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Topic: Alkene / Acid-catalyzed  (Read 6516 times)

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Offline viet

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Alkene / Acid-catalyzed
« on: March 04, 2010, 07:22:26 PM »
cyclobutylmethanol + H2SO4 (plus Heat)  proceed in E1 mechanism which gives 2 alkenes. One alkene would be methylenecyclobutane, I'm not sure how to find the other alkene.

Offline Smrt guy

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Re: Alkene / Acid-catalyzed
« Reply #1 on: March 04, 2010, 09:00:39 PM »
The major product would likely be derived from a hydride shift.  The subsequent deprotonation to give the alkene could happen at any of the three alpha positions.  Two of them would give 1-methylcyclobutene and the other gives methylenecyclobutane.

Offline orgopete

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Re: Alkene / Acid-catalyzed
« Reply #2 on: March 04, 2010, 11:40:13 PM »
It is often difficult to predict rearrangement reactions. However, I have suggested that ring expansions are an often used example. Although the direct elimination often occurs as well, the ring expansion product should be anticipated here to give cyclopentene.
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Offline Schrödinger

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Re: Alkene / Acid-catalyzed
« Reply #3 on: March 05, 2010, 07:30:06 AM »
Can we say anything about the relative amounts of the products?
Since a 5-membered ring suffers lesser angle strain, can we conclude that cyclopentene would be the major product?
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Offline orgopete

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Re: Alkene / Acid-catalyzed
« Reply #4 on: March 05, 2010, 09:31:11 AM »
"Addition of hydrogen bromide to 3-methyl-1-butene gives after rearrangement, 2-bromo-2-methylbutane. Often, I am uncertain whether a rearrangement should take place. This is a frequently used example. You can decide from the product mixture how difficult it can be to predict whether a rearrangement should take place."
This is from A Handbook of Organic Chemistry Mechanisms. The ratio was 55% rearranged and 45% unrearranged.

I don't recall where I got the data from. I was mainly using secondary sources to find reactions and many of them simply indicate major products. For me, a textbook should give actual percentages rather than major and minor. I realize that some reactions are quite variable and this may be somewhat misleading also.

Because I was searching for this data, I found it difficult to find examples in which 100% rearrangement occurs. In this example, I would expect cyclopentene to be the major product, but I would also expect that unrearranged products would also be present.
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Offline viet

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Re: Alkene / Acid-catalyzed
« Reply #5 on: March 05, 2010, 10:02:13 AM »
thank guys, cyclopentene is correct.

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