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Chapter 8
                                    Lecture
                             Structure and Synthesis
                                    of Alkenes
                          Alkenes are also called olefins,
                             meaning ―oil-forming gas.



 © 2013 Pearson Education, Inc.


© 2013 Pearson Education, Inc.
Bonding in Ethylene




  C=C bond is shorter than C—C bond
  a) sp2 hybrid orbitals have _______ s character than the sp3
  hybrid orbitals. They are __________ to the nucleus.
  b) Pi overlap brings carbon atoms _______
         Bonding requires two p orbitals overlapping __________




© 2013 Pearson Education, Inc.    Chapter 7                       2
Cis and trans isomers cannot be
                interconverted without breaking the pi
                         bond (264 kJ/mole).




      No rotation around the carbon–carbon bond is possible
        giving rise to Cistrans geometric isomers




© 2013 Pearson Education, Inc.   Chapter 7                    3
Elements (degrees) of Unsaturation. Also
             called index of hydrogen deficiency.



   • Unsaturation: A structural element that decreases the number of
     hydrogens in the molecule by two.
   • Double bonds and rings are elements of unsaturation (can not distinguish
     with calculation

   Calculate the degrees of unsaturation for a compound with formula C5H8 and
     propose a structure
   •     Reference (saturated compound CnH2n+2)
   •     Compound


© 2013 Pearson Education, Inc.      Chapter 7                           4
• For each Group VA atom increase Hs in reference by one
       • For each halogens decrease Hs atoms in reference by
       one
       • Group of oxygen does not change the C:H ratio, ignore
       oxygen in the reference formula.

       Calculate the IHD for a Compound with Formula

       C4H7Br.



       C6H7N.




       C5H4O.
© 2013 Pearson Education, Inc.   Chapter 7                    5
IUPAC and New IUPAC




                                 In a ring the
                                 double bond is
                                 given carbon 1
                                 and carbon 2.

© 2013 Pearson Education, Inc.       Chapter 7    6
Polyenes: se di-, tri-, tetra- before the ending -ene to specify how many double
                                  bonds are present.


                         Give the double bonds the lowest numbers possible.




© 2013 Pearson Education, Inc.                Chapter 7                       7
EZ nomenclature: Use the Cahn–Ingold–Prelog rules to assign priorities to
  groups attached to each carbon in the double bond.
  If high-priority groups are on the same side, the name is Z (for zusammen).
  If high-priority groups are on opposite sides, the name is E (for entgegen).




                                 E-1-bromo-1-chloropropene




© 2013 Pearson Education, Inc.             Chapter 7                       8
Heat of Hydrogenation can provide valuable data as to the stability of the
   double bond. The more substituted the double bond, the ___________its
   heat of hydrogenation.




© 2013 Pearson Education, Inc.       Chapter 7                              9
Stability: cis-2-butene<iso-butene<trans-2-butene
                     -120 kJ >-117 kJ > -116 kJ




    Cyclopropene is highly strained.
    Rings that are five-membered or larger can easily
    accommodate double bonds, and these cycloalkenes react
    much like straight-chain alkenes.




© 2013 Pearson Education, Inc.      Chapter 7            10
• Cis isomer is ______ stable than trans in small cycloalkenes.
• Must have at least _________carbons to form a stable trans double
  bond.
• For cyclodecene (and larger), the trans double bond is almost as stable
  as the cis.
• All cycloalkenes are assumed to be cis unless otherwise specifically
  named trans.




© 2013 Pearson Education, Inc.     Chapter 7                            11
Bredt’s Rule: A bicyclic compound cannot have a double
   bond at a bridgehead position unless one of the rings
   contains at least eight carbon atoms.




                         Which of the following alkenes are
                         stable?




© 2013 Pearson Education, Inc.             Chapter 7          12
Summary stability

          •    How is it quantitated?
          •    Substitution
          •    Stereochemistry
          •    Ring size
          •    Cisvs trans cyclos
          •    Bredt’s rule

                                        Chapter 7    13
© 2013 Pearson Education, Inc.
Alkenes have relatively low boiling points which depend on
       surface area and polarizability (______________ forces
    _______ dense than water _________ on water
    Slightly polar:
         Pi bond is polarizable, so instantaneous dipole–
          dipole interactions occur.
         Alkyl groups are electron-donating toward the pi
          bond, so may have a small dipole moment.
    • Cisalkenes have a greater dipole moment than trans
       alkenes, so they will be slightly polar and have higher bp




© 2013 Pearson Education, Inc.   Chapter 7                      14
Alkene Synthesis
                                     Overview
    1. R-X ----E2 dehydrohalogenation (-HX) Alkene

    2. R-X ----E1 dehydrohalogenation (-HX) Alkene

    3. Vic dibromide---Dehalogenation (-X2) Alkene

    4. Alcohol ---Dehydration (-H2O) Alkene


© 2013 Pearson Education, Inc.         Chapter 7      15
1. Dehydrohalogenationby the E2
                   Mechanism



   • Strong_________ abstracts H+ as double bond forms
     and X- ___________ from the adjacent carbon.

