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Chapter 12 Alkene Reactions

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Chapter 12 Alkene Reactions. Catalytic Hydrogenation Thermodynamics of addition reactions C=C p -bond is weak and thus reactive Addition reactions: D H = (DH o p + DH o AB ) – (DH o CA + DH o CB ) = - D H CA and CB single s -bonds stronger than AB + p -bond

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chapter 12 alkene reactions
Chapter 12 Alkene Reactions
  • Catalytic Hydrogenation
    • Thermodynamics of addition reactions
      • C=C p-bond is weak and thus reactive
      • Addition reactions:
      • DH = (DHop + DHoAB) – (DHoCA + DHoCB) = -DH
      • CA and CB single s-bonds stronger than AB + p-bond
      • Additions usually occur spontaneously and -DH is released (Table 12-1)
    • Hydrogenation of Alkenes
      • Addition of H2 to C=C requires a catalyst to lower Ea
      • Reaction occurs at the metal surface (Pd/C or PtO2, or Ra Ni)
      • Solvent is usually MeOH, EtOH, or HOAc
Steric Bulk may dictate which side can approach the metal surface
  • p-bond as Nucleophile: HX Additions
    • A p-bond is an e- rich cloud that electrophiles can attack
      • H+ is a strong electrophile
      • Low temperature reduces chance of rearrangement
Markovnikov Rule
    • H+ goes to the least substituted C, and X- goes to the most substituted C
    • Formation of the most stable carbocation directs the reaction. Initial protonation gives the most stable carbocation.



Alcohol Synthesis bye Electrophilic Hydration
    • Strong aqueous mineral acid gives H2O addition
      • This reaction obeys Markovnikov Rule
      • Mechanism is the reverse of the acid catalyzed alcohol dehydration
    • Alkene Hydration vs. Alcohol Dehydration
      • All steps in the mechanism are reversible: Equilibrium
      • H+ acts as catalyst and is not consumed
      • Favor alcohol with low temperature and excess water
      • Favor alkene with concentrated acid and heat
Thermodynamic Control
      • When reversible protonation can happen, an equilibrium mixture exists
      • The most stable product will be major, because minor products will be converted back to the cation, then to the most stable product
      • We can use acid to interconvert alkene isomers to most stable one
  • Halogen Addition
    • Halogen gases (Cl2) don’t seem very electrophilic, but will add to alkenes
      • Cl2 and Br2 in CCl4 solvent at room temperature best conditions
      • F2 reacts violently; I2 doesn’t react at all (DHo = 0)
      • Disappearance of red-brown Br2 upon addition to unknown signals alkene
Halogen Addition Mechanism
    • Anti addition is always observed
    • Bromonium ion
      • Br—Br has a very large, polarizable s-bond
      • C=C p-bond nucleophile attacks the d+ end of Br—Br (like SN2)
      • The result is a Bromonium cation and Br- anion



The last step is nucleophilic attack by Br- on the bromonium ion
  • Other Additions
    • Halonium cation can trap other nucleophiles
      • Cl2 works just like Br2 = chloronium ion
      • Mixed products can be useful synthetic intermediates
Regioselectivity of Halonium ion mixed products
    • Halogen ends up on the less-substituted C. Greater d+ on more subst. C
    • Nucleophile ends up on more substituted C
    • Markovnikov-like addition because electrophile (H+, Br+) behaves same
    • Other reagents behave as electrophile-nucleophile pair (in that order)
      • Br—Cl b. Br—CN
      • I—Cl d. RS—Cl
      • XHg—X (X = acetate)
C. Oxymercuration—Demercuration
  • Oxymercuration proceeds in an anti addition, just like Br2 addition
  • Demercuration replaces the Hg with H
  • The result is Markovnikov addition just like acidic hydration reaction
  • The advantage is that no rearrangement can take place