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Organic : Mechanism help for aromatic compound.
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 Message 1 of 7 in Discussion 
From: MSN NicknameSOSTrooper  (Original Message)Sent: 4/16/2005 1:32 AM
I'm stuck with the mechanism for the following reaction:

http://img.photobucket.com/albums/v225/sostrooper/stuff/ochem01.jpg

Only H3O+ is used to produce the ketone product. But I can't seem to figure out the step by step for it. I did an attempt and this is what I got:

http://img.photobucket.com/albums/v225/sostrooper/stuff/ochem02.jpg

From then on I have no idea what to do.


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Reply
 Message 2 of 7 in Discussion 
From: MSN Nickname·Steve·Sent: 4/16/2005 6:44 AM
Hi, that's a very good problem!  I think you are on the right track, because it is possible, using some of the reactions shown below, to take your enol product, 1,4-cyclohexadien-1-ol, and get to the final product 2-cyclohexen-1-one.

In your mechanism, you formed a vinylic carbocation.  The stability of a 2° vinylic carbocation is comparable to that of a normal 1° carbocation, and as such, its formation is not very favorable.  I can't rule it out entirely, but in the following mechanism, I used an alternate pathway not involving a vinylic carbocation, by starting with electrophilic addition of H+ to the double bond that has the methoxy group present.  This could occur in competition with protonation of the methoxy oxygen, but if the vinylic carbocation cannot form, protonation of the oxygen would be a dead end.  Here is a (proposed!) overall mechanism:
 
 

Shown in the first line, the resulting carbocation from addition of H+ to the double bond is stabilized by donation of the oxygen lone pair, spreading the positive charge out over two atoms.  Thus, this may be a relatively favorable initial step.

In the second line, addition of water yields a protonated hemiacetal from which we can get to the ketone 3-cyclohexen-1-one (the last compound in line 2).  Note that this compound is just the keto tautomer which your final enol product 1,4-cyclohexadien-1-ol would rearrange to (a standard acid or base-catalyzed process).

The third line shows the acid-catalyzed rearrangement of this keto tautomer to the enol tautomer, but this time with the enol double bond on the other side of the OH group, resulting in an enol of a conjugated diene, which should be more stable and therefore more favorable to form (or so my thinking goes!).

Finally, in the fourth line, I first drew the other resonance structure of the previous enol and then protonated it, "trapping" the product, an a,b-unsaturated protonated ketone, which, upon deprotonation of the oxygen, gives the a,b-unsaturated ketone product.

Works on paper anyway!
 

Steve

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 Message 3 of 7 in Discussion 
From: MSN Nickname·Steve·Sent: 4/16/2005 6:55 AM
>> . . . a,b-unsaturated ketone product . . . <<
 
That should be a,b (forgot to switch to symbol font.  )

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 Message 4 of 7 in Discussion 
From: MSN NicknameSOSTrooperSent: 4/17/2005 3:47 AM
Wow thank you very much Steve. This is my first time on these chemistry boards; are you a teacher or professor? Seems like you know the solution to every chemistry question that is thrown at you. This is going to help me out a lot since it will be on the test this coming Tuesday. Thanks again Steve.

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 Message 5 of 7 in Discussion 
From: MSN Nickname·Steve·Sent: 4/18/2005 1:22 AM
OK, glad to help!  I teach first and second year chemistry at Houston Community College, but believe me, I don't know the solution to every chemistry question!  Many times I have to search my textbooks or the Internet to find information about a problem.  Your question was a "mainstream" Organic II topic, but it was still rather involved.  You may yet see an alternate mechanism, but the one I gave is based on pretty sound principles so I believe it's OK.  Where mechanisms are concerned, the only proof is in experiemental support which is not always available or mentioned in the textbook.  So, just because a mechanism works on paper doesn't automatically mean it's correct!
 
 
Steve

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 Message 6 of 7 in Discussion 
From: MSN NicknameSOSTrooperSent: 4/20/2005 4:05 AM
Hey Steve,

Guess what, that very same mechanism was a bonus question on the test I had today. It is worth 5 pts, half a grade so to speak. And guess what, it is correct... the professor confirmed it after he told me to draw it out on the black board during lab. Just following up on what I've done w/ your mechanism. :) Thanks again!

Danny

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 Message 7 of 7 in Discussion 
From: MSN Nickname·Steve·Sent: 4/20/2005 5:01 AM
Hey Danny, that's great!  You never know what your prof might toss out on the test, but it sounds like you were ready for just about anything he threw at you.  Aside from the mass of reactions to memorize, mechanisms are certainly the hardest part of organic chemistry to learn and understand.  Good luck on the rest of your semester!
 
Steve

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