Created byProfessor William Tam & Dr. Phillis

Chang Ch. 13 - 1

Chapter 13Chapter 13

Conjugated PiConjugated PiSystemsSystems

Ch. 13 - 2

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

Ch. 13 - 3

X

H XX2

high temp(and low conc.

of X2)

+

2. Allylic Substitution and the Allyl Radical

vinylic carbons (sp2)

X

X

X2

low tempCCl4

(hi X2 conc.)

allylic carbon (sp3)

Ch. 13 - 4

2A.2A. Allylic ChlorinationAllylic Chlorination(High Temperature)(High Temperature)

Cl H Cl+ Cl2 +400oC

gas phase

Ch. 13 - 5

Mechanism●Chain initiation:

Cl Cl 2 Cl

●Chain propagation:

H H Cl++ Cl

(allylic radical)

Ch. 13 - 6

Mechanism●Chain propagation:

●Chain termination:

Cl Cl Cl+ + Cl

Cl+ Cl

Ch. 13 - 7

+ HH

DHo = 369 kJmol-1

DHo = 465 kJmol-1

H + H

Allylic vs vinyl bond energies:

Ch. 13 - 8

+ HXH + XEact

(low)

H +Eact

(high)HX+X

Relative stabilityof radicals:

allylic > 3o > 2o > 1o > vinylic

Allylic vs vinyl activation energies:

Ch. 13 - 9

Radical stabilities:

Ch. 13 - 10

2B.2B. Allylic Bromination with N-Bromo-Allylic Bromination with N-Bromo-succinimide (Low Concentration of Brsuccinimide (Low Concentration of Br22))

NBS is a solid and nearly insoluble in CCl4.● Low concentration of Br•

H N

Br

OO

Br N

H

OO

h or ROORheat, CCl4

+

+

(NBS)

Ch. 13 - 11

Examples:Br

ROOR, CCl4heat

NBS

BrROOR, CCl4heat

NBS

Ch. 13 - 12

3. The Stability of the Allyl Radical

3A.3A. Molecular Orbital Description of Molecular Orbital Description of the Allyl Radicalthe Allyl Radical

Ch. 13 - 13

Molecular

orbitals:

Ch. 13 - 14

3B.3B. Resonance Description of the Resonance Description of the Allyl RadicalAllyl Radical

12

3 12

3

1

23

1

2

3

Ch. 13 - 15

4. The Allyl Cation Relative order of Carbocation

stability.

(3o allylic) (allylic)(3o)

(2o) (1o) (vinylic)

> >

>>>

Ch. 13 - 16

5. Resonance Theory Revisited

5A. 5A. Rules for Writing Resonance StructuresRules for Writing Resonance Structures Resonance structures exist only on

paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate.

We connect these structures by double-headed arrows (), and we say that the hybrid of all of them represents the real molecule, radical, or ion.

Ch. 13 - 17

In writing resonance structures, one may only move electrons.

H

H

resonance structures

not resonance structures

Ch. 13 - 18

All of the structures must be proper Lewis structures.

O O: :10 electrons!X

not a proper Lewis structure

Ch. 13 - 19

All resonance structures must have the same number of unpaired electrons.

X

Ch. 13 - 20

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

no delocalizationof -electrons

delocalizationof -electrons

Ch. 13 - 21

The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.

Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization.

Ch. 13 - 22

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

(3o allylic cation)

greater contribution

(2o allylic cation)

Ch. 13 - 23

5B.5B. Estimating the Relative Stability Estimating the Relative Stability of Resonance Structuresof Resonance Structures

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

(more stable) (less stable)

O O

(more stable) (less stable)

Ch. 13 - 24

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

Ch. 13 - 25

Charge separation decreases stability.

(more stable) (less stable)

OMe OMe

Ch. 13 - 26

6. Alkadienes and Polyunsaturated Hydrocarbons

1,3-Butadiene

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

1,3-Cyclohexadiene

12

3

4

1

2

3

4

5

6

1

2 3

4

56

Alkadienes (“Dienes”):

Ch. 13 - 27

Alkatrienes (“Trienes”):

1

2

3

4

5

6

7

8

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

Ch. 13 - 28

Alkadiynes (“Diynes”):

1 2 3 4 5 6

2,4-Hexadiynes

1

23

456 1

2

3

4

5 6 7 8

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

Alkenynes (“Enynes”):

Ch. 13 - 29

Cumulenes:

(Allene)(a 1,2-diene)

C C C

H

HH

H

C C C

H

HH

H

enantiomers

Ch. 13 - 30

Conjugated dienes:

Isolated double bonds:

Ch. 13 - 31

7. 1,3-Butadiene: Electron Delocalization

1

2

3

4

7A.7A. Bond Lengths of 1,3-Butadiene Bond Lengths of 1,3-Butadiene

1.34 Å

1.47 Å

1.54 Å 1.50 Å 1.46 Å

sp3 sp3spsp3sp2

Ch. 13 - 32

7B.7B. Conformations of 1,3-ButadieneConformations of 1,3-Butadiene

(s-cis) (s-trans)

H H

(less stable)

cis

transsinglebond

singlebond

Ch. 13 - 33

7C.7C. Molecular Orbitals of 1,3-ButadieneMolecular Orbitals of 1,3-Butadiene

Ch. 13 - 34

8. The Stability of Conjugated Dienes

Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes.

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

H o (kJmol-1)

=-239

Difference 15

+ 2 H2

Ch. 13 - 35

Stability due to conjugation:

Ch. 13 - 36

9. Ultraviolet–Visible Spectroscopy

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.

