Wade chemistry

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Chapter 16  Aromatic Co mpounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District 2003, Prentice Hall Organic Chemistry , 5 th  Edition L. G. Wade, Jr.

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Chapter 16 Aromatic Compounds

Jo BlackburnRichland College, Dallas, TX

Dallas County Community College District 2003, Prentice Hall

Organic Chemistry , 5th Edition

L. G. Wade, Jr.

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Chapter 16 2

Discovery of Benzene

• Isolated in 1825 by Michael Faraday

who determined C:H ratio to be 1:1.

• Synthesized in 1834 by EilhardMitscherlich who determined molecular

formula to be C6H6.

• Other related compounds with low C:Hratios had a pleasant smell, so they

were classified as aromatic.

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Chapter 16 3

Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé, shortly

after multiple bonds were suggested.

• Failed to explain existence of only one isomerof 1,2-dichlorobenzene.

C

C

CC

C

C

H

H

H

H

H

H

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Chapter 16 4

Resonance Structure

Each sp2 hybridized C in the ring has anunhybridized p orbital perpendicular tothe ring which overlaps around the ring.

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Chapter 16 5

Unusual Reactions

•  Alkene + KMnO4  diol (addition)

Benzene + KMnO4   no reaction.

•  Alkene + Br 2/CCl4  dibromide (addition)Benzene + Br 2/CCl4  no reaction.

• With FeCl3 catalyst, Br 2 reacts with

benzene to form bromobenzene + HBr(substitution!). Double bonds remain.

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Chapter 16 6

Unusual StabilityHydrogenation of just one double

bond in benzene is endothermic!

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Chapter 16 7

 Annulenes

•  All cyclic conjugatedhydrocarbons wereproposed to be aromatic.

• However, cyclobutadiene

is so reactive that itdimerizes before it canbe isolated.

•  And cyclooctatetraene

adds Br 2 readily.

• Look at MO’s to explainaromaticity. =>

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Chapter 16 8

MO Rules for Benzene

• Six overlapping p orbitals must form six

molecular orbitals.

• Three will be bonding, three antibonding.

• Lowest energy MO will have all bonding

interactions, no nodes.

•  As energy of MO increases, the number of

nodes increases. =>

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Chapter 16 9

MO’s for Benzene 

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Chapter 16 10

Energy Diagram for

Benzene• The six electrons fill three bonding pi orbitals.

• All bonding orbitals are filled (“closed shell”),

an extremely stable arrangement.

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Chapter 16 11

MO’s for Cyclobutadiene 

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Chapter 16 12

Energy Diagram for

Cyclobutadiene

• Following Hund’s

rule, two electronsare in separateorbitals.

• This diradical would

be very reactive.

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Chapter 16 13

Polygon Rule

The energy diagram for an annulene has

the same shape as the cyclic compound

with one vertex at the bottom.

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Chapter 16 14

 Aromatic Requirements

• Structure must be cyclic with conjugated

pi bonds.

• Each atom in the ring must have anunhybridized p orbital.

• The p orbitals must overlap continuously

around the ring. (Usually planar structure)• Compound is more stable than its open-

chain counterpart. =>

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Chapter 16 15

 Anti- and Nonaromatic

•  Antiaromatic compounds are cyclic,

conjugated, with overlapping p orbitals

around the ring, but the energy of thecompound is greater than its open-chain

counterpart.

• Nonaromatic compounds do not have acontinuous ring of overlapping p orbitals

and may be nonplanar. =>

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Chapter 16 16

Hückel’s Rule 

• If the compound has a continuous ring

of overlapping p orbitals and has 4N  + 2

electrons, it is aromatic.• If the compound has a continuous ring

of overlapping p orbitals and has 4N

electrons, it is antiaromatic.

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Chapter 16 17

[N ]Annulenes

• [4]Annulene is antiaromatic (4N  e-’s) 

• [8]Annulene would be antiaromatic, but

it’s not planar, so it’s nonaromatic. • [10]Annulene is aromatic except for the

isomers that are not planar.

• Larger 4N  annulenes are notantiaromatic because they are flexible

enough to become nonplanar. =>

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Chapter 16 18

MO Derivation of

Hückel’s Rule• Lowest energy MO has 2 electrons.

• Each filled shell has 4 electrons.

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Chapter 16 19

Cyclopentadienyl Ions

• The cation has an empty p orbital, 4 electrons,so antiaromatic.

• The anion has a nonbonding pair of electronsin a p orbital, 6 e-’s, aromatic.

