- By
**Rita** - Follow User

- 426 Views
- Uploaded on

Download Presentation
## PowerPoint Slideshow about '11.18 Polymerization of Styrene' - Rita

**An Image/Link below is provided (as is) to download presentation**

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

CHC6H5

CH2

CH

CH2

CH

CH2

CH

C6H5

C6H5

C6H5

Polymerization of Styrenepolystyrene

Dr. Wolf\'s CHM 201 & 202

11-108

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 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

cyclooctatetraene is not planar

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

Dr. Wolf\'s CHM 201 & 202

11-124

(CH3)3C

138 pm

151 pm

(CH3)3C

CO2CH3

Structure of Cyclobutadienestructure of a stabilized derivative is characterizedby alternating short bonds and long bonds

Dr. Wolf\'s CHM 201 & 202

11-123

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.

notaromatic

aromatic

Antiaromatic when square

Antiaromatic when planar

Requirements for Aromaticitycyclic conjugation is necessary, but not sufficient

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

11.20Hückel\'s Rule:Annulenes

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

Dr. Wolf\'s CHM 201 & 202

11-126

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

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

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

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 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)

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 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)

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 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

6 p electrons

4 p electrons

8 p electrons

notaromatic

notaromatic

aromatic

Dr. Wolf\'s CHM 201 & 202

11-136

H

Completely Conjugated Polyenes6 p electrons;not completelyconjugated

6 p electrons;completely conjugated

notaromatic

aromatic

Dr. Wolf\'s CHM 201 & 202

11-137

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

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]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]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 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 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

H

H

H

H

H

[18]Annulene18 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

H

H

H

+

H

H

H

Cycloheptatrienyl Cation6 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

H

Br

Cycloheptatrienyl CationTropylium 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

H

••

H

H

–

H

Cyclopentadienide Anion6 p electrons delocalizedover 5 carbons

negative charge dispersedover 5 carbons

stabilized anion

Dr. Wolf\'s CHM 201 & 202

11-149

H

H

H

+

H+

••

H

H

H

H

–

H

H

H

Acidity of CyclopentadieneCyclopentadiene 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

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

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

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

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Compare Acidities ofCyclopentadiene and CycloheptatrienepKa = 16

Ka = 10-16

pKa = 36

Ka = 10-36

Dr. Wolf\'s CHM 201 & 202

11-157

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

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

Download Presentation

Connecting to Server..