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I came across this question in a test:

The enol form is the ________ form for 3-hydroxy pent-2-enoate? (major/minor)

My argument is that the enol form must be more stable because of hydrogen bonding in the enol form and subsequent six membered ring formation (chelation). However the keto form has no such stabilization factor. Hence according to me the enol form must be more stable. But sadly, that is the wrong answer. What's going wrong?

Structure for reference:

enol form

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quick googling shows that for a very similar compound ethyl acetoacetate enol and ketoforms are in equilibrium with 10% of enol form. However, this equilibrium is influenced by solvent significantly. Actually, the percentage shows that the difference in stability between these two forms is veeery little.

The difference may be attributed to the difference in stability of the bonds, i.e. C=O + C-H is better then C=C + C-O + O-H. However, internal hydrogen bonding DOES exist and participates in stability of enol form. For example, for hexan-1,3-one, the percentage of enol form is much lower.

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  • $\begingroup$ Is my thinking about the hydrogen bond stability incorrect? Also, the solvent wasn't mentioned in the question. $\endgroup$ – Apurv Jun 4 '14 at 7:42
  • $\begingroup$ @Apurv see edit. It is correct, it exists, but it is not strong enough in this particular case $\endgroup$ – permeakra Jun 4 '14 at 7:44
  • $\begingroup$ why isn't it strong in this case? $\endgroup$ – Apurv Jun 4 '14 at 9:27
  • $\begingroup$ @Apurv Unfortunately, no idea. $\endgroup$ – permeakra Jun 4 '14 at 13:52
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    $\begingroup$ I believe, question "is enol form is more stable then keto-form for this particular compound?" cannot be answered without specific knowledge about this one or similar compound and cannot be reasoned from general principles taught in common organic chemistry course. However, asking to determine realtive stability of enol form (i.e. for wich compound percentage of enol form is more) for several clearly differend compounds, say, cyclohexa-1,3-one and acethylacetone, would be a valid question that can be answered from general principles. Is this answer OK with you? $\endgroup$ – permeakra Jun 4 '14 at 16:56

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