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Topic: Dehydration of 4-methylcyclohexanol  (Read 20010 times)

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Offline Marcus

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Dehydration of 4-methylcyclohexanol
« on: March 22, 2010, 10:07:53 AM »
Seems pretty straightforward. Yield should be 4-methylcyclohexene. Yet, a simple Google produces some sources that claim this reaction would also produce other cyclohexenes.

I was wondering if anyone has had any practical experience with this actually occurring? Are these claims of secondary products illegitimate, or are they founded in carbocation rearrangement?

Dehydration of the alcohol should only produce a double bond between its adjacent carbons. Does secondary formation occur because the acid catalyst attacks the double bond? I guess that would only happen using a strong acid like H2SO4, but H3PO4 shouldn't do that.

Offline orgopete

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Re: Dehydration of 4-methylcyclohexanol
« Reply #1 on: March 22, 2010, 02:22:06 PM »
While this seems straight forward, I am also imagine it not being so simple.

Let's analyze this further, fast protonation, slow cleavage of C-O bond to form water and carbocation. While water can be expected to pull proton, it could reform alcohol. Not only that, but the carbocation may have been stabilized by neighboring electrons. Could water open to give isomeric alcohol? Less likely. (I recall some labeling studies that quantized neighboring group participation, but I do not recall details.)

Deprotonation to form alkene. Reprotonation of alkene to give isomeric alcohol. Dehydration to give isomeric alkene, repeat.

I am not arguing that 4-methylcyclohexene is not the major product, but given the nature of this reaction, I should not be surprised to find small amount of isomers.
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Offline Schrödinger

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Re: Dehydration of 4-methylcyclohexanol
« Reply #2 on: March 22, 2010, 02:40:53 PM »
@orgopete : So, you mean the double bond just keeps shifting towards the methyl group.
Well, isn't that something unlikely? I mean, once the alkene is formed, why would the acid attack the double bond? Are double bonds equally vulnerable to all acids?
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Offline dudeman

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Re: Dehydration of 4-methylcyclohexanol
« Reply #3 on: March 22, 2010, 03:02:58 PM »
What kind of stuff can you make out of 4-methylcyclohexene? I read that it is used to manufacture flavorings and fragrances. Sounds pretty cool...

Offline orgopete

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Re: Dehydration of 4-methylcyclohexanol
« Reply #4 on: March 22, 2010, 04:34:59 PM »
Alkene plus acid:
http://www.cem.msu.edu/~reusch/VirtualText/addene1.htm#add1b

The way this reaction was done in the lab was to treat cyclohexanol with acid and distill the cyclohexene. I infer two things. One, it is hot. That can enable other reactions as well as an elimination. Two, it could react again with the acid. If I had the information handy, I would pursue more information about the labeling experiments that show the degree of label migration. I don't know which would be a greater participant in this example (nor have the actual identities and yields of by-products been given). 
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Offline azmanam

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Re: Dehydration of 4-methylcyclohexanol
« Reply #5 on: March 22, 2010, 09:20:36 PM »
Yes, orgopete is on the right track.  Every step in the acid-catalyzed dehydration of an alcohol is reversible.  The reverse is the acid catalyzed hydration of an alkene.  Typically, these follow Markovnikov's rule to form the more substituted carbocation.  When the alkene is equally-substituted, the carbocation can form on either side.  If it protonates to form the carbocation at the 3-position, it can re-eliminate to form the isomeric alkene. 

It all has to do with the reversibility of the deprotonation/reprotonation step and the position of the resulting carbocation.  Draw some mechanisms to prove this to yourself.  You can form all possible isomeric alkenes through this reprotonation/deprotonation mechanism.
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Offline Marcus

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Re: Dehydration of 4-methylcyclohexanol
« Reply #6 on: March 22, 2010, 09:53:51 PM »
Are even weak acids, like H3PO4, strong enough to reprotonate the alkene?

Offline azmanam

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Re: Dehydration of 4-methylcyclohexanol
« Reply #7 on: March 22, 2010, 09:57:55 PM »
doesn't have to be a favorable reprotonation... note that the 'desired' product is probably still the major product.  You asked for rationale to get to isomeric alkenes.  That's the rationale.  Also, H3PO4 is in equilibrium with H3O+ which is strong enough to reprotonate alkenes.
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Offline nj_bartel

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Re: Dehydration of 4-methylcyclohexanol
« Reply #8 on: March 22, 2010, 09:59:04 PM »
Any proton source theoretically can.  I did this lab.  Don't remember the exact ratios, but the lab involved varying the reaction duration and temperature to get different ratios (higher temperature/longer reaction = more of the thermodynamic 1-methylcyclohexene).

Offline Marcus

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Re: Dehydration of 4-methylcyclohexanol
« Reply #9 on: March 22, 2010, 11:19:36 PM »
I guess my real issue is with the question that was posed in my lab manual:

"Is there a constitutional isomer of 2-methylcyclohexanol that would yield a single
dehydration product? If so, what is it?"

4-methylcyclohexanol is apparently the answer; at the time, I had considered it because it was the strongest answer, but I thought it was still wrong because of the capability for the product to undergo reprotonation, yielding minor products. I guess my TA's logic is in the very high yield of the major product? I don't know, it seems a little misleading to suggest a reaction will produce only one product when in reality it doesn't happen.

Offline azmanam

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Re: Dehydration of 4-methylcyclohexanol
« Reply #10 on: March 23, 2010, 06:18:03 AM »
Quote
it seems a little misleading to suggest a reaction will produce only one product when in reality it doesn't happen.

Welcome to undergraduate organic chemistry...  Many concepts in the course are much more nuanced than initially taught.  The concept we're discussing here is relatively higher-order and well beyond the scope of the lab you did.  Your TA's right.  4-methylcyclohexanol is the right answer.  You're right.  Technically, the (single) product has the capability of reprotonating and isomerizing.  The question is asking for the product of the first (and textbook) elimination.
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Offline Schrödinger

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Re: Dehydration of 4-methylcyclohexanol
« Reply #11 on: March 24, 2010, 03:00:03 AM »
"Is there a constitutional isomer of 2-methylcyclohexanol that would yield a single dehydration product? If so, what is it?"
If it's a constitutional isomer we're looking for, then why not consider cycloheptanol?
It is a constitutional isomer, and yields exactly one product.

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Offline nj_bartel

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Re: Dehydration of 4-methylcyclohexanol
« Reply #12 on: March 24, 2010, 11:08:11 AM »
Thinking outside the box for the win.

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