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cook.chem.ndsu.nodak.edu/chem341 ©2012 Gregory R Cook Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 Prof. Greg Cook cook.chem.ndsu.nodak.edu/chem341 Friday, April 27, 12
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Page 1: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

cook.chem.ndsu.nodak.edu/chem341

©2012 Gregory R Cook

Chapter 11Arenes and Aromaticity

CHEM 341: Spring 2012

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem341

Friday, April 27, 12

Page 2: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Aromatic Compounds

2

CHO CO2H CH3 CH3

CH3

OH NH2

benzaldehyde benzoic acid toluene xylene

phenol aniline naphthalene

polyaromatic hydrocarbons (PAH's)carcinogens formed from combustionThis PAH is the main carcinogen found in Tobacco smoke (so quit for goodness sake!)

cinnamaldehyde

CHO

Friday, April 27, 12

Page 3: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene - a new hydrocarbon (1800’s)

3

• 1825 - Michael Faraday isolated a new hydrocarbon from whale oil

• Empirical Formula determined to be CnHnMF found to be C6H6

• All the C’s and H’s are equivalent

Friday, April 27, 12

Page 4: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Reactivity

4

• Highly unsaturated - but UNREACTIVE toward Br2 . . . But . . .

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Page 5: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

Friday, April 27, 12

Page 6: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

HHH

HH H

bonds identical1.39 Å

Friday, April 27, 12

Page 7: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

HHH

HH H

bonds identical1.39 Å

Friday, April 27, 12

Page 8: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Stability

6

• Benzene is unusually stable

H2Pd/C NO REACTION

H2Ni

100-200°C100 atm pressure

Friday, April 27, 12

Page 9: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene Stability

7

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Page 10: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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pi Molecular Orbitals of Benzene

8

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Page 11: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Nomenclature for Benzene Derivatives

9

CHO CO2H CH3 CH3

CH3

OH NH2

benzaldehyde benzoic acid toluene xylene

phenol aniline naphthalene

polyaromatic hydrocarbons (PAH's)carcinogens formed from combustionThis PAH is the main carcinogen found in Tobacco smoke (so quit for goodness sake!)

cinnamaldehyde

CHO

Friday, April 27, 12

Page 12: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Nomenclature for Benzene Derivatives

10

CH3

toluenemethylbenzene

Br

bromobenzene

NO2

nitrobenzene

OH

phenolhydroxybenzene

CH3

o-xylene1,2-dimethylbenzene

CH3CH3

m-xylene1,3-dimethylbenzene

CH3

p-xylene1,4-dimethylbenzene

CH3CH3

p-nitrobenzaldehyde o-difluorobenzene TNT2,4,6-trinitrotoluene

Br

Friday, April 27, 12

Page 13: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Benzene as a substituent

11

Ph

4-phenyloctane biphenyl

H2C

benzyl substitutent

4-phenyloctane

Friday, April 27, 12

Page 14: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Polycyclic Aromatic Hydrocarbons

12

naphthalene

Anthracene

phenanthrene benzo[a]pyrene

Friday, April 27, 12

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C60 - Buckminsterfullerene

13

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Page 16: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Graphene

14

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Page 17: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Carbon Nanotubes

15

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Page 18: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Arene Reactivity

16

• Reduction

• Electrophilic Aromatic Substitution

• Reactions of substituents

H2Ni

100-200°C100 atm pressure

Br2+FeBr3 HBr+

Br

Friday, April 27, 12

Page 19: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Birch Reduction

17

Na, NH3

CH3OH

R RH H

H H

Friday, April 27, 12

Page 20: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Oxidation of Alkylbenzenes

18

Na2Cr2O7

H2SO4, H2O, heatNO REACTION

Na2Cr2O7

H2SO4, H2O, heatNO REACTION

Na2Cr2O7

H2SO4, H2O, heat

OH

O

Friday, April 27, 12

Page 21: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Aromatic Stability of Benzene

19

6 π electrons 6 π electrons

Friday, April 27, 12

Page 22: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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What about other sized rings?

20

highly unstable and Reactive

1.56 Å

1.35 Å

no "special" stabilityeasily hydrogenated

not flat

Friday, April 27, 12

Page 23: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Hückel’s Rule

21

• Three requirements for aromatic stability

• Fully planar ring

• Fully conjugated

and

• Must have 4n+2 number of pi electrons

Friday, April 27, 12

Page 24: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Hückel’s Rule

22

4 π electrons6 π electrons 8 π electrons

Friday, April 27, 12

Page 25: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Aromatic Ions

23

H H

pKa = 16

H H

H H

Friday, April 27, 12

Page 26: Chapter 11 Arenes and Aromaticity - The Cook Groupcook.chem.ndsu.nodak.edu/chem341/wp-content/uploads/2012/04/Chem...Chapter 11 Arenes and Aromaticity CHEM 341: Spring 2012 ... Benzene

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Larger rings - Annulenes

24

HH

HH

HH

C10H10not planar C18H18

aromatic

Friday, April 27, 12

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Aromatic Heterocycles

25

N NH

O

N

NH

N H O N HN N

N

NN

N

NH2

HN

NN

N

O

H

H

NH2 N

N

NH2

OH

N

N

OH

O

H3C

HGuanine Thymine CytosineAdenine

Friday, April 27, 12

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Aromatic Heterocycle Basicity

26

N N H

N

NH

N N H

HA

Friday, April 27, 12


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