   • __________ and hindered ______________ alkyl
     halides give alkenes in good yields.Tertiary halides are
     the best E2 substrates because they cannot undergo
     __________substitution.
© 2013 Pearson Education, Inc.   Chapter 7                      16
Bulky Bases for E2 Reactions




          • If the substrate is prone to substitution, a bulky base
            can minimize the amount of substitution.
          • Large alkyl groups on a bulky base hinder its
            approach to attack a carbon atom (substitution), yet it
            can easily abstract a proton (elimination).

© 2013 Pearson Education, Inc.     Chapter 7                          17
Hofmann Product
         Bulky bases, such as potassium tert-butoxide, abstract the
         least hindered H+,giving the less substituted alkene as the
         major product (Hofmann product).




© 2013 Pearson Education, Inc.         Chapter 7                  18
E2 Reactions Are Stereospecific: The stereochemistry of the
      product depends on the stereochemistry of the reactant




© 2013 Pearson Education, Inc.   Chapter 7                   19
Show that the dehalogenation of 2,3-dibromobutane by
             iodide ion is stereospecific by showing that the two
            diastereomers of the starting material give different
                         diastereomers of the product.
  Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and
  coplanar, we find that the product will be trans-2-butene. A similar conformation of either
  enantiomer of the (±) diastereomer shows that the product will be cis-2-butene. (Hint: Your
  models will be helpful.)
   Meso-2,3-dibromobutane




   (R,R)-2,3-dibromobutane




© 2013 Pearson Education, Inc.              Chapter 7                                     20
E2 Reactions of
                  bromocyclohexane is slow




        • An anti-coplanar conformation (180°) can only be
          achieved when both the hydrogen and the halogen
          occupy ___________ positions.
        • The chair must flip to the conformation with the axial
          halide in order for the elimination to take place.
© 2013 Pearson Education, Inc.    Chapter 7                        21
Explain why the following deuterated 1-bromo-2-
      methylcyclohexane undergoes dehydrohalogenation by the
       E2 mechanism, to give only the indicated product. Two
                  other alkenes are not observed.




  In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial
  relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the
  bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing
  a trans-diaxial arrangement.




© 2013 Pearson Education, Inc.              Chapter 7                                      22
2. Debrominationof Vicinal Dibromides: reduction
             with iodide ion in acetone follows an an E2
                              mechanism




© 2013 Pearson Education, Inc.   Chapter 7               23
3. E1 reaction
  •     Tertiary reacts faster than secondary alkyl halides:
  •     Carbocationintermediate; Rearrangements are possible.
  •     Solvolysis reactions in solvents as water or alcohols
  •     Usually have SN1 products, too, since the solvent can attack
        the carbocation directly.




© 2013 Pearson Education, Inc.        Chapter 7                  24
4. Dehydration of Alcohols




• Use concentrated H2SO4 or H3PO4 and remove low-boiling
  alkene as it forms to shift the equilibrium and increase the yield
  of the reaction.
• E1 mechanism.
• Carbocation intermediate. Rearrangements can occur.
• Reaction obeys Zaitsev’s rule.
© 2013 Pearson Education, Inc.   Chapter 7                     25
Dehydration Mechanism: E1
  Step 1:Protonation of the hydroxyl group (fast equilibrium).




    Step 2: Ionization to a carbocation (slow; rate limiting).




   Step 3: Deprotonation to give the alkene (fast).




© 2013 Pearson Education, Inc.         Chapter 7                 26
In acid-catalyzed mechanisms,
                    the first step is often addition of
                      H+, and the last step is often
                                 loss of H+.



© 2013 Pearson Education, Inc.     Chapter 7              27
Propose a mechanism for the sulfuric acid–catalyzed dehydration of t-butyl alcohol.




   The first step is protonation of the hydroxyl group, which converts it to a good leaving group.




   The second step is ionization of the protonated alcohol to give a carbocation.




   Abstraction of a proton completes the mechanism.




© 2013 Pearson Education, Inc.                Chapter 7                                        28
Other Industrial methods:
                        a) Catalytic Cracking of Alkanes



  • Long-chain alkane is heated with a catalyst to
    produce an alkene and shorter alkane.
  • Complex mixtures are produced.