Ch. 13 - 37

9A.9A. The Electromagnetic SpectrumThe Electromagnetic Spectrum

Ch. 13 - 38

9B.9B. UVUV––Vis SpectrophotometersVis Spectrophotometers

Ch. 13 - 39

Ch. 13 - 40

Beer’s law

A = absorbance= molar absorptivityc = concentrationℓ = path length

A = x c x ℓ A

c x ℓor =

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

Ch. 13 - 41

9C.9C. Absorption Maxima for NonconjugatedAbsorption Maxima for Nonconjugatedand Conjugated Dienesand Conjugated Dienes

Ch. 13 - 42

O OAcetone

Ground state

n

max = 280 nmmax = 15

* Excited state

O

n

max = 324 nm,max = 24

max = 219 nm,max = 3600

Ch. 13 - 43

9D. 9D. Analytical Uses of UVAnalytical Uses of UV––Vis SpectroscopyVis 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, however, has to do with determining the concentration of an unknown sample.

Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions.

Ch. 13 - 44

10. Electrophilic Attack on ConjugatedDienes: 1,4 Addition

Cl

HCl

H

1

2

3

4 H Cl

25oC

+

(78%)(1,2-Addition)

(22%)(1,4-Addition)

Ch. 13 - 45

(a)

Cl

H

Mechanism:

Cl H + H

(a)

H

(b)

H

X

H+ +

Cl

(b)

ClH

(a)

(b)

Ch. 13 - 46

10A.10A. Kinetic Control versus Kinetic Control versus Thermodynamic Control of a Thermodynamic Control of a Chemical ReactionChemical Reaction

+

HBr

Br

Br+

(80%)

-80oC

(20%)

(80%)40oC

Br

Br+

(20%)

Ch. 13 - 47

Br

Br

40oC, HBr

1,2-Additionproduct

1,4-Additionproduct

Ch. 13 - 48

The 1,4-product is thermodynamically more stable.

Ch. 13 - 49

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

[4+2]+

(diene) (dienophile) (adduct)

Ch. 13 - 50

O

O

O

O

O

O

1,3-Butadiene(diene)

Maleicanhydride

(dienophile)

Adduct(100%)

+benzene

100oC

e.g.

Ch. 13 - 51

11A.11A. Factors Favoring the DielsFactors Favoring the Diels––AlderAlderReactionReaction

EDG

EWG

EDG

EWG

+

Type A

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

+

Type B

EDG

EWG EWG

EDG

Ch. 13 - 52

EWG

EDG

EWG

EDG

+

Type C

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

+

Type D

EWG

EDG EDG

EWG

Ch. 13 - 53

Relative rate:

Diene D.A. cycloadduct+30oC

O

O

O

OMe

> >Diene

t1/2 20 min. 70 min. 4 h.

Ch. 13 - 54

Relative rate:

Dienophile D.A. cycloadduct+20oC

> >Dienophile

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

NC CN

NC CN

CN

CN

CN

Ch. 13 - 55

Steric effects:

> >Dienophile:

Relative rate: 1 0.14 0.007

COOEt COOEt COOEt

Ch. 13 - 56

11B.11B. Stereochemistry of the Stereochemistry of the DielsDiels––Alder ReactionAlder Reaction

O

O

OMe

OMeH

H

OMe

O

OMe

OH

H

+

Dimethyl maleate(a cis-dienophile)

Dimethyl cyclohex-4-ene-cis-1,2-dicarboxylate

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

Ch. 13 - 57

O

OMeH

OMe

O

OMe

OH

H

+

Dimethyl fumarate(a trans -dienophile)

Dimethyl cyclohex-4-ene-trans -1,2-

dicarboxylate

HMeO

O

Ch. 13 - 58

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

s-cis Configuration s-trans Configuration

R

O

+

O

R

Highly strained

X

Ch. 13 - 59

e.g.COOMe COOMe

heat+

(diene lockedin s-cis

conformation)

COOMe

+ No Reaction

(diene lockedin s-trans

conformation)

heat

Ch. 13 - 60

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

Relative rate:

Diene D.A. cycloadduct+30oC

O

O

O

> >Diene

t1/2 11 sec. 130 sec. 4 h.

Ch. 13 - 61

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

Ch. 13 - 62

Alder-Endo Rule:●If a dienophile contains

activating groups with bonds they will prefer an ENDO orientation in the transition state.

X

XX

X

HH

Ch. 13 - 63

e.g.

OO O

O

O

O

HH

+

100% endo

Ch. 13 - 64

Stereospecific reaction:

X

X

X

X

+

X X

X

+

X

(i)

Ch. 13 - 65

Stereospecific reaction:

+

+

(ii) Y

Y

Y

Y

Y

Y

Y

Y

Ch. 13 - 66

Examples:

CN

CN

+

Me

NC

NC

CN

CNCN

CNMe(A)

D.A.

CN

+

NC

Me

Me

NC

CN

CN

CN

CN

CN

MeMe(B)

D.A.

Ch. 13 - 67

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

Me

Me

H

Me

Me

(s-cis) (s-trans)

Ch. 13 - 68

Examples:

+

(C)

O

O

O

O

H

H

O

O

D.A.

+

(D)

O

O

O

O

H

H

O

O

D.A.

Ch. 13 - 69

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, t-Bu group electron donating group increase rate

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

Ch. 13 - 70

END OF CHAPTER 13