1 11.4 Resonance energy of benzene as estimated from heats of - - PDF document

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Chapter 11 - Arenes and Aromaticity

11.1 Increasing Unsaturation in 6-Membered Rings

cyclohexane cyclohexene 1,3-cyclohexadiene 1,3,5-cyclohexatriene ?? isomers? resonance forms

11.2 Evidence of structure: all C-C bonds are the same length, all H’s are equivalent.

Kekule (1866) - rapidly interconverting isomers? Robinson (1920) - the two Kekule forms are resonance contributors

11.2-11.3 Bond distances and bond angles of benzene

Figure 11.1

Robinson - “Aromatic Sextet” depiction

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11.4 Resonance energy of benzene as estimated from heats of hydrogenation

Figure 11.2

11.5 The σ bonds (a), the delocalized π system, and the electrostatic potential map (c) of benzene

Figure 11.3

i.e. each carbon experiences the same electron density, the six pi electrons are delocalized over the entire molecule 11.6 The π molecular orbitals of benzene arranged in order of increasing energy Figure 11.4

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11.7 Nomenclature of Substituted Benzenes

Br Cl F NO2 NH2 OH CH3 C(CH3)3 CO2H SO3H

Many common names, however IUPAC systematic names often easier to work out

11.7 Nomenclature of Disubstituted Benzenes

Br CH3 CH3 NO2 F NH2 Br CH3 CH3 CH3 F Br Can use numbering or o, m, p nomenclature systems

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11.8 11.9 11.11

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11.12 Free-Radical Halogenation of Alkylbenzenes

C H H H C H H

+ H

C H H H

+ H

C H H C H H H

+ H

C H H

∆H = 91 kcal/mol ∆H = 88 kcal/mol ∆H = 85 kcal/mol C H H H C H H

Br Br2

CCl4, 80 oC (+ HBr) 71% yield

a 11.13 Oxidation of Alkylbenzenes

CH3 C Na2Cr2O7 H2O, H2SO4 Heat HO O CH(CH3)2 C Na2Cr2O7 H2O, H2SO4 Heat HO O CH(CH3)2 C KMnO4 H2O, Heat (then HCl) HO O NO2 NO2

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11.14 Nucleophilic Substitution in Benzylic Halides

C Cl H H C H H OCH3 NaOCH3 CH3OH C Cl CH3 CH3 C CH3 CH3 OCH3 CH3OH

SN2 applies with good nucleophiles on 1o and 2o carbons SN1 applies with weak nucleophiles – good carbocation E2 competes with more basic nucleophiles on 2o and 3o

11.15 Preparation of Alkenylbenzenes 11.16 Additrion to Alkenylbenzenes

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11.17 11.18

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11.19 Hückel’s Rule

Aromatic = 4n+2 π electrons and flat π system

• Fig. 11.1

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11.20

11.21 Aromatic Ions

a a

Br

Ag+ NaOH

H

Cation easy to form pKa ~ 16

• Fig. 11.12
• Fig. 11.13

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11.22 Heterocyclic Aromatic Compounds

N N H O S N N N H O S pyridine pyrrole furan thiophene quinoline isoquinoline indole benzofuran benzothiophene

11.23 Heterocyclic Aromatic Compounds – Hückel’s Rule N N H pyridine pyrrole

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