Keto-Enol Tautomerization

[MedPR]

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This keeps popping up in TBR/EK/Wikipremed as a very important topic and something we should definitely know for the MCAT.

I've always thought it was a simple concept, but the fact that it keeps showing up is starting to make me wonder if I don't know it as well as I should.

To what extent do we need to know about keto-enol tautomers? This image basically sums up everything I know about it. Is this adequate?

 

[ljc]

I've seen this pop up a couple times in various places (including practice mcats), and I *vaguely* know the principles you've listed, yet I've still gotten every tautomerization question right thus far. So while I can't say for certain, I'd guess you'd be golden if you know everything you've listed here.

A lot of this can be intuited, also.

 

[MT Headed]

I came up with a list of gotchas and important points before looking at the diagram, and everything I was gonna say was already on the diagram. Nice diagram!

 

[SaintJude]

I have a follow up question. Which ketone (see picture) will favor the enol form more?

View attachment Picture 3.png


Is the answer A, then?

 

[mcloaf]

I have a follow up question. Which ketone (see picture) will favor the enol form more?

View attachment 18486


Is the answer A, then?

I'd imagine so, as the more substituted alkene will be more stable.

 

[SaintJude]

And yet another follow-up question: The enol form is not prevalent in solution. But why? I understand that the higher bond energies of the ketone makes it more thermodynamically stable than the enol.
Is there are another major reason apart from that?

 

[ljc]

And yet another follow-up question: The enol form is not prevalent in solution. But why? I understand that the higher bond energies of the ketone makes it more thermodynamically stable than the enol.
Is there are another major reason apart from that?

Here's a good rundown of what is favored and when:
http://masterorganicchemistry.com/2010/04/12/keto-enol-tautomerism-key-points/

Edit:

I'd imagine so, as the more substituted alkene will be more stable.

Can anyone give me a quick one sentence reminder why a more substituted alkene is more stable? I keep thinking more substituted = more electron donating alkyl groups = more negative carbons = more unhappy carbons. But this must be wrong?

 

[MT Headed]

Here's a good rundown of what is favored and when:
http://masterorganicchemistry.com/2010/04/12/keto-enol-tautomerism-key-points/

Edit:



Can anyone give me a quick one sentence reminder why a more substituted alkene is more stable? I keep thinking more substituted = more electron donating alkyl groups = more negative carbons = more unhappy carbons. But this must be wrong?

For the purposes of the mcat, I would think empirical evidence is sufficient enough. Here's the real reason. Caution- it is kinda scary:

http://en.wikipedia.org/wiki/Hyperconjugation

 

[mcloaf]

Can anyone give me a quick one sentence reminder why a more substituted alkene is more stable? I keep thinking more substituted = more electron donating alkyl groups = more negative carbons = more unhappy carbons. But this must be wrong?

Well, the double bond is an accumulation of more negative charge density in the region, so whatever is going on must have to do with spreading out that density. You're correct in that alkyl groups are generally electon-donating for the purposes of acidity and aromatic substitution etc. In this case though, I think they'd actually remove some electron density. Carbon is more electronegative than hydrogen, so a C-H bond is going to have more of the electron cloud towards the carbon, while a C-C bond will have it evenly distributed, all else being equal. I'd imagine that surrounding an alkene carbon with C-C bonds will be more stable that surrounding it with C-H bonds for this reason.

Keep in mind, I just made all of that up from what I do know.

 

[ljc]

I'd imagine that surrounding an alkene carbon with C-C bonds will be more stable that surrounding it with C-H bonds for this reason.

This is correct. Just re-read Bruice and it says:

C(CH3)2=C(CH3)2 > CH(CH3)=C(CH3)2 > trans-CH(CH3)=CH(CH3) > cis-CH(CH3)=CH(CH3), which is approximately equal to CH2=C(CH3)2. Trans is more stable than cis because the CH3 groups in cis interfere with each other, causing hindrance.

Doesn't go too much into the why of it, so I suppose I'll just remember it rather than try to intuit it.

 

[gettheleadout]

Can anyone give me a quick one sentence reminder why a more substituted alkene is more stable? I keep thinking more substituted = more electron donating alkyl groups = more negative carbons = more unhappy carbons. But this must be wrong?

For the purposes of the mcat, I would think empirical evidence is sufficient enough. Here's the real reason. Caution- it is kinda scary:

http://en.wikipedia.org/wiki/Hyperconjugation

Yeah, "due to hyperconjugation" is what I have in my notes from Organic I.