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Tautomerism
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Given
Which of the given compounds can exhibit tautomerism?Keto-enol tautomerism requires at least one $\alpha$-hydrogen next to the C=O group.
All three ketones have an $\alpha$-H on the other side of the carbonyl group, so I, II and III all exhibit tautomerism. -
The order of stability of the following tautomeric compounds is:
III is the most stable: it has intramolecular H-bonding and a conjugated system, and it forms through the more acidic $-CH_2-$ hydrogen.
II (the diketo form) has no conjugation.
I is the least stable: it forms through the less acidic H and its C=C is not conjugated with C=O.
Stability: III > II > I