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Topic: Multiple functional group reaction problem  (Read 3345 times)

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Offline Biostar

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Multiple functional group reaction problem
« on: March 04, 2015, 01:34:43 AM »


This is the problem I need help with. The first two steps I know. EtO- adds to the least substituted carbon via backside attack since basic conditions and the protic workup adds H+ to the R-O-, which gives this molecule.



The issue is the third step. I know the addition of HBr to a alcohol or ether but I'm not sure if aqueous HBr will react with them both to give a dibromide molecule, just one of them, or some other thing. I believe the fourth step would be a E2 step.

I would appreciate if someone could indicate which it is and if my thinking is accurate..

Offline jstownse

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Re: Multiple functional group reaction problem
« Reply #1 on: March 04, 2015, 01:53:17 AM »
Aqueous strong acids such as HBr and HI are known to protonate not only alcohols, but ethers as well... That being said, I believe your thought process is correct. You would end up with a dibrominated material that would readily undergo e2 to yield 1,3 - cyclohexene.

Offline Dan

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Re: Multiple functional group reaction problem
« Reply #2 on: March 04, 2015, 02:02:00 AM »
I know the addition of HBr to a alcohol or ether but I'm not sure if aqueous HBr will react with them both to give a dibromide molecule, just one of them, or some other thing.

Careful with your terminology, this reaction is a substitution, not an addition.

I disagree with jstownse on this one. What is the mechanism of the substitution? Based on this mechanism would you expect a rate difference between substitution of the tertiary alcohol compared with the secondary ether?
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Offline Biostar

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Re: Multiple functional group reaction problem
« Reply #3 on: March 04, 2015, 02:21:34 AM »
I believe it's a SN1 mechanism, so the tertiary alcohol would react quicker.

Offline Dan

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Re: Multiple functional group reaction problem
« Reply #4 on: March 04, 2015, 04:33:13 AM »
I believe it's a SN1 mechanism, so the tertiary alcohol would react quicker.

That sounds sensible to me. Can you attempt the rest of the question now?
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Offline Biostar

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Re: Multiple functional group reaction problem
« Reply #5 on: March 04, 2015, 11:48:08 AM »


This is what I came up with.

Offline Dan

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Re: Multiple functional group reaction problem
« Reply #6 on: March 04, 2015, 12:02:35 PM »
Ok. In your bromide intermediate, there are 3 different H adjacent to the Br - abstraction of one of these three protons and elimination of bromide (E2) gives a different alkene product in each case. Why did you choose the one you chose?
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Offline Biostar

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Re: Multiple functional group reaction problem
« Reply #7 on: March 04, 2015, 12:11:17 PM »
If my model is accurate, then that H is the only one that was anti to Br.

Edit:Oh, I forgot about the methyl group.I guess that would be easier to abstract a proton from.
« Last Edit: March 04, 2015, 01:09:02 PM by Biostar »

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