Question

Tautomerism is due to:
A. Delocalization of sigma electrons
B. Delocalization of pi electrons
C. Migration of active H-atom
D. None is correct

Answer 1

Hint: Tautomerism: Inter convertible change in the nature of the functional group due to the position of an atom is called tautomerism. The carbon skeleton of the compound is unchanged.

Complete step by step answer:

Tautomers are those molecules that have the same molecular formula that interconverts rapidly. In such compounds, the single alcohol bond is weaker than the carbonyl double bond. So, the alcohol on an ${ sp }^{ 2 }$ carbon will interconvert into its keto form favorably.
 The most important form of tautomerism is Keto-enol tautomerism. In such a type of tautomerism, one form of the tautomer contains a keto ${ -CO }$ group and the form contains an enolic ${ -C=C-OH }$ group.

One of the basic condition for tautomerism is that there should be at least one ${ S-H }$, ${ O-H }$, ${ C-H }$, ${ N-H }$, bond whose lone pair can enter into resonance with a double bond of a functional group, doesn't matter whether its at alpha or beta or gamma position.
Hence, we can say that tautomerism occurs due to the migration of H-atom.
The correct option is C.

Additional Information:
1. The stability of keto-enol tautomers depends on various factors such as intramolecular hydrogen bonding, inductive effect, resonance effect, substituents attached to the ketone, etc.
2. One of the main conditions for tautomerism to occur is that it must contain at least one alpha-hydrogen.

Note: The possibility to make a mistake is that you may confuse between migration and delocalization of electrons. Here, delocalization can occur before or after protonation but tautomerism occurs due to proton transfer.