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Chapter 16 20

 Acidity of Cyclopentadiene

pK a of cyclopentadiene is 16, much more

acidic than other hydrocarbons.

=>

 p K a = 19 p K a = 16

HOC(CH3)3+

H

OC(CH3)3

 _ 

+

H H

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Chapter 16 21

Tropylium Ion

• The cycloheptatrienyl cation has 6 p 

electrons and an empty p orbital.

•  Aromatic: more stable than open chain ion

=>

H OH

H+ , H2O

H

+

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Chapter 16 22

Dianion of [8]Annulene

• Cyclooctatetraene easily forms a -2 ion.

• Ten electrons, continuous overlapping p 

orbitals, so it is aromatic.

=>

+ 2 K + 2 K  +

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Chapter 16 23

Pyridine

• Heterocyclic aromatic compound.• Nonbonding pair of electrons in sp2 

orbital, so weak base, pK b = 8.8.

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Chapter 16 24

Pyrrole

 Also aromatic, but lone pair of electrons isdelocalized, so much weaker base.

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Chapter 16 25

Basic or Nonbasic?

 N N

Pyrimidine has two basic

nitrogens.

 N N HImidazole has one basic

nitrogen and one nonbasic.

 N

 N

 N

 N

H

Purine?=>

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Chapter 16 26

Other Heterocyclics

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Chapter 16 27

Fused Ring Hydrocarbons• Naphthalene

• Anthracene

• Phenanthrene=>

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Chapter 16 28

Reactivity of

Polynuclear Hydrocarbons As the number of aromatic rings increases,

the resonance energy per ring decreases,

so larger PAH’s will add Br 2.

H Br 

H   Br H Br 

Br 

H

(mixture of cis and trans isomers) =>

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Chapter 16 29

Fused Heterocyclic

CompoundsCommon in nature, synthesized for drugs.

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Chapter 16 30

 Allotropes of Carbon•  Amorphous: small particles of graphite;

charcoal, soot, coal, carbon black.

• Diamond: a lattice of tetrahedral C’s. 

• Graphite: layers of fused aromatic rings.

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Chapter 16 31

Some New Allotropes

• Fullerenes: 5- and 6-membered ringsarranged to form a “soccer ball” structure.

• Nanotubes: half of a C60 sphere fused to a

cylinder of fused aromatic rings.

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Chapter 16 32

Common Names of

Benzene DerivativesOH OCH3 NH2

CH3

 phenol toluene aniline anisole

C

H

CH2 C

O

CH3

C

O

H

C

O

OH

styrene acetophenone benzaldehyde benzoic acid

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Chapter 16 33

Disubstituted Benzenes

The prefixes ortho-, meta-, and para- are

commonly used for the 1,2-, 1,3-, and 1,4-

positions, respectively.Br 

Br 

o-dibromobenzene o1,2-dibromobenzene

HO

 NO2

 p-nitrophenol or 4-nitrophenol

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Chapter 16 34

3 or More Substituents

Use the smallest possible numbers, but

the carbon with a functional group is #1.

 NO2

 NO2

O2 N

1,3,5-trinitrobenzen

 NO2

 NO2

O2 N

OH

2,4,6-trinitrophenol

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Chapter 16 35

Common Names for

Disubstituted Benzenes

CH3

CH3

CH3

CH3H3C

CH3

CO OH

OH

H3Cm-xylene mesitylene   o-toluic acid   p-cresol

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Chapter 16 36

Phenyl and Benzyl

Br 

 phenyl bromide

CH2Br 

 benzyl bromide

Phenyl indicates the benzene ring

attachment. The benzyl group has

an additional carbon.

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Chapter 16 37

Physical Properties

• Melting points: More symmetrical thancorresponding alkane, pack better into

crystals, so higher melting points.

• Boiling points: Dependent on dipolemoment, so ortho > meta > para, for

disubstituted benzenes.

• Density: More dense than nonaromatics,less dense than water.

• Solubility: Generally insoluble in water. =>

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Chapter 16 38

IR and NMR Spectroscopy

• C=C stretch absorption at 1600 cm-1.

•  sp2 C-H stretch just above 3000 cm-1.

• 1H NMR at 7-8 for H’s on aromaticring.

• 13C NMR at 120-150, similar to alkene

carbons.

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Chapter 16 39

Mass Spectrometry

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Chapter 16 40

UV Spectroscopy

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Ch t 16 41

End of Chapter 16