Is rearrangement possible

[chiddler]

I'm thinking what it means for rearrangement to be "possible".

Consider cyclohexanol + acid. It certainly is theoretically possible for it to occur, but there is no thermodynamic reason for it.

Or what about 2-propanol. Here it's thermodynamically unfavorable for rearrangement to occur, but surely it must happen in very very small frequency. It is theoretically possible, though unlikely.

So. "Rearrangement is possible during the reaction." T/F?

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[milski]

Who is asking the question? For all normal purposes, my answer would be 'no.' It's unlikely enough that considering the possibility does not have any practical value. The only time when 'yes' might be an answer would be a wacky professor in a thermodynamics class trying to make a point or being a witness in court...

L: "Are you sure, beyond any reasonable doubt, that the OH group did not rearrange from C2 to C1?"
W: "Well, your honor, it's unlikely but there is some possibility that it happened..."

 

[chiddler]

Who is asking the question? For all normal purposes, my answer would be 'no.' It's unlikely enough that considering the possibility does not have any practical value. The only time when 'yes' might be an answer would be a wacky professor in a thermodynamics class trying to make a point or being a witness in court...

L: "Are you sure, beyond any reasonable doubt, that the OH group did not rearrange from C2 to C1?"
W: "Well, your honor, it's unlikely but there is some possibility that it happened..."

SCIENCE COURT!

 

[chiddler]

then i'm doubting what i previously thought. Maybe what I wrote is not so applicable:

is rearrangement possible?

 

[milski]

Ochem is not my strongest subject but what exactly do you mean by re-arrangement? What you have in the last pic is dehydration (I think). Are you asking if it can happen spontaneously without the acid?

What I was thinking of was rearrangement similar to the H/CH3 rearrangements that happen to carbocations. For your second example 2-butanol turning into butanol.

 

[chiddler]

Ochem is not my strongest subject but what exactly do you mean by re-arrangement? What you have in the last pic is dehydration (I think). Are you asking if it can happen spontaneously without the acid?

What I was thinking of was rearrangement similar to the H/CH3 rearrangements that happen to carbocations. For your second example 2-butanol turning into butanol.

Your thinking is correct. After OH is protonated, water leaves and carbocation forms. The question TBR asks is if rearrangement, or shifting of the carbocation, is possible.

My first impression is that it is not possible because there is no reason for it to occur.

The explanation is as follows: "...a carbocation is formed, so rearrangement is possible. Whether it is observed depends on the compound, but it is possible."

 

[milski]

Ok, my fault then, I was not thinking the right way about it. I thought that they/you were asking if a re-arrangement without going through the carbocation is possible. As in moving the switching H/OH between two carbon atoms on its own without any other interaction. That's the one that's theoretically possible but so unlikely that it does not happen in real life.

I see what you are asking though. I could not find anything in my textbook about the case where the carbocation is formed at the already favorable positions and if the re-arrangement is some sort of equilibrium which can go either way with certain probability. It goes through a transition state which has the H or CH3 sort of half-bonded to the two C atoms, so that certainly sounds like a possibility. Especially if you're running the reaction with plenty of energy like the one you have above.

 

[chiddler]

you're right about the high energy, i didn't think of that. i still don't like this question though.

thanks.

 

[mcloaf]

Well, the double bond is going to stabilize the carbocation so you'll get two products...but I'm not sure that's what you're asking about. To be honest, the product you've got shown there isn't even the major one I'd expect, since flipping the double bond up there will move the carbocation to a more stable tertiary carbon instead of a secondary.

As I said, I feel like I may be totally missing what you're asking. If so, ignore me and carry on.

 

[chiddler]

Well, the double bond is going to stabilize the carbocation so you'll get two products...but I'm not sure that's what you're asking about. To be honest, the product you've got shown there isn't even the major one I'd expect, since flipping the double bond up there will move the carbocation to a more stable tertiary carbon instead of a secondary.

As I said, I feel like I may be totally missing what you're asking. If so, ignore me and carry on.

yes you're right! thanks

 

[milski]

Well, the double bond is going to stabilize the carbocation so you'll get two products...but I'm not sure that's what you're asking about. To be honest, the product you've got shown there isn't even the major one I'd expect, since flipping the double bond up there will move the carbocation to a more stable tertiary carbon instead of a secondary.

As I said, I feel like I may be totally missing what you're asking. If so, ignore me and carry on.

The question is if you start with the more stable carbocation will you still have some rearrangement to the less favorable one. In other words, is rearrangement an equilibrium between the two cations or one way street only?

 

[chiddler]

The question is if you start with the more stable carbocation will you still have some rearrangement to the less favorable one. In other words, is rearrangement an equilibrium between the two cations or one way street only?

yes, and i failed to realize that with my compound it's not a one way street.

 

[MedPR]

So the consensus is that it's possible, but not likely? And, given a multiple choice question asking for the product of a reaction, the answer will be the product formed from rearrangement to the most stable carbocation?

 

[chiddler]

So the consensus is that it's possible, but not likely? And, given a multiple choice question asking for the product of a reaction, the answer will be the product formed from rearrangement to the most stable carbocation?

correctumundo