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#1 2010-02-02 13:30:41

orgopete
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Unexpected reactivity in dehydration of alcohols

The following problem posted on Chemical Forum.
The poster speculated that the order of reactivity would be 4>3>2>1 (but structure 3 was incorrect) based upon basicity of the alcohol. The further from the C=O, the more basic and the faster it would react.

The poster then provided the answer to the problem with the following reaction rates: 1>4>3>2.

This was my answer:
This is an interesting post. I am going to assume this data is correct. The answer is more interesting than the question. I have also taken the liberty to provide what I think is the structure for III.

Let's do this problem from the answer, because I don't know that I knew that was going to be the rates. The rate determining step in the reactions is the loss of water from the protonated alcohol. If carbocation stability were equal, then the rate would reflect the basicity of the alcohol as the most basic alcohol would have the greatest concentration of oxonium ion.

The data does not agree with that premise. Another premise is that the rate of the reaction is dependent on the stability of the carbocation. The carbocations are shown below the alcohols. If that were the case, then the order might be expected to be IV>I>II or III.

That doesn't agree either because I reacts the fastest. If I were in equilibrium with its enol, then I might be expected to react faster as it is an allylic alcohol with a pair of electrons that can resonance stabilize the carbocation.

From the answer, we can learn that enolization of the ketone is an important process that is accompanying the dehydration.

       http://www.curvedarrow.com/chem/ZZ6B098C16.jpg

Note, this is not a proof. It is simply a conjecture.

Structures: 2-hydroxy-2-pentanone, 3-hydroxy-2-pentanone, 4-hydroxy-2-pentanone,  and 2-pentanol.

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