© 2013 Pearson Education, Inc.        Chapter 7            29
b) Dehydrogenation of Alkanes




          • Dehydrogenation is the removal of H2 from a
            molecule, forming an alkene (the reverse of
            hydrogenation).
          • This reaction has an unfavorable enthalpy
            change but a favorable entropy change.
© 2013 Pearson Education, Inc.         Chapter 7           30
Methods to synthesize alkenes




                                 Chapter 7   31
© 2013 Pearson Education, Inc.

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08 alkesynth

  • 1. Chapter 8 Lecture Structure and Synthesis of Alkenes Alkenes are also called olefins, meaning ―oil-forming gas. © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc.
  • 2. Bonding in Ethylene C=C bond is shorter than C—C bond a) sp2 hybrid orbitals have _______ s character than the sp3 hybrid orbitals. They are __________ to the nucleus. b) Pi overlap brings carbon atoms _______ Bonding requires two p orbitals overlapping __________ © 2013 Pearson Education, Inc. Chapter 7 2
  • 3. Cis and trans isomers cannot be interconverted without breaking the pi bond (264 kJ/mole). No rotation around the carbon–carbon bond is possible giving rise to Cistrans geometric isomers © 2013 Pearson Education, Inc. Chapter 7 3
  • 4. Elements (degrees) of Unsaturation. Also called index of hydrogen deficiency. • Unsaturation: A structural element that decreases the number of hydrogens in the molecule by two. • Double bonds and rings are elements of unsaturation (can not distinguish with calculation Calculate the degrees of unsaturation for a compound with formula C5H8 and propose a structure • Reference (saturated compound CnH2n+2) • Compound © 2013 Pearson Education, Inc. Chapter 7 4
  • 5. • For each Group VA atom increase Hs in reference by one • For each halogens decrease Hs atoms in reference by one • Group of oxygen does not change the C:H ratio, ignore oxygen in the reference formula. Calculate the IHD for a Compound with Formula C4H7Br. C6H7N. C5H4O. © 2013 Pearson Education, Inc. Chapter 7 5
  • 6. IUPAC and New IUPAC In a ring the double bond is given carbon 1 and carbon 2. © 2013 Pearson Education, Inc. Chapter 7 6
  • 7. Polyenes: se di-, tri-, tetra- before the ending -ene to specify how many double bonds are present. Give the double bonds the lowest numbers possible. © 2013 Pearson Education, Inc. Chapter 7 7
  • 8. EZ nomenclature: Use the Cahn–Ingold–Prelog rules to assign priorities to groups attached to each carbon in the double bond. If high-priority groups are on the same side, the name is Z (for zusammen). If high-priority groups are on opposite sides, the name is E (for entgegen). E-1-bromo-1-chloropropene © 2013 Pearson Education, Inc. Chapter 7 8
  • 9. Heat of Hydrogenation can provide valuable data as to the stability of the double bond. The more substituted the double bond, the ___________its heat of hydrogenation. © 2013 Pearson Education, Inc. Chapter 7 9
  • 10. Stability: cis-2-butene<iso-butene<trans-2-butene -120 kJ >-117 kJ > -116 kJ Cyclopropene is highly strained. Rings that are five-membered or larger can easily accommodate double bonds, and these cycloalkenes react much like straight-chain alkenes. © 2013 Pearson Education, Inc. Chapter 7 10
  • 11. • Cis isomer is ______ stable than trans in small cycloalkenes. • Must have at least _________carbons to form a stable trans double bond. • For cyclodecene (and larger), the trans double bond is almost as stable as the cis. • All cycloalkenes are assumed to be cis unless otherwise specifically named trans. © 2013 Pearson Education, Inc. Chapter 7 11
  • 12. Bredt’s Rule: A bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. Which of the following alkenes are stable? © 2013 Pearson Education, Inc. Chapter 7 12
  • 13. Summary stability • How is it quantitated? • Substitution • Stereochemistry • Ring size • Cisvs trans cyclos • Bredt’s rule Chapter 7 13 © 2013 Pearson Education, Inc.
  • 14. Alkenes have relatively low boiling points which depend on surface area and polarizability (______________ forces _______ dense than water _________ on water Slightly polar:  Pi bond is polarizable, so instantaneous dipole– dipole interactions occur.  Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. • Cisalkenes have a greater dipole moment than trans alkenes, so they will be slightly polar and have higher bp © 2013 Pearson Education, Inc. Chapter 7 14
