[srihari]

1-hydroxy-1-methyl-cyclohex-2-ene   on   reaction with chromic acid(H₂CrO₄) produces what?

Is this right ?

My attempt : first the protonation of the hydroxyl group . then there is a allyl carbocation formation , due to curtin-hammet priniciple , the secondary carbocation reacts faster and this will lead to  the product

3-methyl-cyclohex-2-enone

this is the major product ??
please comment about whether this the correct usage of curtin-hammet principle

[srihari]

Please reply !!
I've also attached the image didn't know how to before .

[stewie griffin]

I'm not sure whether you are wanting to find an example of the curtin-hammett principle or just figure out the product of the reaction.
I would not invoke curtin-hammett here. If you want a real world example of curtin-hammett, look into E2 eliminations with alkyl bromides by focusing on the various conformers.
The product of the reaction is indeed the enone that you describe. However I don't believe this reaction goes through a carbocation. I could be wrong, but I was pretty sure it starts out like a normal Jones oxidation whereby the alcohol attacks the chromium center. Then, since there's no proton to be removed like in the normal Jones, we see allylic transposition. Then we finally remove a proton like in the normal Jones. See JOC 1977, 42, 813-819.

[srihari]

hey buddy
First of all , thanks for replying !!
okie . I was first after the product ,then we were taught curtin-hammet .
Just thought whether it would be possible to apply .
the mentioned product will be major .
If possible do provide the mechanism .

I'd be grateful if you could tell me the instances when curtin-hammet is applicable
with regards,

[stewie griffin]

As I said, an example of the Curtin-Hammett principle would be the E2 elimination of secondary alkyl bromides. Another example would be a kinetic resolution. I'm sure you can google these topics and get more info that you ever wanted.
A basic mechanism is shown below. But again, I do not think Curtin-Hammett applies here (b/c we don't have two rapidly interconverting species in equilibrium with each other, where each one can go on irreversibly to product).

[orgopete]

I wouldn't exactly jump ship just yet. While that is a fine mechanism suggested by Stewie (though a little sloppy), the alternate carbocation mechanism has not been disproven. If you were to do a Jones oxidation of but-3-en-2-ol, I think the major product is methyl vinyl ketone (this is not the preferred MVK prep). I don't know that crotonic acid does not form, but I don't think it is a significant product. I think the equilibrium of the chromate ester should still favor the more substituted alkene if a facile electrocyclic equilibrium were present. I am not ruling out the electrocyclic reaction per se, but I'd like to see stronger proof for it. I don't think of pyridinium chlorochromate as acidic. Does it give the same product? We could also dispute the leaving group, whether it is water or chromic acid, or chromate.

I am also not so troubled by the Curtin-Hammett thinking either. Obviously, if there was an oxidation path from the tertiary alcohol, then it could be invoked more rationally, but if one were considering the energy levels of the intermediate alcohols in relation to their predicting (or not) the product, then I am not too troubled by it. I would consider understanding the implications of the Curtin-Hammett principle of greater value than a perfect application of it.

[stewie griffin]

First off I should say I was at home without access to the reference I had listed and thus could not see then entire article. It looked useful thus I referenced it in case the original poster wanted to view it on their own time.
Anyway, now I can see that the article proposes ionization to an ion pair which can recombine via two different pathways to go to product. Different from what I was taught but that's fine. However, the paper does not propose that there is a pair of rapidly interconverting interemediates in equilibrium with each other. So while I too think it's important to understand the energy levels of the intermediates involved and the overall energy diagram, I have a problem with labeling it Curtin-Hammett. By definition, Curtin-Hammett must have a pair of rapidly interconverting intermediates. I would be fine if we were to say that this is a "Curtin-Hammett like" situation.

[srihari]

okie thank you

got a lot to learn in this discussion , further replies are always helpful

regards