Tautomerism

Tautomerism is a special type of isomerism in which two constitutional isomers interconvert rapidly by the relocation of a proton accompanied by the reorganisation of pi bonds. In keto-enol tautomerism the keto form has a carbonyl group and an alpha C-H bond, while the enol form has a C=C double bond and an O-H group. The conversion proceeds by transfer of the alpha hydrogen from carbon to the carbonyl oxygen, together with movement of the double bond from C=O to C=C. The option describing migration of a hydroxyl group is wrong because tautomerism moves a hydrogen, not an intact -OH. The shift of a methyl group between carbons would be a skeletal change, not tautomerism, so that is incorrect. Movement of a full carbonyl carbon would break the molecular skeleton and is not what occurs. This proton-shift mechanism is exactly how NCERT describes keto-enol equilibrium, in which the keto form usually predominates because the carbonyl C=O bond is stronger than the enol C=C bond. The enol content rises sharply in systems such as 1,3-dicarbonyl compounds, where the enol is stabilised by conjugation and intramolecular hydrogen bonding. Tautomers are genuine isomers, not resonance structures, because atoms actually move. Plausibility check: only a small, mobile hydrogen can migrate fast enough to give the dynamic equilibrium characteristic of tautomers, confirming the hydrogen transfer.