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Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah

Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah Islamic University of Gaza. Effect of double bond on reactions (Double bond as a substituent). A) Radical and nucleophilic substitution B) Conjugated diens. Free Radical Halogenation of Alkenes.

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Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah

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  1. Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

  2. Effect of double bond on reactions(Double bond as a substituent) • A) Radical and nucleophilic substitution • B) Conjugated diens

  3. Free Radical Halogenation of Alkenes • Reactions at the double bond

  4. Substitution at the Alkane

  5. Free Radical addition does not occur at high temperature Why? • Because the allyl radical formed leading to substitution is more stable than the secondary one leading to addition

  6. Free Radical substitution in alkenesorientation and reactivity

  7. Bromination of Alkenes at the allylic position by N-Bromosuccinimide

  8. Allylic Rearrangement

  9. Orbital picture of the allyl Radical

  10. Hyperconjugation and stability of Radicals • Hyperconjugation: Delocalization of electrons involving sigma bond orbitals over three atoms (two carbons and one hydrogen). • Three possible hyperconjugation structures for the ethyl radical, six for isopropyl and nine for tertiary butyl

  11. The allylcation as a resonance hybrid • SN1 Substitution in allylic substrates

  12. The allyl carbocation is another example of a conjugated system. • Conjugation stabilizes the allyl carbocation.

  13. Example: SN1 substitution using a weak nucleophile (ethanol)

  14. Example: SN2 substitution using a strong nucleophile (EtO-)

  15. Nucleophilic substitution in vinylic substrates is very difficult • To solve the problem, you need • a) very good leaving group • b) Electron release in the carbocation being formed

  16. Example

  17. Resonance in Conjugated Dienes

  18. Having three or more p orbitals on adjacent atoms allows p orbitals to overlap and electrons to delocalize. • 1,4-Pentadiene is an isolated diene. • The  bonds in 1,4-pentadiene are too far apart to be conjugated.

  19. Conjugation occurs whenever p orbitals can overlap on three or more adjacent atoms. • The four p orbitals on adjacent atoms make a 1,3-diene a conjugated system.

  20. Stability of substituted alkenes due to hyperconjugation • The greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the alkene

  21. Electrophilic addition to conjugated diene

  22. Mechanism

  23. Example: suggest a mechanism

  24. 1,2- vs. 1,4-addition. Rate vs. equilibrium • The 1,2-product is the kinetic product (formed faster) • The 1,4- product is the thermodynamic product (more stable)

  25. Volcanization (Formation of sulfur bridges between different chaines)

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