Which of the following best describes what happens when a tautomeric shift occurs?

• A. A hydrogen atom and a double bond move positions within a molecule, typically interconverting between $\text{keto}$ and $\text{enol}$ forms.
• B. A molecule undergoes complete dissociation into its constituent ions.
• C. A molecule loses a methyl group through a radical mechanism.
• D. A carbon-carbon single bond is converted into a carbon-carbon triple bond.

Answer: (A)

Verified step by step guidance

1. Understand that tautomeric shifts involve the relocation of a hydrogen atom and a change in the position of a double bond within the same molecule.
2. Recognize that this process typically interconverts two isomeric forms called tautomers, most commonly the keto and enol forms.
3. Recall that in the keto form, the molecule contains a carbonyl group (C=O), while in the enol form, there is an alcohol group (OH) adjacent to a carbon-carbon double bond (C=C).
4. Note that the shift involves the movement of a proton (H) from the alpha carbon (next to the carbonyl) to the oxygen atom, accompanied by the migration of the double bond from C=O to C=C.
5. Conclude that this tautomerization is a dynamic equilibrium process and does not involve breaking the molecule into ions, losing groups, or changing bond orders beyond the double bond shift.