH

+

100% endo


Explanation of Endo Rule: There are two possible approaches between the dieneand dienophile in this reaction.

H

H

HH

H

H

O

HH

H

H

H

H

HH

H

H

O

HH

H

H

H

OHC

CHOH

Endo - major Exo - minor

HOMOdiene

LUMOdienophile

Additional interactionlowers Ea – reacts faster

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93

Endo RuleIf the electron-withdrawing group(s) on the dienophile have a pi bond, there is secondary overlap with the p orbitals of C2 and C3 in the diene.

=>

More stable transition state

Stereospecific reaction

X

X

X

X

+

X X

X

+

X

(i)


Stereospecific reaction

+

+

(ii) Y

Y

Y

Y

Y

Y

Y

Y


Examples

CN

CN

+

Me

NC

NC

CNCN

CN

CNMe(A)

D.A.

CN+

NC

Me

MeNC

CN

CNCN

CN

CN

MeMe(B)

D.A.


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Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups

Me

MeH

Me

Me

(s-cis) (s-trans)


Examples

+

(C)

O

O

O

O

H

H

O

O

D.A.

+

(D)

O

O

O

O

H

H

O

O

D.A.


Examples

+

(E)

O

O

O

D.A.No Reaction

● Rate of Diene C > Diene D (27 times), but Diene D >> Diene E

● In Diene C, tBu group electron donating group increase rate

● In Diene E, 2 tBu group steric effect, cannot adopt s-cis conformation


10C. How To Predict the Products of a Diels–Alder Reaction

(s-cis)

COOMe


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10D. How to Use a Diels–Alder Reaction in a Retrosynthetic Analysis

RetrosynthesisCH3

CH3

CH3

CH3

CH2Br

CH2BrCH3

CH3

A

CO2H

+CO2H

D


Solution

CO2H

+CO2H

CH2Br

CH2BrCH3

CH3

PBr3


Note: use EWG’s ondienophile to ensure afacile D.A. reaction atlow temperature; usingmethyl groups directlywould be a more difficult reaction


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ch13 - 호환성 모드 - Konkukhome.konkuk.ac.kr/~parkyong/Classes/ch13.pdf · 7 O _ O _ _ O _ O _ E N E R G Y MAJOR CONTRIBUTORS MINOR CONTRIBUTORS RESONANCE HYBRID UNEQUAL CONTRIBUTIONS