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

Explore the formation, structure, and reactivity of aromatic compounds, including Alkenes, Dienes, and Electrophilic Aromatic Substitution. Learn about the criteria for aromaticity, electrophilic and nucleophilic substitution reactions, and the molecular orbitals involved.

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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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