Chemistry 3719 Chapter 6

  1. What intermediates are formed in electrophilic addition of HBr to alkenes?
  2. Why does HBr react faster than HCl in addition reactions with alkenes?
    HBr is a stronger acid than HCl
  3. What is Markovnikov's rule?
    When HX adds to an alkene, the H adds to the carbon with the most H atoms and X adds to the carbon with the fewest H atoms.
  4. What is the major product formed when HBr adds to 2-methylpropene?

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  5. Which reaction sequence may be used to completely cleave an alkene?
    Use ozonolysis followed by reaction with Zn in H2O
  6. What intermediates are formed when HBr adds to alkenes in the presence of peroxides?
    Free radicals
  7. What stereochemical outcome is observed when Br2 adds to alkenes in organic solvents?
    Trans alignment of the Br atoms via anti addition
  8. What type of product results when alkenes react with Br2 in H2O?
    A bromohydrin, e.g.:

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  9. What type of product results when alkenes react with m-CPBA?
    An epoxide, e.g.:

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  10. What product results when 1,2-dimethylcyclohexene reacts with H2 and Pd?

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  11. Why is the addition of H2 to an alkene generally exothermic?
    A weaker pi bond is swapped for stronger sigma bonds
  12. Why are tertiary carbocations generally more stable than primary carbocations?
    Tertiary carbocations have more adjacent sigma bonds which contribute to extra stabilization through hyperconjugation
  13. What driving forces lead to carbocation rearrangements in electrophilic addition reactions?
    The chance to go from a secondary to a tertiary carbocation, as well as the chance to alleviate ring strain in small cycles
  14. What role does an acid play the hydration of alkenes?
    The acid catalyzes the reaction by protonating the double bond. The proton is regenerated in a subsequent step.
  15. What conditions are required to add H2O to an alkene in the "anti-Markovnikov" manner?
    Use a borane (H-BR2) to add to the alkene, then oxidize the resulting alkylborane to the alcohol using NaOH and H2O2.
  16. How is the stereochemistry of addition retained in the oxidation step of the hydroboration-oxidation process?
    The migration of the carbon framework from B to O is concerted, e.g.:

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  17. What is the name of the major product formed from the reaction of cyclohexene with Br2 in a non-polar organic solvent?
  18. When HBr adds to 1-butene in the presence of peroxides, why is 1-bromobutane formed more than 2-bromobutane?
    The Br radical adds to the double bond at C-1 preferably to give a secondary radical, which results in 1-bromobutane; the other product would be formed via a less stabilized primary radical.
  19. What is Le Chatelier's principle?
    A system at equilibrium adjusts so as to minimize any stress applied to it
  20. Why does running certain reactions at high temperature overcome seemingly unfavourable enthalpic changes?
    High temperatures can make the entropic component of the Gibbs free energy equation (TdeltaS) overcome the enthalpic component (deltaH) thus resulting in a favourable deltaG.
Card Set
Chemistry 3719 Chapter 6
Chemistry 3719