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:
- Intramolecular Hydrogen Bonding
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:
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.
- Has conjugation extending only to 3 atoms & only 2 alpha hydrogens for hyperconjugation.
- Has conjugation extending only to 5 atoms & only 3 alpha hydrogens for hyperconjugation.
- Has conjugation extending only to 6 atoms & only 4 alpha hydrogens for hyperconjugation.
- 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.
- 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.