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Topic: Dehydration of Alcohols  (Read 12389 times)

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Offline Nitin_Naudiyal

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Dehydration of Alcohols
« on: July 16, 2012, 03:46:03 AM »
Which of the following will have the Fastest Rate of Dehydration ?




I know that the ease of Dehydration of alcohols is Tertiary > Secondary > Primary Alcohols.

But still i couldnt understand why the answer is Option (C).
are there any other factors for dehydration ?

Offline discodermolide

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Re: Dehydration of Alcohols
« Reply #1 on: July 16, 2012, 05:11:43 AM »
You have not depicted any stereochemistry in your pictures, nor reaction conditions.
But the leaving group must be trans and anti-periplanar to the, in this case, hydrogen. (see picture).
Note compounds a & b are the same!
If this is acid catalyzed compound c will be favored because the carbonium ion generated is more stable that that of compound d.
Plus you have the extra effect of conjugation of the resulting double bond with the carbonyl group two give a compound which is thermodynamically favored.
So compound c is correct.


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Offline Nitin_Naudiyal

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Re: Dehydration of Alcohols
« Reply #2 on: July 16, 2012, 05:32:53 AM »
yes it is acid catalysed Conc H2SO4 .

Quote
If this is acid catalyzed compound c will be favored because the carbonium ion generated is more stable that that of compound d.

Please can you show me which will be the leaving group and how will the carbonium ion look like .


In my text book it is written that when an alcohol having a β - Hydrogen is heated with a Dehydrating agent like Conc H2SO4 an alkene is formed by loss of water.

But i am finding it difficult to relate it with this Example.

Offline discodermolide

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Re: Dehydration of Alcohols
« Reply #3 on: July 16, 2012, 06:40:24 AM »
yes it is acid catalysed Conc H2SO4 .

Quote
If this is acid catalyzed compound c will be favored because the carbonium ion generated is more stable that that of compound d.

Please can you show me which will be the leaving group and how will the carbonium ion look like .


In my text book it is written that when an alcohol having a β - Hydrogen is heated with a Dehydrating agent like Conc H2SO4 an alkene is formed by loss of water.

But i am finding it difficult to relate it with this Example.

The beta hydrogen is the one attached to the carbon with the methyl group on.
In the picture is my attempt at drawing.
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Offline Nitin_Naudiyal

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Re: Dehydration of Alcohols
« Reply #4 on: July 16, 2012, 07:03:11 AM »
Thank you .

That was really well explained

Offline discodermolide

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Re: Dehydration of Alcohols
« Reply #5 on: July 16, 2012, 07:08:57 AM »
Thank you .

That was really well explained

Just remember that the acyl carbonium ion is not tremendously stable, I don't remember where it lies in the stability league. You don't have to do a 1,2-H shift to get the product, just eliminating the H next to the methyl group will malso suffice.
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Offline orgopete

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Re: Dehydration of Alcohols
« Reply #6 on: July 16, 2012, 09:57:45 AM »
While the scheme posted by discodermolide could be the mechanism, I think it is very unlikely. If it were true, then a) should be the correct answer. I see no reason for a hydride shift to form the carbocation intermediate. If it were more stable than the initial carbocation, then the option of forming it directly should be a faster reaction. For example, dehydration of 3-methyl-2-butanol can give a tertiary carbocation intermediate as it is more stable than the secondary carbocation. If you compared rates, I'm sure the tertiary alcohol could be dehydrated faster than the secondary.

I think you should approach this problem differently. If you know that c) is the correct answer, then what is the rate determining step? For most alcohols, the rate determine step is the formation of the carbocation intermediate. I don't think that is the case here or the 4-OH should be the fastest. The rate determine step must be the loss of the proton. Whether this occurs from the enol of the ketone or directly from the ketone, I don't know.
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Offline milo2112

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Re: Dehydration of Alcohols
« Reply #7 on: July 26, 2012, 01:37:52 PM »
It is assumed that the protonation of the alcohol followed by its susbsequent elimination is the only chemistry occuring. What happens to ketones in the presence of acid? Can the resulting product facillitate elimination of protonated OH?

Offline discodermolide

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Re: Dehydration of Alcohols
« Reply #8 on: July 26, 2012, 01:52:51 PM »
It is assumed that the protonation of the alcohol followed by its susbsequent elimination is the only chemistry occuring. What happens to ketones in the presence of acid? Can the resulting product facillitate elimination of protonated OH?

Ketones can enolise in the presence of Lewis acids. As far as I know the enolate OH does not eliminate.
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Offline milo2112

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Re: Dehydration of Alcohols
« Reply #9 on: July 26, 2012, 02:07:35 PM »
I would imagine the enol form would have a much faster rate of dehydration as allylic alcohols are quite easy to dehydrate.

Offline discodermolide

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Re: Dehydration of Alcohols
« Reply #10 on: July 26, 2012, 02:28:09 PM »
I would imagine the enol form would have a much faster rate of dehydration as allylic alcohols are quite easy to dehydrate.

But this is an enrol, in equilibrium with the keto form.
It will not dehydrate.
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Offline milo2112

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Re: Dehydration of Alcohols
« Reply #11 on: July 26, 2012, 04:14:19 PM »
Here is what I am suggesting:



Offline orgopete

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Re: Dehydration of Alcohols
« Reply #12 on: July 26, 2012, 05:10:20 PM »
@milo2112

That is what I was thinking would best explain the more rapid dehydration of the 3-OH. For what it is worth, it doesn't mean the protonation of the alcohol isn't occurring. It simply means that the rate determining step of breaking the C-O bond does not occur very readily. Enolization of the ketone accelerates the cleavage with no carbocation in the reaction pathway. If we had to rate the other alcohols, the 4-OH would be the next fastest, with formation of a carbocation. The 2-OH would be the slowest due to the destabilization of a carbocation at C2.
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