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11.18 Polymerization of Styrene. Dr. Wolf\'s CHM 201 & 202. 11-107. H 2 C. CH C 6 H 5. CH 2. CH. CH 2. CH. CH 2. CH. C 6 H 5. C 6 H 5. C 6 H 5. Polymerization of Styrene. polystyrene. Dr. Wolf\'s CHM 201 & 202. 11-108. •. •. •. H 2 C. CH C 6 H 5. Mechanism. ••. RO.

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11 18 polymerization of styrene
11.18Polymerization of Styrene

Dr. Wolf\'s CHM 201 & 202

11-107

polymerization of styrene

H2C

CHC6H5

CH2

CH

CH2

CH

CH2

CH

C6H5

C6H5

C6H5

Polymerization of Styrene

polystyrene

Dr. Wolf\'s CHM 201 & 202

11-108

mechanism

H2C

CHC6H5

Mechanism

••

RO

Dr. Wolf\'s CHM 201 & 202

11-109

mechanism4

Mechanism

••

RO

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-110

mechanism5

H2C

CHC6H5

Mechanism

••

RO

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-111

mechanism6

H2C

CHC6H5

Mechanism

••

RO

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-112

mechanism7

Mechanism

••

RO

H2C

CHC6H5

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-113

mechanism8

H2C

CHC6H5

Mechanism

••

RO

H2C

CHC6H5

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-114

mechanism9

H2C

CHC6H5

Mechanism

••

RO

H2C

CHC6H5

H2C

CHC6H5

Dr. Wolf\'s CHM 201 & 202

11-115

mechanism10

••

RO

H2C

CHC6H5

H2C

CHC6H5

H2C

CHC6H5

Mechanism

Dr. Wolf\'s CHM 201 & 202

11-116

mechanism11

••

RO

H2C

CHC6H5

H2C

CHC6H5

H2C

CHC6H5

H2C

CHC6H5

Mechanism

Dr. Wolf\'s CHM 201 & 202

11-117

mechanism12

••

RO

H2C

CHC6H5

H2C

CHC6H5

H2C

CHC6H5

H2C

CHC6H5

Mechanism

Dr. Wolf\'s CHM 201 & 202

11-118

11 19 cyclobutadiene and cyclooctatetraene
11.19Cyclobutadiene and Cyclooctatetraene

Dr. Wolf\'s CHM 201 & 202

11-119

heats of hydrogenation
Heats of Hydrogenation

to give cyclohexane (kJ/mol)

heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of cyclohexene

120

231

208

heats of hydrogenation15
Heats of Hydrogenation

to give cyclooctane (kJ/mol)

heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene

97

205

303

410

Dr. Wolf\'s CHM 201 & 202

11-122

structure of cyclooctatetraene
Structure of Cyclooctatetraene

cyclooctatetraene is not planar

has alternating long (146 pm)and short (133 pm) bonds

Dr. Wolf\'s CHM 201 & 202

11-124

structure of cyclobutadiene

C(CH3)3

(CH3)3C

138 pm

151 pm

(CH3)3C

CO2CH3

Structure of Cyclobutadiene

structure of a stabilized derivative is characterizedby alternating short bonds and long bonds

Dr. Wolf\'s CHM 201 & 202

11-123

stability of cyclobutadiene
Stability of Cyclobutadiene

Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in thecustomary way.

Not only is cyclobutadiene not aromatic, it is antiaromatic.

An antiaromatic substance is one that is destabilizedby cyclic conjugation.

requirements for aromaticity

notaromatic

notaromatic

aromatic

Antiaromatic when square

Antiaromatic when planar

Requirements for Aromaticity

cyclic conjugation is necessary, but not sufficient

conclusion
Conclusion

there must be some factor in additionto cyclic conjugation that determines whether a molecule is aromatic or not

Dr. Wolf\'s CHM 201 & 202

11-125

11 20 h ckel s rule annulenes
11.20Hückel\'s Rule:Annulenes

the additional factor that influences aromaticity is the number of p electrons

Dr. Wolf\'s CHM 201 & 202

11-126

h ckel s rule
Hückel\'s Rule

among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic)

n 4n+2

0 2

1 6

2 10

3 14

4 18

Dr. Wolf\'s CHM 201 & 202

11-127

h ckel s rule23
Hückel\'s Rule

among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic)

n 4n+2

0 2

1 6 benzene!

