Enol content tautomers

Question

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.

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 — PCK, Jun 13 '19 at 19:24

Chakravarthy Kalyan · Answer 1 · 2019-06-15T09:15:36.633

2

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.

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

edited Jun 15 '19 at 09:15

answered Jun 14 '19 at 16:01

Chakravarthy Kalyan

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Pretty sure cyclohexanone doesn’t have a pKa of 11.3! According to Bordwell (https://www.chem.wisc.edu/areas/reich/pkatable/) it’s 26.4, and acetone is 26.5... – PCK Jun 15 '19 at 08:32
@ PCK i will give source shortly – Chakravarthy Kalyan Jun 15 '19 at 08:57
@ PCK , https://www.drugbank.ca/drugs/DB02060 ,iwill try to include snapshot of the pka ,which you will find here. – Chakravarthy Kalyan Jun 15 '19 at 09:00
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? – PCK Jun 15 '19 at 09:52
@PCK not according to my numbes,but from references as ptesented. – Chakravarthy Kalyan Jun 15 '19 at 11:21
Shouldn’t we critically analyse data rather than just accepting it blindly? 11.3 is implausible – PCK Jun 15 '19 at 11:52
But maybe I’m being an idiot here - anyone else care to comment? – PCK Jun 15 '19 at 11:57
@ PCK https://owl.oit.umass.edu/departments/OrganicChemistry/appendix/pKaTable.html gives Pka of cyclohexanone as 16.7 – Chakravarthy Kalyan Jun 15 '19 at 13:54
So what’s the actual value? ¯_(ツ)_/¯ – PCK Jun 15 '19 at 14:15
@ PCK https://pubchem.ncbi.nlm.nih.gov/compound/Cyclohexanone#section=Computed-Properties pka 11.3 – Chakravarthy Kalyan Jun 15 '19 at 14:53

Enol content tautomers

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