Skip to main content
Chemistry LibreTexts

14.2: Keto-Enol Equilibria

  • Page ID
    201239
  • Acidity of Alpha Hydrogens

    Functional groups, such as aldehydes, ketones and esters, contain a carbonyl group which is made up of a sp2 hybridized carbon and oxygen. Because they are sp2 hybridized the carbon and oxygen both have unhybridized p orbitals which can overlap to form the C=O \(\pi\) bond.

    1.jpg

    Keto-enol Tautomerism

    Because of the acidity of α hydrogens carbonyls undergo keto-enol tautomerism. Tautomers are rapidly interconverted constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen atom and a differently located double bond. The equilibrium between tautomers is not only rapid under normal conditions, but it often strongly favors one of the isomers (acetone, for example, is 99.999% keto tautomer). Even in such one-sided equilibria, evidence for the presence of the minor tautomer comes from the chemical behavior of the compound. Tautomeric equilibria are catalyzed by traces of acids or bases that are generally present in most chemical samples.

    3.jpg

    Mechanism for Enol Formation

    Acid conditions

    1) Protonation of the Carbonyl

    2) Enol formation

    Basic conditions

    1) Enolate formation

    2) Protonation

    How Enols React

    Contributors

    • William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry
    • Prof. Steven Farmer (Sonoma State University)

    Due to the acidic nature of α hydrogens they can be exchanged with deuterium by reaction with D2O (heavy water). The process is accelerated by the addition of an acid or base; an excess of D2O is required. The end result is the complete exchange of all α hydrogens with deuterium.

    General reaction

    Example 1: Deuterium Exchange

    Mechanism in basic conditions

    1) Enolate Formation

    2) Deuteration

    Problems

    1) Please draw the product for the following reactions.

    2) Answers

    Contributors