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Alkenes: Formation by elimination reactions (McM 11.12, 11.13)

Alkenes: Formation by elimination reactions (McM 11.12, 11.13) Substitution reactions: Cleavage of ethers (McM 18.5) Organometallic coupling react. (McM 10.9, lab ex 10) Dienes: MO description, conjugated diene (McM 14.2) Aromatic compounds: Aromaticity (repetition; McM 15.3-15.8)

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Alkenes: Formation by elimination reactions (McM 11.12, 11.13)

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  1. Alkenes: Formation by elimination reactions (McM 11.12, 11.13) Substitution reactions: Cleavage of ethers (McM 18.5) Organometallic coupling react. (McM 10.9, lab ex 10) Dienes: MO description, conjugated diene (McM 14.2) Aromatic compounds: Aromaticity (repetition; McM 15.3-15.8) Electrophilic aromatic subst and substituent effects (McM 16.5-16.7, 16.12) Nucleophilic aromatic subst (McM16.8-16.) Other react on aromatic compounds (McM 16.10-16.11)

  2. Dienes (McM 14.2)

  3. Molecular orbitals of H2 (cf fig 1.20) - s-bond Molecular orbitals of p-bond (cf fig. 1.21) MO 1,3-butadiene cf fig 14.2 Energy absorb UV spectra LUMO hn 1 e- HOMO 4 p electron in 2 bonding orbitals (ground state)

  4. MO orbitals benzene - aromaticity (McM 15.4 - 15.5) 1,3,5-hexatriene only bonding MO shown Benzene

  5. Criteria for Aromaticity (Hückel) • (Monocyclic) ring • Planar • No of p-electrons in conjugation 4n+2 (n: 0, 1, 2,....) Benzene: The 3 bonding MOs are filled Filled shell of MOs (cf. filled shell of atomic orbitals nobel gasses) Less than 6 p-electrons: Half-filled orbital(s) - radical character More than 6: Electron in antibonding orbitals Unstable, high-enegy species

  6. Ex. of an anti-aromatic compound - Cyclobutadiene

  7. Electrophilic Aromatic Substitution and Substituent Effects (McM. 16.5 - 16.6) • Halogenation (bromination) • Nitration • Sulfonation • Alkylation (Friedel Craft) • Acylation (Friedel Craft) • R influence: • Reactivity • Regiochemistry

  8. R: Electron donating By resonance ex. RO-, HO-, O-, R2N-, RHN, H2N-. RCOHN- The ring is activated for E-fil ar subst More reactive than benzene

  9. Reaction in o or p pos. Activating, o/p directing

  10. R: Electron donating The ring is activated for E-fil ar subst More reactive than benzene By inductive effect ex. alkyls Activating, o/p directing

  11. R: Electron withdrawing By resonance effect: -CN, -COR, COH, CO2H, CO2R, -NO2, -SO2R etc Especially electron poor in o/p pos The ring is deactivated for E-fil ar subst Less reactive than benzene

  12. Deactivating, m directing

  13. R: Electron withdrawing By inductive effect: Alternative A: -NR3 Deactivating, m directing Alternative B: -Halogens Deactivating, but o/p directing!! Electron donor by resonance (lone pairs) weak effect Inductively strong withdrawing effect

  14. Regiochemistry in E-fil aromatic subst of disubst. benzene derivs. (McM 16.7) Synth of trisubst. benzene deriv. - Planning of a good reaction sequence (McM 16.12)

  15. Nucleophilic Aromatic Substitution (McM 16.8 - 16.9)

  16. Mechanisms: • SNAr • SN1: Via diazonium salts (McM 24.8) • Benzyne • (SRN1: Involves radicals) • (VNS: Vicarious nucl. Subst.)

  17. SNAr Only on electron defficient arenes (EWG o/p to X, Anion stabilizing effect) (Aromatic heterocycles, McM 28.6) 1st step rate limiting (Aromaticity broken) X=F>Cl>Br>I

  18. Benzyne “Triple bond” between sp2 C p-p overlap sp2-sp2 overlap - weak bond Benzynes unstable / reactive intermed. • Reactivity of benzynes: • Adds nucleophiles • Dienophile in Diels Alder react.

  19. Reactivity of aromatic side chains • Ox. of alkyl side chains • Halogenation benzylic position (c.f. Lab ex. 1) • No ox. cleavage of “double bonds” in benzene • Reduction of benzene (hydrogenation) difficult

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