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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: science123 on March 28, 2010, 12:19:07 PM

Title: Diels-Alder and Dehydration of Alcohols Reaction rates
Post by: science123 on March 28, 2010, 12:19:07 PM
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I know that butadiene would be the slowest of the three since it doesn't have ring strain or any alkyl groups to donate the e- density but i am not sure which one would undergo diels alder faster between cyclopentadiene and 1,3 dimethylbutadiene. Does ring strain beat the extra 2 electron donating methyl groups?

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For the dehydration, i know it has something to do with carbocation stability after the protonated OH leaves.. the middle carbocation would be aromatic hence most stable. I am not sure about the first and the last one. The acyclic alcohol has three double bonds - don't know if that affects anything or not

Title: Re: Diels-Alder and Dehydration of Alcohols Reaction rates
Post by: Schrödinger on March 28, 2010, 12:59:13 PM
Quote from: science123 on March 28, 2010, 12:19:07 PM
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For the dehydration, i know it has something to do with carbocation stability after the protonated OH leaves.. the middle carbocation would be aromatic hence most stable.
Yes that's correct

Quote
I am not sure about the first and the last one. The acyclic alcohol has three double bonds - don't know if that affects anything or not
You should be able to account for the the high instability of the first one.
For the last one, of course it is stabilized by delocalisation of electrons, but not as stable as the aromatic one. Aromatics are just super-stable.
Title: Re: Diels-Alder and Dehydration of Alcohols Reaction rates
Post by: bolo on March 28, 2010, 03:47:47 PM
It is noteworthy that cyclopentadiene exist as a dimer (dicyclopentadiene). Before any reaction it must be decomposed and redistilled which goes in one process.
Title: Re: Diels-Alder and Dehydration of Alcohols Reaction rates
Post by: w00tage19 on March 31, 2010, 05:30:08 PM
For the first question, think about what conformation the diene must be in to undergo a diels-alder reaction.  If the diene can switch (or possibly... rotate...) into another conformation, what would that do to the reaction rate?

Edit: I'm not sure if ring strain is particularly important to this question.
Title: Re: Diels-Alder and Dehydration of Alcohols Reaction rates
Post by: pacifyer on April 01, 2010, 12:39:23 PM
For D-A to work, the diene MUST be in an s-cis conformation...


Regarding the dehydrations, think cation stabilities. Think on AROMATIC character...  ;)