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Chapter 5

Chapter 5. Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics. Paula Yurkanis Bruice University of California, Santa Barbara. Molecular Formula and the Degree of Unsaturation.

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Chapter 5

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

  2. Molecular Formula and the Degree of Unsaturation molecular formula = CnH2n+2 – 2 hydrogens for every π bond or ring degree of unsaturation = the total numbers of π bonds and rings

  3. Compounds with Molecular Formula = C8H14 CnH2n+2 = C8H18 Therefore, C8H14 has two degrees of unsaturation. compounds with six hydrogens and two degrees of unsaturation

  4. Saturated and Unsaturated Hydrocarbons Saturated hydrocarbons have no double bonds. Unsaturated hydrocarbons have one or more double bonds.

  5. Nomenclature of Alkenes Replace “ane” of alkane with “ene.” The functional group gets the lowest possible number.

  6. Stereoisomers are Named Using a cis or trans Prefix

  7. Nomenclature of Dienes two double bonds = diene

  8. Nomenclature of Alkenes Number in the direction so thatthe functional group gets the lowest number.

  9. Nomenclature of Alkenes Substituents are stated in alphabetical order.

  10. Nomenclature of Alkenes

  11. Nomenclature of Cyclic Alkenes A number is not needed to denote the position of the functional group; it is always between C1 and C2.

  12. Nomenclature of Alkenes

  13. Nomenclature of Alkenes

  14. Vinylic and Allylic Carbons vinylic carbon: the sp2 carbon of an alkene allylic carbon: a carbon adjacent to a vinylic carbon

  15. Double Bonds Have Restricted Rotation Rotation about a double bond breaks the π bond.

  16. Six Atoms of an Alkene are in the Same Plane

  17. Alkenes Have Cis–Trans Isomers cis trans Cis: The hydrogens are on the same side of the ring. Trans: The hydrogens are on opposite sides of the ring. They have different configurations; they can be separated.

  18. Which Is Cis and Which Is Trans?

  19. The E,Z System of Nomenclature Z = Zusammen (together) E = Entgegen (opposite)

  20. Relative Priorities The relative priorities of the two groups depends on the atomic numbers of the atoms attached to the sp2 carbon.

  21. The E and Z Isomers

  22. E and Z

  23. E and Z

  24. Isomers for Compounds with Two Double Bonds

  25. Reactions of Organic Compounds Organic compounds can be divided into families. All members of a family react in the same way. The family a compound belongs to depends on its functional group.

  26. Electrophiles An electrophile has a positive charge, a partial positive charge, or an incomplete octet.

  27. Nucleophiles A nucleophile has a negative charge or a lone pair.

  28. A Nucleophile Reacts with an Electrophile Your first organic reaction is a two-step reaction.

  29. Curved Arrows first step of the reaction Curved arrows are used to show the mechanism of a reaction. The mechanism of a reaction is the step-by-step description of the process by which reactants are converted into products.

  30. Curved Arrows second step of the reaction The curved arrow shows where the electrons start from and where they end up.

  31. How to Draw Curved Arrows

  32. How to Draw Curved Arrows

  33. How to Draw Curved Arrows

  34. How to Draw Curved Arrows

  35. Thermodynamics and Kinetics

  36. A Reaction Coordinate Diagram A reaction coordinate diagram shows the energy changes that take place in each step of a reaction.

  37. The Equilibrium Constant The equilibrium constant gives the concentration of reactants and products at equilibrium.

  38. Exergonic and Endergonic Reactions

  39. Gibbs Free-Energy Change (∆G°)

  40. Calculating ∆H°

  41. Reaction Coordinate Diagrams for Fast and Slow Exergonic and Endergonic Reactions

  42. Free Energy of Activation (∆G‡) The free energy of activation is the energy barrier of the reaction.

  43. Rate of a Reaction Increasing the concentration increases the rate. Increasing the temperature increases the rate. The rate can also be increased by a catalyst.

  44. The Rate of a Reaction versus The Rate Constant for a Reaction

  45. The Rate of a Reaction versus The Rate Constant for a Reaction

  46. The Arrhenius Equation The value of A pertains to the frequency and orientation of collisions. e–Ea/RT The term e–Ea/RT provides the reaction of molecules with the needed activation energy at a given temperature (T).

  47. An Electrophilic Addition Reaction Transition states have partially formed bonds.

  48. Reaction Coordinate Diagram for Each Step of the Addition of HBr to 2-Butene

  49. Reaction Coordinate Diagram forthe Addition of HBr to 2-Butene The rate-limiting step of the reaction is the step that has its transition state at the highest point of the reaction coordinate diagram.

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