2 10

3 14

4 18

Dr. Wolf\'s CHM 201 & 202

11-128

h ckel s rule24
Hückel\'s Rule

Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenes

he found that the p molecular orbitals ofthese compounds had a distinctive pattern

one p orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highestand the lowest

Dr. Wolf\'s CHM 201 & 202

11-129

p mos of benzene
p-MOs of Benzene

Antibonding

6 p orbitals give 6 p orbitals

3 orbitals are bonding; 3 are antibonding

Benzene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-130

p mos of benzene26
p-MOs of Benzene

Antibonding

6 p electrons fill all of the bonding orbitals

all p antibonding orbitals are empty

Benzene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-131

p mos of cyclobutadiene square planar
p-MOs of Cyclobutadiene(square planar)

Antibonding

4 p orbitals give 4p orbitals

1 orbital is bonding, one is antibonding, and 2 are nonbonding

Cyclo-butadiene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-132

p mos of cyclobutadiene square planar28
p-MOs of Cyclobutadiene(square planar)

Antibonding

4 p electrons; bonding orbital is filled; other 2p electrons singly occupy two nonbonding orbitals

Cyclo-butadiene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-133

p mos of cyclooctatetraene square planar
p-MOs of Cyclooctatetraene(square planar)

Antibonding

8 p orbitals give 8 p orbitals

3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding

Cyclo-octatetraene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-134

p mos of cyclooctatetraene square planar30
p-MOs of Cyclooctatetraene(square planar)

Antibonding

8 p electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-filled

Cyclo-octatetraene

Bonding

Dr. Wolf\'s CHM 201 & 202

11-135

p electron requirement for aromaticity
p-Electron Requirement for Aromaticity

6 p electrons

4 p electrons

8 p electrons

notaromatic

notaromatic

aromatic

Dr. Wolf\'s CHM 201 & 202

11-136

completely conjugated polyenes

H

H

Completely Conjugated Polyenes

6 p electrons;not completelyconjugated

6 p electrons;completely conjugated

notaromatic

aromatic

Dr. Wolf\'s CHM 201 & 202

11-137

annulenes
Annulenes

Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel\'s rule.

Dr. Wolf\'s CHM 201 & 202

11-138

10 annulene
[10]Annulene

predicted to be aromatic by Hückel\'s rule,but too much angle strain when planar and all double bonds are cis

10-sided regular polygon has angles of 144°

Dr. Wolf\'s CHM 201 & 202

11-139

10 annulene36
[10]Annulene

incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity

Dr. Wolf\'s CHM 201 & 202

11-140

10 annulene37
[10]Annulene

van der Waalsstrain betweenthese two hydrogens

incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity

Dr. Wolf\'s CHM 201 & 202

11-141

14 annulene
[14]Annulene

14 p electrons satisfies Hückel\'s rule

van der Waals strain between hydrogens insidethe ring

H

H

H

H

Dr. Wolf\'s CHM 201 & 202

11-142

16 annulene
[16]Annulene

16 p electrons does not satisfy Hückel\'s rule

alternating short (134 pm) and long (146 pm) bonds

not aromatic

Dr. Wolf\'s CHM 201 & 202

11-143

18 annulene

H

H

H

H

H

H

[18]Annulene

18 p electrons satisfies Hückel\'s rule

resonance energy = 418 kJ/mol

bond distances range between 137-143 pm

Dr. Wolf\'s CHM 201 & 202

11-144

11 22 aromatic ions
11.22Aromatic Ions

Dr. Wolf\'s CHM 201 & 202

11-145

cycloheptatrienyl cation

H

H

H

H

+

H

H

H

Cycloheptatrienyl Cation

6 p electrons delocalizedover 7 carbons

positive charge dispersedover 7 carbons

very stable carbocation

also called tropylium cation

Dr. Wolf\'s CHM 201 & 202

11-146

cycloheptatrienyl cation43

H

H

H

H

H

H

H

H

+

+

H

H

H

H

H

H

Cycloheptatrienyl Cation

Dr. Wolf\'s CHM 201 & 202

11-147

cycloheptatrienyl cation44

+

H

Br

Cycloheptatrienyl Cation

Tropylium cation is so stable that tropyliumbromide is ionic rather than covalent.

