4-Hydroxy-2-mercapto-5-

Phosgen(III)289 entstehen 1,2-Dihydroxy- bzw. 1-Hydroxy-2-mercapto-imidazole ... 

Multifunctional Hydroxy, Mercapto, and Amino Compounds. These are used to cross-link halogenated polymers. Depending on the labihty of the halogen, the cross-linking agents can be capped to reduce reactivity or used in combination with accelerators to increase the rate of reaction. Benzoyl capping is common with hydroxy and mercapto compounds forming the carbamate by reaction with one equivalent of carbon dioxide is used with diamines. 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently... 

For hydroxy- and mercaptoazoles, hydroxy-oxo and thiol-thione tautomeric equilibria depend strongly on the orientation of the substituents and heteroatoms. Most 2- and 5-hydroxy(mercapto)azoles exist mainly as 0X0 (thione) tautomers, but the corresponding 3- and 4-isomers often prefer the hydroxy (mdrcapto) forms. 

Whereas vicinal hydroxy, mercapto, and hydroselenoaldimines of azoles strongly prefer the aminomethylene tautomeric form (Section II,E,2), their metal chelates 380 are characterized by.pronounced equalization of bond lengths within the chelate ring, which makes their structures similar to those expected for the aldimine tautomeric type. 

Often, the exact position of tautomeric equilibria (Section III,A,4,a) is uncertain for fused troponoids substituted by hydroxy, mercapto, or amino groups. In such cases the structures preferred by the authors or the most probable tautomeric forms will be used here. 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

Simple considerations such as these account adequately for many of the familiar reactions of substituted 7r-deficient heterocycles, such as nucleophilic displacement, tautomerism in hydroxy, mercapto and amino heterocycles, facile deprotonation of alkyl substituents, decarboxylation of carboxymethyl groups and electrophilic substitution of benzo-fused and aryl-substituted heterocycles. These individual effects are discussed separately in the following subsections. 

Bei Imidazolen, die an einem oder zwei Ring-C-Atomen Hydroxy-, Mercapto- oder Amino-Gruppen tragen, sind durch Keto-Enol-Tautomerie mehrere tautomere Formen moglich. Als Beispiel mogen die vier moglichen Tautomeren von 2-Hydroxy-imidazol dienen ... 

Hydroxy-, Mercapto- oder Amino-benzimidazole besitzen weitere acide Protonen, die am Gleichgewicht tcilnchmcn z. B.33 ... 

MI4 95CCR313), 8-hydroxy(mercapto)quinolines (50), azomethines (51) and (52) (93MI5 96MI14), and formazanes (53) (91KK1011 96MI12). 

Chemistry of heteroaromatic hydroxy-, mercapto-, and selenolo-aldimines 92KGS5. 

Poly(pyrazolyl)borate complexes of Sn(II) and Pb(II) 92SL469. Synthesis, structure, and properties of vicinal hydroxy-, mercapto-, and hydroselenoaldimines of the pyrazole series 92MI22. 

Some of dicarboxylic acids can also be distilled, without decomposition, under reduced pressures. This is at least a theoretical ground for the possibility of their direct GC analysis. Few successful attempts have been described, but these analytes require on-column injection of samples and extremely high inertness of chromatographic systems. Many types of polyfunctional carboxylic acids (hydroxy-, mercapto-, amino-, etc.) cannot be analyzed in free, underivatized form, owing to either non volatility and/or absence of thermal stability. These features are the principal reasons for the conversion of carboxylic adds, before their GC analysis, into less polar derivatives without active hydrogen atoms. 

For purposes of classification, 4-hydroxy-, 4-mercapto- and 4-amino-3-pyrazo in-5-ones, except 4-amino having no hydrogen on the nitrogen, have been considered to be 3,4-pyrazolidinediones or derivatives thereof. These compounds could theoretically exist as the 4-oxo, 4-thiono or 4-imino forms but do exist largely, if not exclusively, as the hydroxy, mercapto and amino isomers. The amino compounds will be named as 4-amino-3-pyrazolin-5-ones. They are listed in Table XLIV. 

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