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Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics

Chapter 5. Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics Paula Yurkanis Bruice University of California, Santa Barbara. Contents of Chapter 5. General Formulae and Nomenclature of Alkenes Reactivity Considerations Curved Arrow Mechanisms

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Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics

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  1. Chapter 5 Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics Paula Yurkanis Bruice University of California, Santa Barbara

  2. Contents of Chapter 5 • General Formulae and Nomenclature of Alkenes • Reactivity Considerations • Curved Arrow Mechanisms • Thermodynamics and Kinetics Chapter 4

  3. General Molecular Formula for Alkenes • General molecular formula for acyclic alkanes is CnH2n+2 CH3CH2CH2CH2CH3 an alkane C5H12 CnH2n+2 Chapter 4

  4. General Molecular Formula for Alkenes • Each  bond introduced, reduces the H content by 2 CH3CH2CH2CH=CH2 an alkene C5H10 CnH2n Chapter 4

  5. General Molecular Formula for Alkenes • Each ring also reduces the H content by 2 a cyclic alkane C5H10 CnH2n Chapter 4

  6. General Molecular Formula for Alkenes • Generalization: The molecular formula for a hydrocarbon is CnH2n+2 minus 2 hydrogens for every  bond and/or ring present in the molecule • Each  bond or ring is considered a unit of unsaturation. C5H8 CnH2n-2 a cyclic alkene with 2 units of unsaturation Chapter 4

  7. Saturated and Unsaturated Hydrocarbons • Alkanes or saturated hydrocarbons contain the maximum number of carbon-hydrogen bonds CH3CH2CH2CH2CH3 a saturated hydrocarbon Chapter 4

  8. Saturated and Unsaturated Hydrocarbons • Alkenes contain fewer than the maximum number of carbon-hydrogen bonds and are therefore referred to as unsaturated hydrocarbons CH3CH2CH2CH=CH2 an unsaturated hydrocarbon Chapter 4

  9. Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene” Chapter 4

  10. IUPAC Rules for Alkene Nomenclature • The longest chain containing the functional group (the double bond) is numbered such that the double bond is the lowest possible number Chapter 4

  11. IUPAC Rules for Alkene Nomenclature • If there are substituents, the chain is still numbered in a direction that gives the double bond the lowest number Chapter 4

  12. IUPAC Rules for Alkene Nomenclature • If chain has more than one substituent, they are cited in alphabetical (not numerical) order. Rules for alphabetizing are the same as for alkanes Chapter 4

  13. IUPAC Rules for Alkene Nomenclature • If the same number for the double bond is obtained in both directions, number in the direction that gives lowest number to a substituent. Chapter 4

  14. IUPAC Rules for Alkene Nomenclature • In cyclic compounds, a number is not needed to denote the position of the functional group The double bond is assumed to be between carbons 1 and 2 Chapter 4

  15. IUPAC Rules for Alkene Nomenclature • If both directions yield same low number for a functional group and for one substituent, number in the direction which yields the lower number for one of the remaining substituents Chapter 4

  16. IUPAC Rules for Alkene Nomenclature • Two groups containing double bonds that are used as names for substituents are the vinyl group and the allyl group Chapter 4

  17. IUPAC Rules for Alkene Nomenclature The sp2 carbons of an alkene are called vinylic An sp3 adjacent carbon is called allylic Chapter 4

  18. IUPAC Nomenclature of Dienes • Find the longest chain containing both double bonds 1 2 3 4 5 3-butyl-1,4-pentadiene Chapter 4

  19. IUPAC Nomenclature of Dienes • Use corresponding alkane name but replace the “ne” ending with “diene” 3-butyl-1,4-pentadiene “pentane” changed to “pentadiene” Chapter 4

  20. IUPAC Nomenclature of Dienes • Number in the direction that gives the lowest number to a double bond 1,5-heptadiene not 2,6-heptadiene Chapter 4

  21. IUPAC Nomenclature of Dienes • List substituents in alphabetical order 5-ethyl-2-methyl-2,4-heptadiene Chapter 4

  22. IUPAC Nomenclature of Dienes • Place numbers indicating the double bond positions either in front of the parent compound or in the middle of the name immediately before the diene suffix 5-ethyl-2-methyl-2,4-heptadiene or 5-ethyl-2-methyl-hepta-2,4-diene Chapter 4

  23. The E, Z System of Nomenclature Left: Z-1-bromo-2-chloropropene Right: E-1-bromo-2-chloropropene Chapter 4

  24. Relative Stabilities of Alkenes Chapter 4

  25. Relative Stabilities of Alkenes • The more alkyl substituents attached to a double bond the more stable the double bond. • Trans alkenes more stable than cis alkenes • Not difficult concepts but should be learned now in order to understand Chapter 9 later. Chapter 4

  26. Reactivity Considerations • Electrophiles react with nucleophiles • An alkene has electron density above and below the  bond making it electron-rich and therefore a nucleophile • Therefore alkenes react with electrophiles Chapter 4

  27. Reaction Mechanisms • We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact Chapter 4

  28. Reaction Mechanisms • Curved arrows are drawn only from the electron-rich site to the electron deficient site Chapter 4

  29. Thermodynamics • When G° is negative the reaction is exergonic Chapter 4

  30. Thermodynamics • When G° is positive the reaction is endergonic Chapter 4

  31. Kinetics • Knowing the G° of a reaction will not tell us how fast it will occur or if it will occur at all • We need to know the rate of reaction • The rate of a reaction is related to the height of the energy barrier for the reaction, G‡, the free energy of activation Chapter 4

  32. Free Energy of Activation Chapter 4

  33. Rate-Determining Step • Formation of the carbocation intermediate is the slower of the two steps • It is the rate-determining step Chapter 4

  34. Rate-Determining Step • Carbocation intermediates are consumed by bromide ions as fast as they are formed • The rate of the overall reaction is determined by the slow first step Chapter 4

  35. Transition States and Intermediates • It is important to distinguish between a transition state and a reaction intermediate • A transition state • is a local maximum in the reaction coordinate diagram • has partially formed and partially broken bonds • has only fleeting existence Chapter 4

  36. Transition States and Intermediates • An intermediate • is at a local minimum energy in the reaction coordinate diagram • may be isolated in some cases Chapter 4

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