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Chapter 5 – Structure and Preparation of Alkenes

Chapter 5 – Structure and Preparation of Alkenes. Double bond - now dealing with sp 2 hybrid carbon. 5.1 Nomenclature. 1-but ene. 1-hex ene. 2-methyl-2-hex ene. 2,3-dimethyl-2-but ene. 5-methyl-4-hex en -1-ol. 6-bromo-3-propyl-1-hex ene. Common Alkene Substituents. vinyl.

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Chapter 5 – Structure and Preparation of Alkenes

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  1. Chapter 5 – Structure and Preparation of Alkenes Double bond - now dealing with sp2 hybrid carbon 5.1 Nomenclature 1-butene 1-hexene 2-methyl-2-hexene 2,3-dimethyl-2-butene 5-methyl-4-hexen-1-ol 6-bromo-3-propyl-1-hexene

  2. Common Alkene Substituents vinyl allyl isopropenyl Cycloalkenes cyclohexene 1-chlorocyclopentene 3-bromocyclooctene

  3. 5.2 Structure and bonding in ethylene Figure 5.1

  4. 5.3-5.4 cis-trans isomerism in alkenes 1-butene 2-methylpropene cis-2-butene trans-2-butene Cinnamaldehyde (trans alkene - E) cis alkene (Z) See Table 5.1 for priority rules

  5. Interconversion of cis and trans-2-butene

  6. 5.5-5.6 Heats of combustion of isomeric C4H8 alkenes Figure 5.3

  7. 5.5-5.6 Heats of combustion of isomeric C4H8 alkenes Figure 5.2 Generally, the moresubstituted an alkene, the more stable

  8. Molecular models of cis-2-butene and trans-2-butene Figure 5.4

  9. 5.7 Cycloalkenes - trans not necessarily more stable than cis C-12 cis and trans ~ equal in energy Sterculic acid (natural product)

  10. 5.8 Preparation of Alkenes - Elimination reactions 5.9 Dehydration of Alcohols

  11. 5.10 Zaitsev Rule Dehydration usually results in more highly substituted alkene being major product - Zaitsev rule (regioselectivity)

  12. 5.11 Stereoselectivity in Alcohol Dehydration One stereoisomer is usually favoured in dehydrations When cis and trans isomers are possible in this reaction the more stable isomer is usually formed in higher yield

  13. 5.12 Acid-catalyzed Alcohol Dehydration E1 and E2 E1 E2 on 1o alcohols – concerted with no cation

  14. 5.13 Carbocation Rearrangements in E1 Reactions Cation rearrangement leads to more stable cation

  15. Orbital representation of methyl migration Figure 5.6

  16. 5.13 Hydride shifts to more stable carbocations 1o carbocation?????

  17. 5.14 Dehydrohalogenation - Elimination with loss of H-X 100% Zaitsev rule followed for regioisomers when a small base such as NaOCH3, NaOCH2CH3 is used. Trans usually favoured over cis.

  18. 5.15 The E2 Mechanism - Elimination Bimolecular • Reaction occurs under basic conditions • Reaction is concerted • Rate depends on [base][alkyl halide] i.e. Bimolecular - E2 • C-H bond breaking, C=C bond forming and C-X bond breaking • events all occur at the same time

  19. The E2 Mechanism - Elimination Bimolecular

  20. 5.16 Anti Elimination faster than Syn Elimination E2 Elimination usually faster when H and leaving group are anti periplanar as opposed to syn periplanar.

  21. Conformations of cis- and trans-4-tert-butylcyclohexyl

  22. Favourable conformations for fast elimination E2 Elimination usually faster when H and leaving group are anti periplanar as opposed to syn periplanar.

  23. Not covering Section 5.17 (Isotope Effects)

  24. 5.18 Different Halide Elimination Mechanism - E1 R.D.S. is now unimolecular, therefore E1 - usually under neutral or acidic conditions (compare with dehydration earlier)

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