[Brook]

I'm treating the first 2,2 dimethyl-1methyl-cyclohexanol molecule (sorry if I did not name it properly)
with sulfuric acid and heat.

Is it okay to do multiple shifts in a row, and achieve extra possible products, even though they are not energetically favorable, or isoenergetic? Also, is elimination a possibility in this reaction, and what would cause elimination to be favored over the rearrangement?

[Archer]

Aliphatic carbocations don't tend to hang around for long so eventually one of the cations you have drawn would eliminate a proton and form a double bond. In this case I would say that the first elimination would be favoured. You have a tertiary carbocation being formed which is more stable than the other species.


Where you have suggested that the methyl group migrates, if this did happen how would you know? if you rotate the product cation it is identical to the starting species. Deuterating the gemial dimethyl group of the stating material would be needed to prove whether this occurs experimentally.

[Brook]

So the first methyl shift would yield the same product. Thanks for catching me on that. But would the hydride shifts still occur as I drew them? The professor's teaching assistant said to expect four or five products so I was just wondering if the structures I proposed are reasonable. Thank you for your answer

Aliphatic carbocations don't tend to hang around for long so eventually one of the cations you have drawn would eliminate a proton and form a double bond. In this case I would say that the first elimination would be favoured. You have a tertiary carbocation being formed which is more stable than the other species.


Where you have suggested that the methyl group migrates, if this did happen how would you know? if you rotate the product cation it is identical to the starting species. Deuterating the gemial dimethyl group of the stating material would be needed to prove whether this occurs experimentally.

[magician4]

though Wagner-Meerwein rearrangement and hydride shift always are an issue with carbocations, in this case the abstraction of the hydroxyl-group already will result in the most stable carbocation this system has to offer

I don't think that aside of elimination much else will happen, esp. as ether formation doesn't seem to be a favoured option due to sterical hindrance


regards

Ingo

[Brook]

Thank you for your answer Ingo. I agree that the secondary carbocations are not as stable as the tertiary , so they would not be as likely to form, I just think the TA meant to show all the possibilities. It is okay that I did multiple hydride shifts though? Just so that the positive charge would ultimately end up in different locations. I can't think of any other way to get different products.

though Wagner-Meerwein rearrangement and hydride shift always are an issue with carbocations, in this case the abstraction of the hydroxyl-group already will result in the most stable carbocation this system has to offer

I don't think that aside of elimination much else will happen, esp. as ether formation doesn't seem to be a favoured option due to sterical hindrance


regards

Ingo

[Archer]

Sigmatropic shifts can occur to produce ring contraction / expansion reactions too but I don't think this is very likely in this case.

I am going on personal experience of tertiary alcohol elimination, perhaps someone else can provide a theory which supports what the teaching assistant has said.

[magician4]

I just think the TA meant to show all the possibilities.

good luck than ...

if only you abuse the title alcohol with sulfuric acid and some heat long enough, you'll end up with an interesting tar of thousands of products: ring opening, ring contraction, rearrangements and olefin polymerisation of all sorts will occur, for example

some of those reactions you did show, and if we discussed side reactions in general, I would agree: all this can happen*⁾ to a certain degree, and much more

regards

Ingo

*⁾
... and will happen, as I once learned whilst doing deuterium-labelling experiments for mechanistical studies.
you wouldn't believe, that stuff went places you even didn't consider to be available .

[Brook]

Boy. Can't wait until I get to take an Ochem lab x)

[limonade]

I just think the TA meant to show all the possibilities.

good luck than ...

if only you abuse the title alcohol with sulfuric acid and some heat long enough, you'll end up with an interesting tar of thousands of products: ring opening, ring contraction, rearrangements and olefin polymerisation of all sorts will occur, for example

some of those reactions you did show, and if we discussed side reactions in general, I would agree: all this can happen*⁾ to a certain degree, and much more

regards

Ingo

*⁾
... and will happen, as I once learned whilst doing deuterium-labelling experiments for mechanistical studies.
you wouldn't believe, that stuff went places you even didn't consider to be available .

The hydride shifts would not likely occur as they are not energetically favorable (ie. going from tertiary carbocation to secondary). However looking into ring contraction....
Here is what I have so far. I think thats five. Please let me know if there are any that I am missing.

[Archer]

You need to check the directions of your mechanism arrows. When double bonds migrate they lose H⁺ from one end and pick up H⁺ at the other.

What you have drawn would involve loss of a hydride from the molecule which can't happen.

[limonade]

You need to check the directions of your mechanism arrows. When double bonds migrate they lose H⁺ from one end and pick up H⁺ at the other.

What you have drawn would involve loss of a hydride from the molecule which can't happen.

Yikes. Thank you.

[Archer]

What you have drawn would involve loss of a hydride from the molecule which can't happen.

"Can't happen" is a bit of a strong statement in science.

I should have said that loss of a hydride is not the generally accepted mechanism for this type of bond migration.