[qw098]

Hi guys,

So... I am lazy... and I was wondering if this was an acceptable way to draw an enol tautomerization . Please find below. I know usually the mechanism is a two step mechanism catalyzed by acid, but why wouldn't the proposed mechanism shown below not be possible.

Well... actually I think it is possible... but in all the textbooks they seem to show an intermolecular proton transfer instead of an intramolecular one.

Thanks!

[discodermolide]

Hi guys,

So... I am lazy... and I was wondering if this was an acceptable way to draw an enol tautomerization . Please find below. I know usually the mechanism is a two step mechanism catalyzed by acid, but why wouldn't the proposed mechanism shown below not be possible.

Well... actually I think it is possible... but in all the textbooks they seem to show an intermolecular proton transfer instead of an intramolecular one.

Thanks!

I don't think you should be lazy. Stick to the textbooks version.

[qw098]

Why wouldn't this work?

In order to make advances in society one has to challenge the "norms" and come up with new ideas

[Jorriss]

At the end of the day, experimental evidence does not agree with yours.

But also, it's a pretty strained transition state.

[Doc Oc]

The biggest problem I see is that you haven't moved the enol proton at all, there are still only 2 protons on the a-carbon in the ketone.  Which is yet another reason why you shouldn't be lazy, it's important to pay attention to small details like that.

[qw098]

The biggest problem I see is that you haven't moved the enol proton at all, there are still only 2 protons on the a-carbon in the ketone.  Which is yet another reason why you shouldn't be lazy, it's important to pay attention to small details like that.

Yah, I forgot to include the last hydrogen in my ketone.. kinda embarrassing. Here's a quick touch-up below to avoid confusion..

At the end of the day, experimental evidence does not agree with yours.

That's interesting that you state experimental evidence doesn't agree with me. Do you have a reference?

But also, it's a pretty strained transition state.

Great point! Thanks!

[Dan]

But also, it's a pretty strained transition state.

This is the main reason the intermolecular mechanisms are favoured, otherwise it's a 4-membered transition state. I used to see this short cut regularly when I was teaching, as well as intramolecular proton transfer in the tetrahedral intermediate of A_AC2 ester hydrolysis, which is an alalogous scenario.

I used to tell my students that I didn't mind them omitting keto-enol tautomerism (or proton transfer) mechanisms when I was happy that they could do it, but to either put no curly arrows or put the full mechanism - nothing in between because it propagates bad habits. It did get a bit excessive when for a carbonyl tutorial the set work would otherwise require them to draw the same mechanism 20+ times. I always told them to draw it out in full for exams though.

[qw098]

Thanks for the nice and clear response Dan!

[orgopete]

@qw098
This is why I don't like it. A proton has a small charge and while it can attract electrons, it is difficult to attract two pairs of electrons. When two pairs of electrons are attracted in a hydrogen bond, the angle is usually 180°. That angle allows the electrons to not repel each other.

There are intramolecular reactions with smaller angles. The decarboxylation of a beta-keto acid results in a proton transfer in a six-membered ring. An elimination of an amine-oxide, sulfoxide or selenoxide involves a proton transfer in a five membered ring. Those are the smallest bona fide examples that I can find.

You could draw your tautomerization with a water or alcohol molecule to make a six-membered ring, but this would increase the entropy of the transfer. If you are in a proton medium, it seems likely that proton transfers can occur with the protic solvent. Perhaps you could think of this analogy. If proton transfers in the NMR are so rapid that the splitting from adjacent protons is not found in the spectrum, then you could expect that proton transfer without tautomerization may also be a fast process and protonation on carbon may be a slower, but thermodynamically favored reaction.

Secondly, you should make a model so you can examine the actual distances though which you are transferring the proton. You should also remain aware that the alkene electron orbitals are perpendicular to the double bond and the proton must reach them.

[CaverKat]

This is my "lazy" way