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Chapter 16 Aromatic Compounds

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Organic Chemistry , 6 th Edition L. G. Wade, Jr. Chapter 16 Aromatic Compounds. Discovery of Benzene. Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6 H 6 . - PowerPoint PPT Presentation
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Chapter 16 Aromatic Compounds Organic Chemistry, 6 th Edition L. G. Wade, Jr.
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Page 1: Chapter 16 Aromatic Compounds

Chapter 16Aromatic Compounds

Organic Chemistry, 6th EditionL. G. Wade, Jr.

Page 2: Chapter 16 Aromatic Compounds

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 Eilhard Mitscherlich who determined molecular formula to be C6H6.

• Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. =>

Page 3: Chapter 16 Aromatic Compounds

Chapter 16 3

Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested.

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

CC

CC

C

C

H

H

HH

H

H

=>

Page 4: Chapter 16 Aromatic Compounds

Chapter 16 4

Resonance StructureEach sp2 hybridized C in the ring has an

unhybridized p orbital perpendicular to the ring which overlaps around the ring.

=>

Page 5: Chapter 16 Aromatic Compounds

Chapter 16 5

Unusual Reactions

• Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction.

• Alkene + Br2/CCl4 dibromide (addition)Benzene + Br2/CCl4 no reaction.

• With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. =>

Page 6: Chapter 16 Aromatic Compounds

Chapter 16 6

Unusual StabilityHydrogenation of just one double

bond in benzene is endothermic!

=>

Page 7: Chapter 16 Aromatic Compounds

Chapter 16 7

Aromatic Requirements

• Structure must be cyclic with conjugated pi bonds.

• Each atom in the ring must have an unhybridized p orbital.

• The p orbitals must overlap continuously around the ring. (Usually planar structure.)

• Compound is more stable than its open-chain counterpart. =>

Page 8: Chapter 16 Aromatic Compounds

Chapter 16 8

Anti- and Nonaromatic

• Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart.

• Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. =>

Page 9: Chapter 16 Aromatic Compounds

Chapter 16 9

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.

=>

Page 10: Chapter 16 Aromatic Compounds

Chapter 16 10

Cyclopentadienyl Ions• The cation has an empty p orbital, 4 electrons, so antiaromatic.• The anion has a nonbonding pair of electrons in a p orbital, 6

e-’s, aromatic.

=>

Page 11: Chapter 16 Aromatic Compounds

Chapter 16 11

Acidity of Cyclopentadiene

pKa of cyclopentadiene is 16, much more acidic than other hydrocarbons.

=>

Page 12: Chapter 16 Aromatic Compounds

Chapter 16 12

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

+

Page 13: Chapter 16 Aromatic Compounds

Chapter 16 13

Pyridine• Heterocyclic aromatic compound.

• Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8.

=>

Page 14: Chapter 16 Aromatic Compounds

Chapter 16 14

PyrroleAlso aromatic, but lone pair of electrons is

delocalized, so much weaker base.

=>

Page 15: Chapter 16 Aromatic Compounds

Chapter 16 15

Other Heterocyclics

=>

Page 16: Chapter 16 Aromatic Compounds

Chapter 16 16

Fused Ring Hydrocarbons• Naphthalene

• Anthracene

• Phenanthrene

=>

Page 17: Chapter 16 Aromatic Compounds

Chapter 16 17

Reactivity of Polynuclear Hydrocarbons

As the number of aromatic rings increases, the resonance energy per ring decreases, so larger PAH’s will add Br2.

H Br

H BrH Br

Br

H

(mixture of cis and trans isomers) =>

Page 18: Chapter 16 Aromatic Compounds

Chapter 16 18

Larger Polynuclear Aromatic Hydrocarbons

• Formed in combustion (tobacco smoke).• Many are carcinogenic.• Epoxides form, combine with DNA base.

pyrene =>

Page 19: Chapter 16 Aromatic Compounds

Chapter 16 19

Common Names of Benzene Derivatives

OH OCH3NH2CH3

phenol toluene aniline anisole

CH

CH2 C

O

CH3C

O

HC

O

OH

styrene acetophenone benzaldehyde benzoic acid=>

Page 20: Chapter 16 Aromatic Compounds

Chapter 16 20

Disubstituted BenzenesThe prefixes ortho-, meta-, and para- arecommonly used for the 1,2-, 1,3-, and 1,4-positions, respectively.

=>

Page 21: Chapter 16 Aromatic Compounds

Chapter 16 21

3 or More Substituents Use the smallest possible numbers, butthe carbon with a functional group is #1.

NO2

NO2

O2N

1,3,5-trinitrobenzene

NO2

NO2

O2N

OH

2,4,6-trinitrophenol

=>

Page 22: Chapter 16 Aromatic Compounds

Chapter 16 22

Common Names forDisubstituted Benzenes

CH3

CH3

CH3

CH3H3C

CH3

CO OH

OH

H3Cm-xylene mesitylene o-toluic acid p-cresol

=>

Page 23: Chapter 16 Aromatic Compounds

Chapter 16 23

Phenyl and Benzyl

Br

phenyl bromide

CH2Br

benzyl bromide

Phenyl indicates the benzene ringattachment. The benzyl group hasan additional carbon.

=>

Page 24: Chapter 16 Aromatic Compounds

Chapter 16 24

Physical Properties

• Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points.

• Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes.

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

• Solubility: Generally insoluble in water. =>

Page 25: Chapter 16 Aromatic Compounds

Chapter 16 25

End of Chapter 16


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