Chapter 11 - Arenes and Aromaticity 11.1 Increasing Unsaturation in 6-Membered Rings cyclohexane cyclohexene 1,3-cyclohexadiene 1,3,5-cyclohexatriene ?? 11.2 Evidence of structure: all C-C bonds are the same length, all H’s are equivalent. Kekule (1866) - rapidly interconverting isomers? isomers? Robinson (1920) - the two Kekule forms are resonance contributors resonance forms 11.2-11.3 Bond distances and bond angles of benzene Figure 11.1 Robinson - “ Aromatic Sextet ” depiction 1
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 2
11.7 Nomenclature of Substituted Benzenes Cl F Br CH 3 C(CH 3 ) 3 NH 2 OH CO 2 H SO 3 H NO 2 Many common names, however IUPAC systematic names often easier to work out 11.7 Nomenclature of Disubstituted Benzenes Br CH 3 CH 3 Br CH 3 CH 3 NO 2 F NH 2 Br F CH 3 Can use numbering or o , m , p nomenclature systems Not Covering 11.8 11.9 11.11 3
11.12 Free-Radical Halogenation of Alkylbenzenes H H C C H ∆ H = 91 kcal/mol + H H H H H C C H + H ∆ H = 88 kcal/mol H H H H H C H H C ∆ H = 85 kcal/mol + H H H Br H C H H C Br 2 ( + HBr ) CCl 4 , 80 o C 71% yield a 11.13 Oxidation of Alkylbenzenes HO O CH 3 C Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 Heat HO O CH(CH 3 ) 2 C Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 Heat HO O CH(CH 3 ) 2 C KMnO 4 H 2 O, Heat (then HCl) NO 2 NO 2 4
11.14 Nucleophilic Substitution in Benzylic Halides H H H C Cl H C OCH 3 NaOCH 3 CH 3 OH CH 3 CH 3 CH 3 C Cl CH 3 C OCH 3 CH 3 OH S N 2 applies with good nucleophiles on 1 o and 2 o carbons S N 1 applies with weak nucleophiles – good carbocation E2 competes with more basic nucleophiles on 2 o and 3 o 11.15 Preparation of Alkenylbenzenes CH 3 CH 3 CH 2 CH 3 C Br C NaOCH 3 CH 3 OH CH 3 CH 3 CH 2 C CH 3 C OH KHSO 4 heat Cl Cl 11.16 Additrion to Alkenylbenzenes Br H CH 2 C H C CH 2 Br Br 2 Cl HCl H Not Covering 11.17 11.18 5
11.19 Hückel’s Rule Fig. 11.1 Aromatic = 4n+2 π electrons and flat π system Not Covering 11.20 11.21 Aromatic Ions Fig. 11.12 Fig. 11.13 a a Br H Ag + NaOH Cation easy to form p Ka ~ 16 6
11.22 Heterocyclic Aromatic Compounds N O S N H pyridine pyrrole furan thiophene N N quinoline isoquinoline N O S H benzofuran benzothiophene indole 11.23 Heterocyclic Aromatic Compounds – Hückel’s Rule pyridine pyrrole .. N N .. H 7
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