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__ of alcohols is a common method for making alkenes. This word literally means __.
- removal of water
__ is reversible, and in most cases, the __is not large. In fact, the reverse reaction (__) is a method for converting alkenes to alcohols.
- equilibrium constant
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
__ 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
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
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)
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
__ 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
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
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.
- concentration of hydrogen
__ 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
The hydrogenation of alkenes is __, with values of deltaH around -80 to -120. Therefore, dehydrogenation is __ and has an unfavorable value of deltaH.
The entropy change for dehydrogenation is stronly favorable because ++/
one alkane becomes two molecules
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
At a high temp, __ is favored.
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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