Reaction Friday: Keto-Enol Tautomerism

by James

in Alkenes, Ketones, Organic Chemistry 2, Organic Reactions

Today’s Reaction Friday is all about keto-enol tautomersm, the chemistry equivalent of Jekyll and Hyde.

Included:

  • Tautomers are constitutional isomers (not resonance forms)
  • The same factors which stabilize alkenes stabilize the enol form
  • How to draw the mechanism for tautomerism (and how NOT to draw it!)

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{ 5 comments… read them below or add one }

John Spevacek April 29, 2012 at 1:02 pm

A very important subject. Too bad Linus Pauling forgot about it, as that cost him a third Nobel prize. (The base structures in DNA undergo a keto-enol rearrangement and he had the wrong form in his structure.)

And this also is critical to understanding the misnomenclature behind the polymer poly(vinyl alcohol). Vinyl alcohol doesn’t exist except as acetaldehyde, so poly(vinyl alcohol) is made by hydrolysis of poly(vinyl acetate).

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Nima Omid-Fard May 3, 2012 at 12:40 am

Why is the keto form generally favored over enol form? I mean, you’ve mentioned how the enol form is stabilized (conjugation, aromaticity, and H-bonding), but why is the enol form less stable to begin with?

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Feynman Liang May 7, 2012 at 7:00 pm

Correct me if I’m wrong (I just finished the second semester).

The keto form is favored under neutral conditions. This is because RO-H is more acidic than RC-H (oxygen is more electronegative) and thus the keto form (which has the acidic hydrogen on the alpha carbon rather than the carbonyl oxygen) is favored.

Conversion of the keto to the enol form is catalyzed by acidic/basic conditions. Acidic conditions will protonate the carbonyl oxygen while basic conditions will deprotonate the alpha carbon, both of which generates a charged intermediate which is stabilized by conversion to the enol form.

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Albert Tam January 6, 2013 at 9:04 pm

Well, there is an exception. Some dicarbonyl compounds prefer the mono-enol form (1,3-propanedial molecules are an example; empirical measurements say 99% of molecules are in the mono-enol form).

The way I see it, more electronegative atoms form stronger bonds.

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james January 7, 2013 at 7:01 pm

True. If you have a carbonyl two carbons away, the enol form can be stabilized by intramolecular hydrogen bonding. The enol form can also be stabilized if it happens to be aromatic (as in phenol). Finally, subtle factors like solvent, increasing the number of alkyl substituents or adding electron withdrawing groups can also affect the keto/enol ratio.

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