Tautomerism

noun

1. Chemical isomerism characterized by facile interconversion of isomeric forms in equilibrium, especially by migration of a hydrogen atom.
2. In organic chemistry, the phenomenon exhibited by certain compounds of reacting as if they possessed two different constitutions, that is, as if they consisted of two substances each structurally different from the other. For example, many compounds containing the group -CH2CO- behave as ketones toward some reagents, but act toward others as if they were unsaturated alcohols, -CH: COH-. In many cases both forms can be isolated.
3. The condition, quality, or relation of metameric substances, or their respective derivatives, which are more or less interchangeable, according as one form or the other is the more stable. It is a special case of metamerism; thus, the lactam and the lactim compounds exhibit tautomerism.
4. A form of isomerism in which a dynamic equilibrium between multiple isomers exists, such as that between an enol and a ketone.