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Alkenes: Structure, Isomerism, and Preparation

This chapter focuses on the structure, nomenclature, isomerism, and preparation of alkenes, including important concepts such as cis-trans isomerism and heats of combustion. Learn about priority rules, stability of alkene isomers, molecular models, and elimination reactions. Understand the Zaitsev rule, stereoselectivity, acid-catalyzed alcohol dehydration, carbocation rearrangements, dehydrohalogenation, and the E2 mechanism. Dive into the bimolecular nature of E2 elimination reactions and factors influencing reaction rates.

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Alkenes: Structure, Isomerism, and Preparation

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  1. Chapter 5 – Structure and Preparation of Alkenes Double bond - now dealing with sp2 hybrid carbon YSU

  2. 5.1 – Structure and Nomenclature of Alkenes YSU 1-butene 1-hexene 2-methyl-2-hexene 6-bromo-3-propyl-1-hexene 2,3-dimethyl-2-butene 5-methyl-4-hexen-1-ol

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

  4. 5.2 Structure and bonding in ethylene Figure 5.1 YSU

  5. 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 YSU

  6. Interconversion of cis and trans-2-butene YSU

  7. 5.5-5.6 Heats of combustion of isomeric C4H8 alkenes Figure 5.3 YSU

  8. 5.5-5.6 Heats of combustion of isomeric C4H8 alkenes Figure 5.2 Generally, the moresubstituted an alkene, the more stable YSU

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

  10. 5.7 Cycloalkenes - trans not necessarily more stable than cis C-12 cis and trans ~ equal in energy Sterculic acid (natural product) YSU

  11. 5.8 Preparation of Alkenes - Elimination reactions YSU 5.9 Dehydration of Alcohols

  12. 5.10 Zaitsev Rule Dehydration usually results in more highly substituted alkene being major product - Zaitsev rule (regioselectivity) YSU

  13. 5.10 Zaitsev Rule YSU

  14. 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 YSU

  15. 5.12 Acid-catalyzed Alcohol Dehydration – E1 E1 YSU

  16. 5.13 Carbocation Rearrangements in E1 Reactions Cation rearrangement leads to more stable cation YSU

  17. Orbital representation of methyl migration Figure 5.6 YSU

  18. 5.13 Hydride shifts to more stable carbocations YSU 1o carbocation?????

  19. 5.14 Dehydrohalogenation - Elimination with loss of H-X YSU 100% Zaitsev rule followed for regioisomers when a small base such as NaOCH3, NaOCH2CH3 is used. Trans usually favoured over cis.

  20. 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 YSU

  21. The E2 Mechanism - Elimination Bimolecular YSU

  22. 5.16 Anti Elimination faster than Syn Elimination YSU E2 Elimination usually faster when H and leaving group are anti periplanar as opposed to syn periplanar.

  23. Conformations of cis- and trans-4-tert-butylcyclohexyl YSU

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

  25. Not covering Section 5.17 (Isotope Effects) YSU

  26. 5.18 Different Halide Elimination Mechanism - E1 R.D.S. is now unimolecular, E1 - usually under neutral/acidic conditions YSU

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