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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Biostar on March 04, 2015, 01:34:43 AM

Title: Multiple functional group reaction problem
Post by: Biostar on March 04, 2015, 01:34:43 AM

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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.

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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..

Title: Re: Multiple functional group reaction problem
Post by: jstownse 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.

Title: Re: Multiple functional group reaction problem
Post by: Dan on March 04, 2015, 02:02:00 AM

Quote from: Biostar on March 04, 2015, 01:34:43 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?

Title: Re: Multiple functional group reaction problem
Post by: Biostar on March 04, 2015, 02:21:34 AM

I believe it's a SN1 mechanism, so the tertiary alcohol would react quicker.

Title: Re: Multiple functional group reaction problem
Post by: Dan on March 04, 2015, 04:33:13 AM

Quote from: Biostar on March 04, 2015, 02:21:34 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?

Title: Re: Multiple functional group reaction problem
Post by: Biostar on March 04, 2015, 11:48:08 AM

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This is what I came up with.

Title: Re: Multiple functional group reaction problem
Post by: Dan 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?

Title: Re: Multiple functional group reaction problem
Post by: Biostar 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.