mp 203 °C; soluble in water; insoluble indiethyl ether

Br–

Ionic

Covalent

Dr. Wolf\'s CHM 201 & 202

11-148

cyclopentadienide anion

H

H

••

H

H

H

Cyclopentadienide Anion

6 p electrons delocalizedover 5 carbons

negative charge dispersedover 5 carbons

stabilized anion

Dr. Wolf\'s CHM 201 & 202

11-149

cyclopentadienide anion46

H

H

H

H

••

H

H

H

H

H

H

Cyclopentadienide Anion

Dr. Wolf\'s CHM 201 & 202

11-150

acidity of cyclopentadiene

H

H

H

H

+

H+

••

H

H

H

H

H

H

H

Acidity of Cyclopentadiene

Cyclopentadiene is unusually acidic for a hydrocarbon.

Increased acidity is due to stability of cyclopentadienide anion.

pKa = 16

Ka = 10-16

Dr. Wolf\'s CHM 201 & 202

11-151

electron delocalization in cyclopentadienide anion

H

H

H

H

H

Electron Delocalization in Cyclopentadienide Anion

••

Dr. Wolf\'s CHM 201 & 202

11-152

electron delocalization in cyclopentadienide anion49

H

H

H

H

••

H

H

H

H

H

H

Electron Delocalization in Cyclopentadienide Anion

••

Dr. Wolf\'s CHM 201 & 202

11-153

electron delocalization in cyclopentadienide anion50

H

H

H

H

••

H

H

H

H

H

H

H

H

H

H

H

Electron Delocalization in Cyclopentadienide Anion

••

Dr. Wolf\'s CHM 201 & 202

11-154

electron delocalization in cyclopentadienide anion51

H

H

H

H

••

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Electron Delocalization in Cyclopentadienide Anion

••

Dr. Wolf\'s CHM 201 & 202

11-155

electron delocalization in cyclopentadienide anion52

H

H

H

H

••

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Electron Delocalization in Cyclopentadienide Anion

••

H

H

••

H

H

H

Dr. Wolf\'s CHM 201 & 202

11-156

compare acidities of cyclopentadiene and cycloheptatriene

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Compare Acidities ofCyclopentadiene and Cycloheptatriene

pKa = 16

Ka = 10-16

pKa = 36

Ka = 10-36

Dr. Wolf\'s CHM 201 & 202

11-157

compare acidities of cyclopentadiene and cycloheptatriene54

H

H

H

H

H

H

••

H

H

••

H

H

H

H

Compare Acidities ofCyclopentadiene and Cycloheptatriene

Aromatic anion

6 p electrons

Anion not aromatic8 p electrons

Dr. Wolf\'s CHM 201 & 202

11-158

cyclopropenyl cation

H

H

H

H

+

+

H

H

Cyclopropenyl Cation

n = 0

4n +2 = 2 p electrons

also written as

Dr. Wolf\'s CHM 201 & 202

11-159

cyclooctatetraene dianion
Cyclooctatetraene Dianion

H

H

H

H

n = 2

4n +2 = 10p electrons

H

H

H

H

••

alsowritten as

2–

••

H

H

H

H

H

H

H

H

Dr. Wolf\'s CHM 201 & 202

11-160

11 23 heterocyclic aromatic compounds
11.23Heterocyclic Aromatic Compounds

Dr. Wolf\'s CHM 201 & 202

11-161

examples

••

••

••

O

S

N

N

••

••

••

H

Examples

Pyridine

Pyrrole

Furan

Thiophene

Dr. Wolf\'s CHM 201 & 202

11-162

examples59

N

N

••

Examples

Quinoline

Isoquinoline

Dr. Wolf\'s CHM 201 & 202

11-163

pyridine

N

••

Pyridine

6 p electrons in ring

lone pair on nitrogen is in ansp2 hybridized orbital;not part of p system of ring

Dr. Wolf\'s CHM 201 & 202

11-165

pyrrole

••

N

H

Pyrrole

lone pair on nitrogen must be part of ring p system if ring is to have6 p electrons

lone pair must be in a p orbitalin order to overlap with ring psystem

Dr. Wolf\'s CHM 201 & 202

11-166

furan

••

O

••

Furan

two lone pairs on oxygen

one pair is in a p orbital and is partof ring p system; other is in an sp2 hybridized orbital and is notpart of ring p system

Dr. Wolf\'s CHM 201 & 202

11-167

end of chapter 11
End of Chapter 11

Dr. Wolf\'s CHM 201 & 202

11-168

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