Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. (Wikipedia)

But looking at the examples provided on that same Wikipedia article, it is obvious that different chemical reactions would take place in each case (for example enol vs. keto).

Tautomer examples (source)

  • 3
    $\begingroup$ You're right, that's why I don't consider tautomers to be the same chemical compound. When I buy a bottle of a acetone, I know both the keto and enol forms are in the bottle. Maybe Wikipedia was saying that when I use acetone, even though I just call it acetone, both the keto and enol forms are present. $\endgroup$
    – ron
    Apr 4, 2015 at 18:09
  • $\begingroup$ Hydronium hydroxide is far more reactive and polar than water, too. Why do some people consider it the "same compound" as water? No matter how much you purify water, you just can't seem to be rid of the hydronium hydroxide. $\endgroup$
    – Curt F.
    Apr 5, 2015 at 2:21

1 Answer 1


There are a number of types of rapid chemical equilibria that basically cannot be turned off. Tautomers are probably the best known examples, although fluxional behavior is observed in many inorganic and organometallic systems.

The origin for most tautomers is that protons are really quantum-mechanical. They can tunnel just like electrons can delocalize. Obviously different tautomers will have different stability, much like some electric resonance structures are clearly more stable.

The comments to your question above indicate that most chemists would just call it "acetone" but of course there's some prop-1-en-2-ol present too.

Yes, the reactivities might be different.

It's a partly a matter of the balance of the equilibrium (i.e., that for many tautomers, only one form predominates). There are exceptions, e.g. phenols (from Wikipedia):

In certain aromatic compounds such as phenol, the enol is important due to the aromatic character of the enol but not the keto form. Melting the naphthalene derivative naphthalene-1,4-diol 2 at 200 °C results in a 2:1 mixture with the keto form.

It's also a matter of the time-scale. For example, at room temperature if I take an NMR, do I see two sets of peaks? What about IR or other characterization techniques? If I can't tell the difference because there are one set of peaks, I think many chemists will consider there to be "one compound" even if multiple tautomers are present.

For an example of this (from the cheminformatics perspective), Marc Nicklaus of the NIH/National Cancer Institute gave a great talk at the 247th ACS national meeting "The NCI/CADD Group's InChI Usage and Analysis of Tautomerism for InChI V2".

The talk focuses on tautomers and what software is trying to do to derive simple rules (as opposed to lots of quantum calculations or experiments) to determine one standard tautomer. But they also measured several tautomers by NMR:

These would obviously be chemically quite different, but one form clearly predominates.

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  • 3
    $\begingroup$ I'll point out that there are plenty of cases where multiple tautomers exist and are observable by characterization techniques. But I think your question falls into what I describe - many chemists think of tautomers as the same compound because there's often one predominant form, you can't usually separate them (e.g., enol-keto) or because you can't observe the two forms via NMR or other characterization technique. $\endgroup$ Apr 23, 2015 at 21:22

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