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Topic: Cyclohexylmethanol to cycloheptene  (Read 5648 times)

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Offline jlanrdy287

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Cyclohexylmethanol to cycloheptene
« on: November 26, 2013, 11:16:23 AM »
here is the mechanism I have come up with. If you see anything that needs fixing, please alert me. Also, any comments are welcome.

Thanks!


Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #1 on: November 26, 2013, 12:46:08 PM »
What else could possibly happen?
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Offline jlanrdy287

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Re: Cyclohexylmethanol to cycloheptene
« Reply #2 on: November 26, 2013, 02:13:23 PM »
Well I was 99% sure that was the correct mechanism, but this is for a take-home-test (that's why I didn't just post the question, I attempted it and asked for suggestions). That is why I want it to be 100% PERFECT!

Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #3 on: November 26, 2013, 03:43:55 PM »
How stable is the initial carbocation upon dehydration?
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Offline jlanrdy287

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Re: Cyclohexylmethanol to cycloheptene
« Reply #4 on: November 26, 2013, 03:59:58 PM »
How stable is the initial carbocation upon dehydration?

very very unstable. I thought the same thing but there is no possible way to make that a tertiary carbocation (unless I do a hydride shift) but instead I did a ring expansion and make a secondary carbocation (there is no possible way to form a more stable carbocation).

Offline zsinger

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Re: Cyclohexylmethanol to cycloheptene
« Reply #5 on: November 26, 2013, 04:01:57 PM »
^^ What Alpha Said.  Looks like an opportunity for a 3 prime Carbocation formation :).  Ill bet there is an alternate mechanism with HSO4 being another Nucleophile.  I don't know how it goes, but it is prob. complicated if I had to guess.  Hope this helps.
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Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #6 on: November 26, 2013, 04:02:02 PM »
Hydride shifts are quite common.  In this case, a shift is what is going to happen.  It is the most energetically favorable.
Nature likes easy.
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Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #7 on: November 26, 2013, 04:03:00 PM »
What about this.
If you're not part of the solution, then you're part of the precipitate

Offline jlanrdy287

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Re: Cyclohexylmethanol to cycloheptene
« Reply #8 on: November 26, 2013, 04:08:19 PM »
What about this.

Ahh okay I understand where you're coming from. However, the question is starting at cyclohexylmethanol and going to cycloHEPTENE (sorry if the reaction is hard to see from the white board drawing). Therefor, a hydride shift would not help, if I am correct. A ring expansion is the only way to go from a 6 carbon ring to a 7 carbon ring. Whenever I expand the ring, it stabilizes the primary carbocation and forms a secondary.

Offline orgopete

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Re: Cyclohexylmethanol to cycloheptene
« Reply #9 on: November 26, 2013, 10:39:45 PM »
I found the mechanism up to the cycloheptyl carbocation to be plausible. A simply deprotonation should give cycloheptene, no zwitterions.
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Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #10 on: November 27, 2013, 09:26:21 AM »
OOOOOOOOhhhhhhh, I misunderstood. The question was asking for the mechanism for the ring expansion.  I got it.  I also believe there would be no zwitterions.  I would draw what I think but am home for the holidays and do not have my drawing program.

The carbocation will still be there after the expansion with water removing a proton and forming the pi bond.

Again I do apologize.  I think I had tunnel vision about the mechanism I saw. 
If you're not part of the solution, then you're part of the precipitate

Offline AlphaScent

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Re: Cyclohexylmethanol to cycloheptene
« Reply #11 on: November 27, 2013, 09:30:20 AM »
I believe we are both correct.  With a third compound possible. 
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Offline spirochete

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Re: Cyclohexylmethanol to cycloheptene
« Reply #12 on: November 29, 2013, 03:03:13 PM »
What about this.

You have drawn a primary carbocation which you labeled as secondary. I think ring expansion should be concerted with the loss of water as a leaving group, to avoid formation of the primary cation.

Ultimately this is all going to depend on the exact conditions, how acidic things really are, what the temperature is, etc. Generally we try to avoid drawing primary cation though, and instead draw funny things like concerted hydride or alkyl shifts to avoid it.

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