Enol content tautomers

I have figured out that 4 one will have the highest enol content of all, and the 1 structure would come after 4 in enol content, i.e 4 > 1. Then I got confused between the 2nd and the 3rd structures.

Please give a brief explanation for the answer as well.

  • $\begingroup$ What are the factors that determine enol formation? Is it easier to deprotonate an Ester or a ketone? Those are the sort of questions you need to consider $\endgroup$ – PCK Jun 13 '19 at 19:24

Enol content in 3 is greater then enol content of 2

Greater the acidic nature of alpha Hydrogen,greater is the tendency for Tautomeric behavior. H at alpha position in ketone is more acidic then H at alpha position in an ester.Pka values are shown below.The withdrawing power of ester is less compared to carbonyl group in ketone. Since lone pair on oxygen is in conjugation with C=O in ester,its withdrawing power decreases(as shown below).A more acidic α-hydrogen implies a weaker C−H bond. Summarizing,the enol content increases with the acidity of the enolic hydrogen.

$$\ce{carbonyl <=> enol}$$ $$\mathrm{K_{eq}=\frac{[enol]}{[carbonyl]}}$$

The equilibrium between keto and enol form is tipped to enol if H at alpha carbon is more acidic.

Hence enol content in 3 is greater then enol content of 2.

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Pka of cyclohexanone is as per this link: https://www.drugbank.ca/drugs/DB02060

Enol of 4th compound is aromatic and should be most stable.

Therefore extent of enol content must be 4>1>3>2. As a general guide line the following can be used for predicting enol content.

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The enol form of cyclohexanone has bond angles around double bond other then 120${^o}$ as shown below using avagadro.

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  • $\begingroup$ Pretty sure cyclohexanone doesn’t have a pKa of 11.3! According to Bordwell (chem.wisc.edu/areas/reich/pkatable) it’s 26.4, and acetone is 26.5... $\endgroup$ – PCK Jun 15 '19 at 8:32
  • $\begingroup$ @ PCK i will give source shortly $\endgroup$ – Chakravarthy Kalyan Jun 15 '19 at 8:57
  • $\begingroup$ @ PCK , drugbank.ca/drugs/DB02060 ,iwill try to include snapshot of the pka ,which you will find here. $\endgroup$ – Chakravarthy Kalyan Jun 15 '19 at 9:00
  • $\begingroup$ Can you give a plausible explanation why (according to your numbers) cyclohexanone is a billion times more acidic than acetone? Is the solvent the same for your acetone and cyclohexanone values? $\endgroup$ – PCK Jun 15 '19 at 9:52
  • $\begingroup$ @PCK not according to my numbes,but from references as ptesented. $\endgroup$ – Chakravarthy Kalyan Jun 15 '19 at 11:21

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