tautomerism

(noun)

A form of isomerism in which a dynamic equilibrium between multiple isomers exists, such as that between an enol and a ketone.

Related Terms

Examples of tautomerism in the following topics:

  • Hydration of Alkynes and Tautomerism

    • The explanation for this deviation lies in enol-keto tautomerization, illustrated by the following equation.
    • Tautomeric equilibria are catalyzed by traces of acids or bases that are generally present in most chemical samples.
    • The tautomerization step is indicated by a red arrow.
    • Two factors have an important influence on the enol-keto tautomerizations described here.
    • The first is the potential energy difference between the tautomeric isomers.

  • Multistep Transformations

    • Initial hydrolysis of the lactone to santoninic acid is followed by isomerism of the α,β-unsaturation to a β,ν-location and tautomerization.

  • By Structural Change

    • Rapid rearrangements of this kind are called tautomerizations.

  • Radical Recombination Reactions

    • Subsequent tautomerism of the nitroso product, followed by hydrolysis, converts this function to a carbonyl group.
    • Hydrogen abstraction, followed by coupling of the CH2• to •NO gave a nitroso compound that tautomerizes to an oxime, 3.

  • Sigmatropic Rearrangements

    • The product of this rearrangement is an enol which immediately tautomerizes to its keto form.
    • The initial cyclohexadienone product immediately tautomerizes to a phenol, regaining the stability of the aromatic ring.

  • Reduction

    • The mechanism of this useful transformation involves tautomerization of the initially formed hydrazone to an azo isomer, and will be displayed in the second diagram below.

  • Mechanism of Electrophilic α-Substitution

    • A full description of the acid and base-catalyzed keto-enol tautomerization process (shown below) discloses that only two intermediate species satisfy this requirement.

  • Aldehydes and Ketones

    • Ketones have alpha-hydrogens which participate in keto-enol tautomerism.

  • The Secondary & Tertiary Structures of DNA

    • A simple model for this tautomerism is provided by 2-hydroxypyridine.
    • Note that this tautomerism reverses the hydrogen bonding behavior of the nitrogen and oxygen functions (the N-H group of the pyridone becomes a hydrogen bond donor and the carbonyl oxygen an acceptor).
    • Similar data for the N-methyl derivative, which cannot tautomerize to a pyridine derivative, is presented on the right.

  • Rearrangements of Cationic or Electron Deficient Nitrogen

    • The initial product from hydration at carbon is an iminol, which immediately tautomerizes to the more stable amide.
    • The Hofmann rearrangement in reaction # 8 provides a novel example of the tautomerism of an acetylenic 1º-amine to a nitrile.