Tautomeric substances

Ethyl acetoacetate is a tautomeric substance which at room temperature exists as an equilibrium mixture of the keto and enol forms, the latter form being present to the extent of about 7%. 

Polarographic techniques have been used by Sturm and Hans to demonstrate that certain amino-thiadiazoles and -benzthiazoles exist in the amino form (cf. also references 62, 63). This method, which involves comparison of the polarographic rdeuction potentials of the potentially tautomeric substance with those of alkylated reference compounds, has not been applied often, but may well prove to be a means to obtain qualitative information quickly. There is a possibility that the method can be modified to yield quantitative data. ... 

The classical theory of methylation with diazomethane was developed by Arndt from a different basis. It depends on the postulate (which can be traced back to von Pechmann " 0 of direct methylation mobile hydrogen in an acid compound is directly replaced by the methyl group, i.e., the methyl group appears in the place which the hydrogen previously occupied. For the reaction of tautomeric substances with diazomethane, the following equation is applicable ... 

This may be expressed somewhat more loosely by saying that, whereas a tautomeric substance is a mixture of two types of molecules, differing in configuration, in general the molecules of a substance showing electronic resonance are all alike in configuration and structure. 

K. H. Meyer s bromine method 1 makes it possible to determine the enol content in almost all solutions of tautomeric substances. 

Whether or no both forms of a tautomeric substance are capable of isolation in the free state depends chiefly on the velocity of rearrangement of the more labile form. The isolation of keto- and enol-forms in a permanent crystalline state was first carried out with unsymmetrical dibenzoylacetone (Claisen, 1896) ... 

Tautomeric substance, 475, 1147 Teflon, 5, 64, 1015 Terephthalic acid, 751, 760 Tertiary amines, 414, 559, 1073 table of, and derivatives of, 661 Tertiary aromatic amines, crystalline derivatives of, 650 reactions of, 649, 1073 Tetrabromophenolsulphonephthalein, 989, 990... 

The indexing and organisation of information relating to tautomeric or potentially tautomeric substances present severe difficulties to all information products. The issue is not how to name or document each individual tautomer, but in what assumptions to make about which structures are tautomeric, and how to organise information that may be imprecisely given in the literature under various structures, often without clear experimental data. 

CAS has fairly precise rules for denoting which structural systems are deemed to be tautomeric, and to which tautomer indexing is directed. In CAS nomenclature 1972-2006, tautomeric substances were often indexed under theoretical tautomers, e.g., the H- form of purines, which had no practical existence. CAS numbers for tautomeric substances therefore often referred to unknown tautomers. 

Keto-enol tautomerism cannot take place without at least a trace of acid or base, " since the acidic or basic center or both in the tautomeric substance is too weak. In this equilibrium, the heteroatom is the basic site the proton is the acidic site. For tautomerism in general (see p 98), the presence of an acid or a base is not necessary to initiate the isomerization since each tautomeric substance possesses amphiprotic properties. Keto-enol tautomerism is therefore the exception. 

Doubtj however, arose as to the validity of the above explanation, and this doubt was confirmed by the isolation of the two isomerides in the solid state, and also by the fact that the velocity of change of the one isomeride into the other could in some cases be quantitatively measured. These and other observations then led to the view, in harmony with the laws of chemical dynamics, that tautomeric substances in the dissolved or fused state represent a mixture of two isomeric forms, and that equilibrium is established not by m/m- but by in r-molecular change, as expressed by the equation... 

The general theory of the dissociation constants for tautomeric acids is the same as that for any equilibrium constant of tautomeric substances. The observed constant is given by... 

Ib.y 577 Lippmann, (i), ii, 458, thought that this foreshadowed the theory of tautomeric substances, but Gerhardt s formulae were not intended to represent actual structures. 

Laar called the phenomenon tautomerism (ravro the same, ficpls a part), pointing out that the two formulae indicated the transfer of a mobile hydrogen atom. A tautomeric substance is not a mixture of two forms but a molecule in which a hydrogen atom oscillates from one position to another. Various modifications of Laar s theory were proposed. ... 

A new phase began when Claisen and W. Wislicenus simultaneously isolated the two forms of tautomeric substances and thus proved that tautomerism is due to the existence of a very mobile equilibrium state between the two forms. Claisen obtained two forms (I, II) of acetyldibenzoylmethane, and of dibenzoylacetone (III, IV) ... 

The high m.p. and b.p. of p3rrazole compared with 1-alkyl or aryl substituted pyrazoles are due to intermolecular hydrogen bonding which results in the dimmer. It is a tautomeric substance. Pyrazole is a weak basic and forms salts with inorganic adds the imino hydrogen... 

The hydroxypyrroles present in their chemical properties the confused picture so typical of tautomeric substances 54-6, 212-3, Lacking alkoxy-carbonyl or acyl substituents, they usually also lack typically phenolic properties. They do not, of course, behave as ketones, for the carbonyl function in the a- or jS-oxopyrroline form is part of an amidic structure. 

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