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I came across an example today while I was studying tautomerism.

It is an example of tautomerism exhibited by acetylactone/1,3-diketone; given under the keto-enol systems under 1-3 migration/triad systems. (G.R.B. Organic Chemistry - O.P. Tandon 4.7.2(i) pg.122)

Soon I wondered - Why out of all the possible enols only this particular one - as shown in the example,
is the most stable form? ( 76% of the total content is only that particular enol, while the total enol content is 80.4%
testifies that it is much stable than the other possible enols)

As per my knowledge, the stability of enol increases in structures exhibiting:

  1. Aromaticity
  2. Intramolecular Hydrogen Bonding
  3. Conjugation
  4. Hyperconjugation

Where, Aromaticity stabilizes the most and hyperconjugation relatively the least with others in between just as they are arranged. (although I am not sure about it because it was just mentioned by my Chemistry teacher and I could not find sources mentioning it in such a hierarchy - so any edit suggestion in the hierarchy is welcome)

I drew all enol structures possible by 1,3-hydrogen migration: I drew all enol structures possible by 1,3-hydrogen migration

Legend for the workings - 3C,5C,6C represent the number of atoms involved in conjugation while the numbers with alpha after them represent the number of alpha hydrogen present to stabilize a double bond.
(Here it is without any workings/markups)

Now, I'll be stating my assumptions about the stability of each of the 5 structures that I've drawn as per my knowledge.

  1. Has conjugation extending only to 3 atoms & only 2 alpha hydrogens for hyperconjugation.
  2. Has conjugation extending only to 5 atoms & only 3 alpha hydrogens for hyperconjugation.
  3. Has conjugation extending only to 6 atoms & only 4 alpha hydrogens for hyperconjugation.
  4. Consecutive double bonds are often unstable so I decided not to bother about this, anyway it is least conjugated although has 6 alpha hydrogen hyperconjugation shared by two double bonds.
  5. 6 atom bad conjugation since it is not spread throughout the chain & 2 alpha hydrogens.

So with all this, I concluded that option 3 is more stable but the book and the teacher said it is option 2. I asked this question to my teacher about why option 3 cannot be counted in to which I got a rather unconvincing answer - the double bond is usually stable if it is placed more towards the center of a chain. Double bonds at the ends are usually unstable.

Having already accounted for hyperconjugation this is such an unconvincing answer! It would be really helpful if someones can tell me the hierarchy of various stabilizing factors that make the enol more stable and how to determine which structure is more stable.

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    $\begingroup$ Firstly, and most importantly, the stability of certain tautomeric forms is very dependent on the solvent. Secondly, counting alpha hydrogens and carbons involved in conjugation is a very, very rough way of guessing. Don't forget about electronegativity and other effects. Third, (intramolecular) hydrogen bonding is usually the most important factor. Everything else pales in comparison. Ketone forms are usually more stable that enol forms (0th order approximation). $\endgroup$
    – Martin - マーチン ♦
    Jan 23, 2021 at 22:28
  • $\begingroup$ Structure 2 is a vinylogous carboxylic acid, which is supported by its 1H NMR spectrum. sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi $\endgroup$
    – user55119
    Jan 24, 2021 at 0:06
  • $\begingroup$ Is there a source where I could read more about this and understand how & how much these stabilizing effects have an influence on the structures? I'm in 12th grade so NMR spectroscopy is like alien to me :( $\endgroup$
    – Desai
    Jan 24, 2021 at 3:36
  • $\begingroup$ related: chemistry.stackexchange.com/q/18904/102629 $\endgroup$
    – cngzz1
    Jan 30, 2021 at 9:34

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