Keto-enol tautomerism

Mechanism

The conversion of an acid catalyzed enol to the keto form proceeds by a two step solution.

First, the exposed electrons of the C=C double bond of the enol are donated to a hydronium ion (H₃O⁺). This addition follows Markovnikov's rule, thus the proton is added to the carbon with more hydrogens. This is a concerted step with the oxygen in the hydroxyl group donating electrons to produce the eventual carbonyl group.

Second, the oxygen in a water molecule donates electrons to the hydrogen in the hydroxyl group, thus relieving the positive charge on the electronegative oxygen atom.

Erlenmeyer rule

One of the early investigators into keto-enol tatomerism was Richard August Carl Emil Erlenmeyer and his Erlenmeyer rule (developed in 1880) states that all alcohols in which the hydroxyl group is attached directly to a double-bonded carbon atom become aldehydes or ketones. This occurs because the keto form is generally more stable than its enol tautomer. As the lower energy form, the keto form is favored at equilibrium.

Significance in biochemistry

Keto-enol tautomerism is important in several areas of thymine can lead to mutation because of their altered base-pairing properties.

In certain aromatic compounds such as isopropyl ether ^[1].

When the enol form is complexed with chromium tricarbonyl, complete conversion to the keto form accelerated and occurs even at room temperature in benzene.

DNA

In solution of the structure for several months ^[2].

Hydration of alkynes

Hydration of sulfuric acid (H₂SO₄) and mercury(II) sulfate (HgSO₄).

Here, stereoselectivity. The reaction immediately continues with keto-enol tautomerization.

In general, the equilibrium lies far toward the keto side; in fact, the enol intermediate cannot be isolated as a product. Hydration of alkynes is unlike hydration of alkenes, where the product is an alcohol rather than an enol; therefore, no such equilibrium occurs.

References

1. ^ Rediscovery, Isolation, and Asymmetric Reduction of 1,2,3,4-Tetrahydronaphthalene-1,4-dione and Studies of Its [Cr(CO)3] Complex E. Peter Kündig, Alvaro Enríquez García, Thierry Lomberget, Gérald Bernardinelli Angewandte Chemie International Edition Volume 45, Issue 1 , Pages 98 - 101 2006 Abstract
2. ^ The Eighth Day of Creation. Judson, Horace Freeland. Simon & Schuster, NY:1979.