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Topic: What would you say for this addition reaction?  (Read 326 times)

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Offline OrganicH2O

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What would you say for this addition reaction?
« on: June 15, 2020, 11:10:16 PM »
I attached two pictures of an addition reaction of H-Cl to an isolated diene. Although unstated, we are meant to assume 1 equivalent of H-Cl. One picture is the answer key, and the other picture shows my work on it.

I think answer given in the official key requires some sketchy logic. I think the trisubstituted endocyclic alkene will react MUCH faster because it is able to form a tertiary benzylic carbocation in the rate limiting step. Therefore, I do not expect the monosubstituted alkene will react at all. After all, the molecule cannot "see the future" and know it will rearrange after the initial addition of the proton to the exocyclic alkene.

And even if the reaction is heated enough to make it equilibrate and allow the monosubstituted alkene to react instead, wouldn't the major product be the conjugated alkene? Resonance is not the same thing as a 1,2 shift. The nucleophile doesn't automatically attack the more stable resonance structure. I think the the monosubstituted alkene would be reacting under some severe thermodynamic control, leading to the more stable alkene as the final product, the conjugated one.

Offline OrganicH2O

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Re: What would you say for this addition reaction?
« Reply #1 on: June 19, 2020, 09:01:17 PM »
Anybody have any opinion on whether that product in the answer key is legit, or if my analysis is correct? If it's too much work to read my block of text you could just say your first instinct...in principle I would imagine there is a thermodynamic product and a kinetic product. It really drives me crazy when I am skeptical of the answers given for these seemingly simple problems that have no literature reference given.

Offline rolnor

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Re: What would you say for this addition reaction?
« Reply #2 on: June 20, 2020, 03:35:03 PM »
I think your answer seems OK. I wonder if not there is a risc you get ringclosure and a 5-membered ring from the terminal alkene and the benzylic position?

Offline OrganicH2O

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Re: What would you say for this addition reaction?
« Reply #3 on: June 20, 2020, 06:33:47 PM »
Just to clarify, you're saying that it makes sense that the trisubstituted alkene inside the ring reacts much faster? And potentially no reaction from the terminal alkene?

I also attached a picture to clarify what you meant by ring closure. Does the picture represent what you meant? This problem was meant for Sophomore undergrad level, so I don't think any professor would expect that at this level. It is definitely fun to speculate about possibilities.


Offline rolnor

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Re: What would you say for this addition reaction?
« Reply #4 on: June 21, 2020, 03:47:05 AM »
Yes and yes.

Offline Marko

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Re: What would you say for this addition reaction?
« Reply #5 on: June 25, 2020, 09:28:35 AM »
In the end, protonation of the vinyl group and rearrangement will result in a more stable cation, but the activation barrier will be higher. Your suggestion will surely be the kinetic product in the reaction.

In any case, I don't believe that the structure given by your teacher is "the" major product. Even if the conditions are such that the cation after rearrangement is the favored one, the (only) product will be the one that is drawn in the bottom left corner; that one is both kinetically and thermodynamically favored.



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