[dorratz]

Hello

I got a question about this reaction:



1.how does the red SN1 mechanism works? why whould the cycloheaxane turn into cyclopentane, if the ring strain of the cyclohexane is 0 and the ring strain of the cyclopentane is higher?

2. can an E1 mechanism compete with the SN1 here?

Thanks
Dor

[mjc123]

What else is happening apart from cyclohexane going to cyclopentane? Is something less stable becoming something more stable?

[Vidya]

1.how does the red SN1 mechanism works? why whould the cycloheaxane turn into cyclopentane, if the ring strain of the cyclohexane is 0 and the ring strain of the cyclopentane is higher?

2. can an E1 mechanism compete with the SN1 here?

Do know the stability of carbocation ? When cyclohexane is converted to cyclopentane what happens to carbocation and its stability...I am sure this will help you in answering your first question why cyclohexane is converted into cyclopentane .
E1 and SN1 are generally always competing but depending on reaction conditions one of the mechanism is going to dominate.

[dorratz]

Hey guys

As for the carbocation, I still don't know why the new form with the cyclopentane is more stabilized.
I read here:
http://www.masterorganicchemistry.com/tips/carbocation-stability/

And couldn't see anything special. There isn't resonance in the new shape.

I would thank you if you could address me some proper material about it.

Thanks
Dor

[Babcock_Hall]

Besides resonance delocalization, what other factor stabilizes carbocations?

[dorratz]

It depends on three things:
Neighboring carbon atoms (inductive effect)

Neighboring carbon-carbon multiple bonds (resonance effect)

Neighboring atoms with lone pairs (resonance effect)


that's why in the upper shift I can see the carbocation shifts from secondary to tertiary.

But what is different in the lower reaction?

I still don't know.

Thanks
Dor

[orgopete]

Hint: analyze the two carbocation intermediates that form. Which one is tertiary?

[dorratz]

The blue is secondary, the yellow is tertiary, so the yellow is more stable, but - still - the cyclo pentane has more ring strain the cyclohexane...so I still don't know how does it compensate that.

Dor

[orgopete]

Look at the norbornyl cation here:
https://en.wikipedia.org/wiki/2-Norbornyl_cation

I think the electron movements are more like the non-classical carbocation. I do not think strain is a factor. The stereochemistry is more likely a factor. Google deamination of 2-aminocyclohexanol. One of the isomers give a ring contracted product.

[Babcock_Hall]

Ring strain for cyclohexane versus cyclopentane (as treated in elementary textbooks) refers to a situation in which all carbon atoms are sp³-hybridized.  Even then, the ring strain is not especially large.  With respect to carbocations, one carbon is sp-2 hybridized, and therefore one would have to take that into account in assessing ring strain.