[dkssudgktpdy]

Can any carbocation rearrangements occur even the consequence does not make it more stable? thanks

[discodermolide]

Can any carbocation rearrangements occur even the consequence does not make it more stable? thanks

Unlikely as you are increasing the energy level, hence decreasing the stability.

[dkssudgktpdy]

ok. Then could you clarify why that is happening in this example?

Hydration of 1,2-dimethylcyclohexene produces 1,2-dimethylcyclohexanol

Few things that I am not clear of:
- why did 1,2-methyl shift occur? Isn't the carbocation already stable in its original form?
- the one that was shift was actually a part of the double bond. Shouldn't we take the alkyl group that is adjacent to the carbocations to make a rearrangement?

Thanks  again for any organic chemistry geeks out there.

[fledarmus]

This sounds like a straight addition reaction to me. Do you have a mechanism to show that has a methyl shift?

[dkssudgktpdy]

Hydration of 1,2-dimethylcyclohexene produces 1,2-dimethylcyclohexanol

Ah I made a mistake..
It's actually:

Hydration fo 1,2-dimethylcyclohexene producing 1,1-dimethyl-2-cyclohexanol.

Sorry about that.
and so how is it possible? thanks for your help.

[fledarmus]

Ah - much better!

So what is your proposed mechanism? (a hint - are any of your intermediates particularly stable?)

[dkssudgktpdy]

What I did was:

I added H3O as a catalyst to the original form; H3O being electrophile
So I attached H on either carbon with a methyl group (because in either way, they are the same due to symmetry)
When I put H on one of them, I had a carbocation on the other carbon with methyl (obviously)
and here is the problem: why was the methyl group with H was shifted to the carbon with carbocation(ig. 1,2-alkyl shift)? This shifted carbon with methyl was involved in the double bond originally. And I thought the carbocation is already tertiary. I thought it does not require any rearrangements at all!

Please tell me if you don't understand what I am trying say here. It's hard to describe in words LOL

[fledarmus]

Ah - now I do understand what you are saying. This is an acid catalyzed aqueous hydration reaction? I haven't been able to find any references that says the 1,1-dimethyl product can be a product of this reaction! Scifinder doesn't show that reaction course, and the references I've found look only at ratios of cis and trans 1,2-dimethylcyclohexanol. See eg Collins and Hammond, Journal of Organic Chemistry vol 25 (1960) ,p 911-913

Is it possible that the original question you were answering asked what the two products of such a reaction would be and you considered 1,1 and 1,2 as answers rather than cis and trans 1,2-dimethylcyclohexanol?

[dkssudgktpdy]

Ah - now I do understand what you are saying. This is an acid catalyzed aqueous hydration reaction? I haven't been able to find any references that says the 1,1-dimethyl product can be a product of this reaction! Scifinder doesn't show that reaction course, and the references I've found look only at ratios of cis and trans 1,2-dimethylcyclohexanol. See eg Collins and Hammond, Journal of Organic Chemistry vol 25 (1960) ,p 911-913

Is it possible that the original question you were answering asked what the two products of such a reaction would be and you considered 1,1 and 1,2 as answers rather than cis and trans 1,2-dimethylcyclohexanol?

Yes, this is an acid-catalyzed hydration reaction of alkene. Sorry I should have mentioned it earlier. The question actually gave a diagram of the reactant reacting with water (with H2SO4) and product. Then it says to propose a mechanisms for this reaction (explaining any transfers of electrons with arrows and so on). And it is not in a dot-line-wedge diagram, so there is no way to figure out what is cis and what is trans.

But do you know possible reasons behind? or is it just way it is to produce that product? (which doesn't make sense)

[fledarmus]

And it showed the 1,1-dimethyl product? I've never seen that reported as a product for this reaction. A mechanism involving a methyl shift to generate a secondary carbocation from a tertiary carbocation is highly unlikely - the energy difference is just too high. Certainly none of my professors would have ever given me credit for a proposed mechanism which involved such a shift. Or, for that matter, for predicting that the 1,1-dimethyl product could have possibly formed from that reaction.

Sorry I can't be of more help - does anybody else on these boards have any clever way of explaining this product? Or any knowledge that the reaction might actually produce this product?