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

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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

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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

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Benzene Reactivity

4

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

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Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

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©2012 Gregory R Cook

Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

HHH

HH H

bonds identical1.39 Å

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©2012 Gregory R Cook

Benzene Structure

5

• Structures proposed for C6H6

1.34 Å1.54 Å

prismane Dewar benzene Kekulé structures

HHH

HH H

bonds identical1.39 Å

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Benzene Stability

6

• Benzene is unusually stable

H2Pd/C NO REACTION

H2Ni

100-200°C100 atm pressure

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©2012 Gregory R Cook

Benzene Stability

7

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pi Molecular Orbitals of Benzene

8

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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

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©2012 Gregory R Cook

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

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Benzene as a substituent

11

Ph

4-phenyloctane biphenyl

H2C

benzyl substitutent

4-phenyloctane

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Polycyclic Aromatic Hydrocarbons

12

naphthalene

Anthracene

phenanthrene benzo[a]pyrene

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C60 - Buckminsterfullerene

13

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Graphene

14

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Carbon Nanotubes

15

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Arene Reactivity

16

• Reduction

• Electrophilic Aromatic Substitution

• Reactions of substituents

H2Ni

100-200°C100 atm pressure

Br2+FeBr3 HBr+

Br

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Birch Reduction

17

Na, NH3

CH3OH

R RH H

H H

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Oxidation of Alkylbenzenes

18

Na2Cr2O7

H2SO4, H2O, heatNO REACTION

Na2Cr2O7

H2SO4, H2O, heatNO REACTION

Na2Cr2O7

H2SO4, H2O, heat

OH

O

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Aromatic Stability of Benzene

19

6 π electrons 6 π electrons

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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

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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

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Hückel’s Rule

22

4 π electrons6 π electrons 8 π electrons

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Aromatic Ions

23

H H

pKa = 16

H H

H H

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Larger rings - Annulenes

24

HH

HH

HH

C10H10not planar C18H18

aromatic

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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

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Aromatic Heterocycle Basicity

26

N N H

N

NH

N N H

HA

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