- Keto-enol tautomerism
organic chemistry, keto-enol tautomerismrefers to a chemical equilibriumbetween a keto form (a ketoneor an aldehyde) and an enol. The enol and keto forms are said to be tautomers of each other. The interconversion of the two forms involves the movement of a protonand the shifting of bonding electrons; hence, the isomerismqualifies as tautomerism.
A compound containing a
carbonylgroup (C=O) is normally in rapid equilibrium with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl(−OH) 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.
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.
One of the early investigators into keto-enol tautomerism was
Richard August Carl Emil Erlenmeyerand 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.
ignificance in biochemistry
Keto-enol tautomerism is important in several areas of
biochemistry. The high phosphate-transfer potential of phosphoenolpyruvateresults 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 guanineand thyminecan lead to mutation because of their altered base-pairing properties (needs reference).
In certain aromatic compounds such as
phenols the enol is important due to the aromaticcharacter of the enol but not the keto form. Melting the naphthalenederivative 1,4-dihydroxynaphthalene 1 at 200 °C results in a 2:1 mixture with the keto form 2. Heating the keto form in benzeneat 120°C for three days also affords a mixture (1:1 with first order reactionkinetics) 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 trifluoroacetic acidand toluene(1:9 ratio) followed recrystallisationfrom isopropyl ether["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 EditionVolume 45, Issue 1 , Pages 98 - 101 2006 [http://www3.interscience.wiley.com/cgi-bin/abstract/112154960/ABSTRACT?CRETRY=1&SRETRY=0 Abstract] ] .
When the enol form is complexed with chromium tricarbonyl, complete conversion to the keto form accelerated and occurs even at room temperature in benzene.
deoxyribonucleic acids(DNA), the nucleotidebases are in keto form. However, James Watsonand Francis Crickfirst believed them to be in the enol tautomeric form, delaying the solution of the structure for several months ["The Eighth Day of Creation". Judson, Horace Freeland. Simon & Schuster, NY:1979.] .
Hydration of alkynes
Hydrationof alkynes(of the general form RC≡CR, where R is an alkylgroup or hydrogen), produces an enol that is in equilibrium with the keto form. If R, R', or both are hydrogen atoms, the keto form is an aldehyde. If both R and R' are alkyl groups, the keto form is a ketone. The most commonly used set of reagents is sulfuric acid(H2SO4) and mercury(II) sulfate(HgSO4).
acetylene( ethyne) is reacted with H2SO4 and HgSO4, adding H to one carbon and OH to the other; this forms the intermediate enol. In this reaction, the carbons are equivalent and there is no 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.
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keto-enol tautomerism — ketoenolinė tautomerija statusas T sritis chemija apibrėžtis Tautomerija, kai vienas tautomeras yra karbonilinis junginys, o kitas – enolis. atitikmenys: angl. keto enol tautomerism rus. кето енольная таутомерия … Chemijos terminų aiškinamasis žodynas
keto-enol tautomerism — ˈkēd.ō+... noun Etymology: ket + enol : tautomerism in which the keto and enol forms of a compound (as ethyl acetoacetate) are in equilibrium … Useful english dictionary
keto-enol tautomerism — tautomerism between two compounds, one an enol and the other a ketone, that equilibrate by transfer of a proton. The keto form usually predominates except where the enol form is stabilized by conjugation with other double bonds … Medical dictionary
keto-enol tautomerism — /ˌkitoʊ ˌinɒl tɔˈtɒmərɪzəm/ (say .keetoh .eenol taw tomuhrizuhm) noun a type of tautomerism involving migration of a proton, in which, at any instant, the individual molecules are present in the mixture as either a keto form or an enol … Australian English dictionary
Keto-Enol-Tautomerie — Die Tautomerie (von gr. tautó = das Gleiche; gr. meros = der Anteil) bezeichnet in der Chemie eine besondere Form der Isomerie. Sie wurde 1876 von Alexander Michailowitsch Butlerow (1828–1886) entdeckt und 1885 von Conrad Peter Laar als Begriff… … Deutsch Wikipedia
tautomerism — tautomeric /taw teuh mer ik/, adj. /taw tom euh riz euhm/, n. Chem. the ability of certain organic compounds to react in isomeric structures that differ from each other in the position of a hydrogen atom and a double bond. [1880 85; TAUTO +… … Universalium
Enol — Enols (also known as alkenols) are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Enols and carbonyl compounds (such as ketones and aldehydes) are in fact isomers; this is called keto enol… … Wikipedia
enol — A compound possessing a hydroxyl group (alcohol) attached to a doubly bonded (ethylenic) carbon atom (–CH=CH(OH)–); properly italicized when attached as a prefix or infix to an otherwise complete name; e.g., e. pyruvate; phosphoe.pyruvate;… … Medical dictionary
enol-keto tautomerism — noun : keto enol tautomerism … Useful english dictionary
tautomerism — Form of isomerism in which there are two or more arrangements usually of hydrogens bonded to oxygen. Keto enol tautomerism is one common example. The balance between two coexisting tautomers may shift with time or as a result of changes in… … Dictionary of molecular biology