Tautomer
The tautomerism (from gr tauto " the same " and meros " part" ) in chemistry a special type of isomerism. It was in 1876 by Aleksandr Mikhailovich Butlerov ( 1828-1886 ) discovered in 1885 and introduced by Peter Conrad Laar as a concept.
If indeed molecules have the same molecular formula, but the individual atoms are linked differently, it is called isomers. As tautomers are called isomers, which merge into each other fast by the migration of individual atoms or groups of atoms, that is, the two isomers in a rapid chemical equilibrium with each other. Due to the rapid equilibrium, the individual tautomers can not be isolated often; the ratio of the tautomers each other is constant.
Tautomers often differ in the position of a group and in the position of a double bond:
When migrating group monovalent cations, such as the proton or monovalent anions such as chloride, hydroxide or acetate ions come into question. If a double bond by a ring formation of single bonds replaced, this is called the ring-chain tautomerism.
The tautomerism should not be confused with the resonance case, in which only the same molecule is described by several resonance structures.
Prototropy
When prototropy a proton changes its position in the molecule:
Keto -enol tautomerism
The most common form of tautomerism is the keto -enol tautomerism. Because of the polarization of the C-O- double bond by the large electronegativity of the oxygen and the ability to delocalize negative charge after deprotonation three atoms, protons at the α -C - atom of the carbonyl group can easily be cleaved. A reprotonation of the enolate ion on oxygen leads to the enol. This tautomerization can by bases that support spin-off of the proton, or by acids, which increase by protonation of the carbonyl oxygen, the polarization of the C -O bond, are catalyzed.
The equilibrium is usually on the side of the keto form. The proportion of enol form in acetone is only 0.00025 %. The pentane -2 ,4- dione (common name acetylacetone ), however, outweigh the enol form in equilibrium. Phenols are predominantly in the enol form, since the formation of the keto form (example: quinoid structure ) cancels the aromatic system.
Example: ethanal and Ethenol are in solution in a tautomeric equilibrium, the equilibrium is, however, clearly on the side of Ethanals. It is in both linkage isomerism (C = C / C = O) and functional isomerism (-CH = O / C- OH) ago.
From the position of the equilibrium can not be close to the reactivity. Often, the enol form is considerably more reactive than the keto form. Because it is constantly recreated but due to the rapidly adjusting equilibrium is observed macroscopically only the reactivity of the enol (see Le Chatelier's principle ). Very nice it can be shown in the reaction of acetone with bromine this. After adding one drop of bromine whose color disappears slowly at first, then faster and faster, as the setting of the keto -enol equilibrium is greatly accelerated by the resulting HBr. Upon further addition of bromine decolorization with the formation of bromoacetone is almost instantaneous.
Ketol enediol tautomerization
α - hydroxy ketones ( acyloins ) have a special form of keto -enol tautomerism.
1, the 2-hydroxy- propanal has the α - carbon atom ( the 2 carbon atom) to a hydroxy group. These polarized additionally by its negative inductive effect of the CH bond at the α - carbon atom and thus facilitates the elimination of a proton. The result is an enediol as a molecule with a double bond and two ( di ) adjacent hydroxy groups. By the rearrangement of a proton, this may again transition to a molecule having a carbonyl oxygen, in this case, a ketone.
Here positional isomerism is present in addition, there are different 2- hydroxy -propanal and 1- hydroxypropanone only in the positions of the hydroxyl and the carbonyl group.
2 in the example of Propanals When the methyl group is replaced by a hydrocarbon chain having four carbon atoms and four hydroxyl groups, there is an aldohexose, which is present on the enediol form with ketohexose in aqueous solution in equilibrium. For instance, in aqueous solution, the epimeric glucose and mannose as aldohexoses with the ketohexose fructose in equilibrium. Also why the Fehling's test is positive even with Fructose: In basic medium of Fehling's solution it converts to glucose or mannose, which are then oxidized by Fehling's solution. But make enediols (or their anions as they arise in the alkaline Fehling's solution) even strong reducing agent is to be dehydrogenated to 1,2- diketones. For example, even benzoin and acetoin (3- hydroxybutanone -2) are highly reducing. In the glycolytic conversion of glucose-6- phosphate into fructose -6-phosphate catalyzed by the enzyme glucose -phosphate isomerase, in a later step of the glycolysis reaction of the glyceraldehyde-3- phosphate to dihydroxyacetone phosphate by the enzyme triose phosphate isomerase.
Other examples: ribose and arabinose are in tautomeric equilibrium with the ketopentose ribulose as aldopentoses.
3 The epimerization is a tautomerization: In aqueous solution to convert epimers, which are aldoses, differing only by the position of the hydroxy group on the second carbon atom into each other. Examples of epimers pairs: glucose / mannose, ribose / arabinose, erythrose / Threose, D- glyceraldehyde / L- glyceraldehyde
See also: ketol enediol tautomerization with phloroglucinol, reductones.
Nitro - aci -nitro tautomerism
Compounds with a nitro group are in acid solution with its aci - form in equilibrium. In this case the equilibrium is usually on the side of the nitro compound.
Nitroso- oxime tautomerism
Compounds with a nitroso group are in acid solution with its oxime form in equilibrium. Here, the balance usually is 100% on the side of the oxime.
Amide imidic tautomerism
Imine -enamine tautomerism
Azo - hydrazo tautomerism
Azo compounds with an enolizable hydrogen atom on the α - carbon atom adjacent to the azo group are in equilibrium with the hydrazo form. In this case, the equilibrium usually lies predominantly on the side of the hydrazo form.
Tautomerism in tetrazoles
Tautomeric equilibrium of the heteroaromatic 1H- tetrazoles and 2H- tetrazoles as compared with 5H- tetrazole (right):
Lactam lactim tautomerism
Thiolactam - Thiolactim tautomerism
Ring-chain equilibria
Oxy - cyclo - tautomerism
Long-chain oxo compounds are cyclic ether could form. Example is the hemiacetal cyclo - tautomerism. Hemiacetals result in an addition reaction of alcohols to carbonyl compounds. If a molecule contains a hydroxyl group, which is far enough away from the carbonyl group, there may be a reaction within the molecule for ring closure. Here, the carbonyl oxygen is replaced by the proton of the hydroxyl group and, because of that the hydroxyl group. Ring closure is effected by the formation of a bond between the negatively polarized oxygen remote hydroxy group and the positively polarized carbon of the carbonyl group. Between the open-chain Aldhehyd or keto form and the ring form in aqueous solution is an equilibrium. This is the case for all aldo- and keto- pentoses and hexoses in aqueous solution.
(see ATP and RNA, see also glucose and fructose)
Tropanol - Cycloheptanone tautomerism
(1R )-1- Tropanol is (R) - (methylamino ) cycloheptanone in a tautomeric equilibrium:
Anionotropie
Such butenols: 3-hydroxy -1-butene is 1 -hydroxy-2 -butene ( crotyl alcohol ) in a tautomeric equilibrium, when it is heated with dilute sulfuric acid at 100 ° C for several hours ( equilibrium ratio 3: 7). This corresponds to only formally a Anionotropie tautomerism in which a hydroxide anion changes its position. The actual reaction mechanism is that initially the hydroxy group is protonated, and then split off as water molecule. What remains is a resonance-stabilized carbenium ion, which is polarized positive at the first and third carbon atom. On one of the two carbon atoms of a water molecule can bind, which is to the hydroxyl group again by elimination of a proton.
Dyadic tautomerism
In the dyadic tautomerism (from gr dyas = duality ) there will be a proton migration between neighboring atoms.