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why is stability of ketonic form>>>enol form in keto-end tautomerism?

why is stability of ketonic form>>>enol form in keto-end tautomerism?

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1 Answers

Arjun Bhattacharya
20 Points
9 years ago

The keto form has a C–H, C–C, and C=O bond whereas the enol has a C=C, C–O an O–H bond. The sum of the first three is about 359 kcal/mol (1500 kJ/mol) and the second three is 347 kcal/mol (1452 kJ/mol). The keto form is therefore more thermodynamically stable by 12 kcal/mol (48 kJ/mol).

 

keto form does not always have to be more stable, other factors like aromaticity, solvents etc play an important role..for example in benzene, the enol form of 2,4-pentanedione predominates in a 94:6 ratio over the keto form, whereas the numbers almost reverse completely in water. In a polar protic solvent like water, the lone pairs will be involved in hydrogen bonding with the solvent, making them less available to hydrogen bond with the enol form...phenol exists in keto form to a very small extent becoz of greater resonance stabilisation of enol form...

hence do not generalise the feature

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