Keto-enol tautomerism
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In organic chemistry, keto-enol tautomerism refers to a chemical equilibrium between a ketone or aldehyde (the keto form) 1 and an enol 2. The interconversion of the two forms involves only the movement of a proton and the shifting of bonding electrons; hence, the isomerism qualifies as a tautomerism.
A compound containing a carbonyl group, C=O, is normally in rapid equilibrium with an enol isomer, which contains a pair of double bonded carbon atoms adjacent to an OH (hydroxyl) group, C=C-OH. The keto form predominates at equilibrium for most ketones. Nonetheless, the enol form is important for some reactions. Furthermore, the deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion, is important in carbonyl chemistry, in large part because it is a strong nucleophile.
Keto-enol tautomerism is important in several areas of biochemistry. The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less stable enol form, whereas after dephosphorylation it can assume the keto form. Rare enol tautomers of the bases guanine and thymine can lead to mutation because of their altered base-pairing properties.
In certain aromatic compounds such as phenols the enol is important due to the aromatic character of the enol but not the keto form. Melting the naphthalene derivative 1,4-dihydroxynaphthalene 1 at 200 °C results in a 2:1 mixture with the keto form 2. Heating the keto form in benzene at 120°C for three days also affords a mixture (1:1 with first order reaction kinetics) The keto product is kinetically stable and reverts back to the enol in presence of a base. The keto form can be obtained in a pure form by stirring the keto form in triflic acid and toluene (1:9 ratio) followed recrystallisation from isopropyl ether .
When the enol form is complexed with chromium tricarbonyl, complete conversion to the keto form accelerated and occurs even at room temperature in benzene.
[edit] DNA
In deoxyribonucleic acids(DNA), the nucleotide bases are in keto form. However, James Watson and Francis Crick first believed them to be in the enol tautomeric form, delaying the solution of the structure for several months .
[edit] Erlenmeyer Rule
One of the early investigators into keto-enol tatomerism was Richard August Carl Emil Erlenmeyer and his Erlenmeyer Rule (1880) states that all alcohols in which the hydroxyl group is attached directly to a double-bonded carbon atom become aldehydes or ketones.
[edit] References
- ↑ 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
- ↑ The Eighth Day of Creation. Judson, Horace Freeland. Simon & Schuster, NY:1979.
