I would like to know the role of tautomerism, if any, in increasing stability of an organic compound. Moreover, how is it different from that of hyperconjugation?

I know that hyperconjugation involves shifting of protons between adjacent atoms in different hypothetical structures to form a practical hybrid structure.

I am not clear with tautomerism, so kindly deliver a precise explanation.


Tautomerism basicially means that you have different isomers that are able to convert into each-other. This is not really similar to hyperconjugation which is an electronic effect in a given molecular geometry. The main difference between them is that hyperconjugation is thought to be a property of a given molecular geometry while tautomerization is an equlibrium between separate, distinct, stable configurations.

There are many types of tautomerizations, as explained for example on the Wikipedia. Probably the most well-known type of tautomerisation is the keto-enol tautomerisation, which is an equilibrium between a ketone molecule and an enol (an alcohol connected to an $sp^2$ carbon). The equilibrium between two isomers is the explanation why enols are only stable under special scenarios: they tautomerise into ketons! Another important example for tautomerism is the equlibrium between the chain and the ring isomers for sugars.

So again: tautomerism is an equilibrium between different (real, existing) isomers as opposed to hyperconjugation which is just defined within a single structure.

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    $\begingroup$ Relevant definitions in the Gold Book: tautomerism, hyperconjugation $\endgroup$ Oct 28 '19 at 14:08
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    $\begingroup$ It would probably be good to mention that tautomerism cannot enhance stability of a given molecular structure as it is dependent on its relative instability towards another isomer. $\endgroup$ Oct 28 '19 at 14:09
  • $\begingroup$ @Martin-マーチン I was thinking about mentioning that, but then I realized that it probably affects the stability of the given tautomer by some second-order topological resonance effects. So I believe that pent-1-ene-1-ol is more stable than pent-2-ene-1-ol due to the possibility of the former one to tautomerise. Since this is just my gut feeling and would be hard to rationalize (at least in a StackOverflow post), I thought I would just not mention it. $\endgroup$
    – user23638
    Oct 28 '19 at 14:16
  • $\begingroup$ Stability is (and cannot be) an absolute concept, you have to know the common state you are referring it to. That is why questions like tho one posed are generally hard to answer completely. However, we know in case of hyperconjugation, which is a special form of resonance, that a description with this effect is energetically lower. We cannot say that for tautomerism. Like you said this is an equilibrium and we have to asses stability for each structure itself. It may happen that other isomers are even lower (or higher) in energy, but that has nothing to do with tautomerism. $\endgroup$ Oct 28 '19 at 14:22
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    $\begingroup$ Let's agree to disagree then. I think this discussion is not helpful to the OP; if you really want to continue it, you can find me in Chemistry Chat. $\endgroup$ Oct 28 '19 at 14:48

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