What distinguishes a tautomer from another isomer?

Tautomers are a specific type of isomer that differ primarily in the placement of a proton and a double bond. This means that while they share the same molecular formula, they transition between forms through the movement of a hydrogen atom and a pi bond. This characteristic distinguishes tautomers from other isomers, which may have different connectivity or arrangement of atoms.

For example, in the case of keto-enol tautomerism, the keto form has a carbonyl group (C=O), while the enol form has a hydroxyl group (O-H) adjacent to a carbon-carbon double bond (C=C). The interconversion between these forms involves the migration of a hydrogen atom and the shift of a double bond, which defines them as tautomers.

Other types of isomers, such as structural isomers or geometric isomers, may involve different atom connectivity or spatial arrangement but do not fit the specific definition of tautomers. Therefore, the distinction of tautomers lies in their unique structural relationship defined by proton and double bond placement, making the correct choice focused on that particular change.