  • 15. Alkene Synthesis Overview 1. R-X ----E2 dehydrohalogenation (-HX) Alkene 2. R-X ----E1 dehydrohalogenation (-HX) Alkene 3. Vic dibromide---Dehalogenation (-X2) Alkene 4. Alcohol ---Dehydration (-H2O) Alkene © 2013 Pearson Education, Inc. Chapter 7 15
  • 16. 1. Dehydrohalogenationby the E2 Mechanism • Strong_________ abstracts H+ as double bond forms and X- ___________ from the adjacent carbon. • __________ and hindered ______________ alkyl halides give alkenes in good yields.Tertiary halides are the best E2 substrates because they cannot undergo __________substitution. © 2013 Pearson Education, Inc. Chapter 7 16
  • 17. Bulky Bases for E2 Reactions • If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. • Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination). © 2013 Pearson Education, Inc. Chapter 7 17
  • 18. Hofmann Product Bulky bases, such as potassium tert-butoxide, abstract the least hindered H+,giving the less substituted alkene as the major product (Hofmann product). © 2013 Pearson Education, Inc. Chapter 7 18
  • 19. E2 Reactions Are Stereospecific: The stereochemistry of the product depends on the stereochemistry of the reactant © 2013 Pearson Education, Inc. Chapter 7 19
  • 20. Show that the dehalogenation of 2,3-dibromobutane by iodide ion is stereospecific by showing that the two diastereomers of the starting material give different diastereomers of the product. Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and coplanar, we find that the product will be trans-2-butene. A similar conformation of either enantiomer of the (±) diastereomer shows that the product will be cis-2-butene. (Hint: Your models will be helpful.) Meso-2,3-dibromobutane (R,R)-2,3-dibromobutane © 2013 Pearson Education, Inc. Chapter 7 20
  • 21. E2 Reactions of bromocyclohexane is slow • An anti-coplanar conformation (180°) can only be achieved when both the hydrogen and the halogen occupy ___________ positions. • The chair must flip to the conformation with the axial halide in order for the elimination to take place. © 2013 Pearson Education, Inc. Chapter 7 21
  • 22. Explain why the following deuterated 1-bromo-2- methylcyclohexane undergoes dehydrohalogenation by the E2 mechanism, to give only the indicated product. Two other alkenes are not observed. In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing a trans-diaxial arrangement. © 2013 Pearson Education, Inc. Chapter 7 22
  • 23. 2. Debrominationof Vicinal Dibromides: reduction with iodide ion in acetone follows an an E2 mechanism © 2013 Pearson Education, Inc. Chapter 7 23
  • 24. 3. E1 reaction • Tertiary reacts faster than secondary alkyl halides: • Carbocationintermediate; Rearrangements are possible. • Solvolysis reactions in solvents as water or alcohols • Usually have SN1 products, too, since the solvent can attack the carbocation directly. © 2013 Pearson Education, Inc. Chapter 7 24
  • 25. 4. Dehydration of Alcohols • Use concentrated H2SO4 or H3PO4 and remove low-boiling alkene as it forms to shift the equilibrium and increase the yield of the reaction. • E1 mechanism. • Carbocation intermediate. Rearrangements can occur. • Reaction obeys Zaitsev’s rule. © 2013 Pearson Education, Inc. Chapter 7 25
  • 26. Dehydration Mechanism: E1 Step 1:Protonation of the hydroxyl group (fast equilibrium). Step 2: Ionization to a carbocation (slow; rate limiting). Step 3: Deprotonation to give the alkene (fast). © 2013 Pearson Education, Inc. Chapter 7 26
  • 27. In acid-catalyzed mechanisms, the first step is often addition of H+, and the last step is often loss of H+. © 2013 Pearson Education, Inc. Chapter 7 27
  • 28. Propose a mechanism for the sulfuric acid–catalyzed dehydration of t-butyl alcohol. The first step is protonation of the hydroxyl group, which converts it to a good leaving group. The second step is ionization of the protonated alcohol to give a carbocation. Abstraction of a proton completes the mechanism. © 2013 Pearson Education, Inc. Chapter 7 28
  • 29. Other Industrial methods: a) Catalytic Cracking of Alkanes • Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane. • Complex mixtures are produced. © 2013 Pearson Education, Inc. Chapter 7 29
  • 30. b) Dehydrogenation of Alkanes • Dehydrogenation is the removal of H2 from a molecule, forming an alkene (the reverse of hydrogenation). • This reaction has an unfavorable enthalpy change but a favorable entropy change. © 2013 Pearson Education, Inc. Chapter 7 30
  • 31. Methods to synthesize alkenes Chapter 7 31 © 2013 Pearson Education, Inc.