Conditionss for aromatic compound to show tautomerism
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The basic necessity for keto-enol tautomerism is the presence of acidic hydrogen at the alpha position of the ketone. I hope this point is well understood.
Coming to the stability of keto-enol tautomers. It depends on various factors like intramolecular hydrogen bonding (IMHB), substituted groups attached to the ketone, resonance and inductive effects etc.
Consider pent-2,4-dione. In the enol form you can see IMHB and a stable 6 membered ring. This makes the enol form more stable than the keto form and thus has more product yield. (Need the mechanism for this?)
Similarly, the above property follows in aceto acetic ester and similar looking compounds. Instead of terminal methyl groups, put up Phenyl group, and think how the molecule and hence the stability would work out. This exercise, I leave it for you. ;-)
Another good example would be cyclohex-1,3,5-trione. (Never mind the IUPAC name ;-) :P )
Here is the keto-enol tautomerism of it.
It is clear from above that the enol form is more stable due to aromatic character. So its yield will be more.
On a whole, you just have to compare between what factors are influential in a keto/enol form of a compound. Then you can easily conclude which form is more stable.
Coming to the stability of keto-enol tautomers. It depends on various factors like intramolecular hydrogen bonding (IMHB), substituted groups attached to the ketone, resonance and inductive effects etc.
Consider pent-2,4-dione. In the enol form you can see IMHB and a stable 6 membered ring. This makes the enol form more stable than the keto form and thus has more product yield. (Need the mechanism for this?)
Similarly, the above property follows in aceto acetic ester and similar looking compounds. Instead of terminal methyl groups, put up Phenyl group, and think how the molecule and hence the stability would work out. This exercise, I leave it for you. ;-)
Another good example would be cyclohex-1,3,5-trione. (Never mind the IUPAC name ;-) :P )
Here is the keto-enol tautomerism of it.
It is clear from above that the enol form is more stable due to aromatic character. So its yield will be more.
On a whole, you just have to compare between what factors are influential in a keto/enol form of a compound. Then you can easily conclude which form is more stable.
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