Orgo 7.10-7.11

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  1. __ of alcohols is a common method for making alkenes. This word literally means __.
    • Dehydration 
    • removal of water
  2. __ is reversible, and in most cases, the __is not large. In fact, the reverse reaction (__) is a method for converting alkenes to alcohols.
    • dehydration
    • equilibrium constant
    • hydration
  3. Dehydration can be forced to completion by doing what?
    The alkene boils at a lower temp than the alcohol becuase the __. A carefully controlled distillation removes the alkene while leaving the alcohol in the reaction mixture.
    • removing the products from the reaction mixture as they form. 
    • alcohol is H bonded
  4. __ and __ are often used as reagents for dehydration because these acids act both as __ and as __. Hydration of these acids is strongly __.
    • concentrated sulfuric acid and or concentrated phosphoric acid
    • acidic catalysts
    • dehydrating agents
    • exothermic
  5. The mechanism of dehydration resembles the E1 mechanism. Why?
    the -OH is a poor leaving group, but protonation by the acidic catalyst converts it to a good leaving group. In the second step, loss of water from the protonated alcohol gives the carbocation, which is a very strong acid: Any weak base such as H2O or HSO4- can abstract the proton in the final ste
  6. Acid-catalyzed dehydration of an alcohol
    • 1) protonation of the -OH group (fast equilibrium)
    • 2) ionization to a carbocation (slow; rate limiting)
    • 3) deprotonation to give the alkene (fast)
  7. Like other E1 reactions, alcohol dehydration follows an order of reactivity that reflects __. What is it?
    • carbocation stability
    • tertiary reacts faster than secondary; primary are the least reactive
  8. __ are common in alc. dehy. In most cases, Zaitsev's rule applies: __.
    • rearrangements of the carbocation intermediates
    • the major product is the most subbed double bond
  9. The least expensive way to make alkenes on a large scale is by __.
    catalytic cracking of petroleum: heating a mixture of alkanes in the presence of a catalyst, forming alkenes by bond cleavage to give alkene and shortened alkane
  10. Cracking is used to make small __. The avg. MW and relative amounts of alkanes and alkenes can be controlled by varying the __,__, and __ in the cracking process.
    • alkenes
    • temp
    • catalyst
    • concentration of hydrogen
  11. __ is the removal of H2 from a molecule.__of an alkane gives an alkene. This reaction has an unfavorable __ but a favorable __.
    • dehydrogenation x2
    • enthalpy change
    • entropy change
  12. The hydrogenation of alkenes is __, with values of deltaH around -80 to -120. Therefore, dehydrogenation is __ and has an unfavorable value of deltaH.
    • exothermic
    • endothermic
  13. The entropy change for dehydrogenation is stronly favorable because ++/
    one alkane becomes two molecules
  14. The equilibrium constant for hte __ depends on the __. At room temp, the enthalyp term predominates and __ is favored. When temp is raised, however, the entropy term becomes larger and eventually dominates the expression.
    • hydrogenation-dehydrogenation equilibrium
    • change in free energy
    • hydrogenation
  15. At a high temp, __ is favored.
  16. How is dehydrogenation similar to catalytic cracking?
    - catalysts lowers Ea and both reactions use high temps to increase a favorable entropy term and overcome an unfavorable enthaly term
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Orgo 7.10-7.11
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