2-Hydroxy-6- ionization constant

For reasons discussed in Section VI, a survey of the purine series (29) is being made in this Department, but so far no example (including 2-hydroxy- and 8-trifluoromethyl-2-hydroxy-purine) of covalent hydration has come to light. An examination of ionization constants disclosed no apparent anomalies, although the interpretation is made more difficult by the ease of anion formation in the 9-position, which often competes with that from other anionic substituents. The only abnormal spectrum seems to be that of the anion of 2-mercaptopurine which is being further examined. 

Ionization constants of czs-3-substituted acrylic acids have been correlated with the Hammett equation by Hogeveen (58) and by Charton (60). Charton has correlated ionization constants for a number of other c/s-vinylene sets with the Hammett equation (60). Charton and Charton have correlated some cw-vinylene sets with the extended Hammett equation [eq. (2)] (73). Sufficient data are available for twelve sets of cis-vinylene equilibria, of which four sets represent ionization constants of hydroxy compounds (sets 12-1 to 12-4) and eight sets represent ionization constants of carboxylic acids (sets 12-5 to 12-12). All sets have been correlated with eq. (24) and eq. (2). Sets studied are reported in Table XII. Results of the correlations are reported in Table XIII. Sets designated A were correlated with eq. (24), sets designated B were correlated with eq. (2). In the case of the second ionization constant of 2,3,5,6-tetrahydroxy-l,4-benzoquinone (set 12-3), it is uncertain which hydroxyl group ionizes therefore, the value for X = OH was excluded from the correlation. All of the sets 12-1 to 12-4 gave significant correlations with both eq. (24) and eq. (2),... 

The electronegative influence of the thiadiazole system greatly increases the acidity of substituents (Table 2). The effect is further enhanced in the 1-monoxide and 1,1 -dioxide analogues. The ionization constants of 4- and 5-hydroxy-2,l,3-benzothiadiazoles were determined in water <70AC(R)80l>. 

Hydroxy-2//-thiopyran as a pseudobase was determined by UV spectrophotometry and discussed with HMO calculations together with other heterocycles.287 Ionization constants of 2//-thiopyran 1,1-dioxides 484 (R = H R1 = Ph R2 = H, Me) were determined spectrophotometrically in H20 and MeOH at 25°C.4S8a... 

The dione, 3, is a yellow crystalline solid that, despite its strained four-membered ring, is much less reactive than 1,2-benzenedione (ort/w-benzoquinone). It cannot be reduced to a cyclobutadienediol, does not undergo Diels-Alder reactions, and with bromine gives a substitution product rather than addition. The bromo compound so formed hydrolyzes rapidly to a hydroxy compound, 4, which is an extraordinarily strong acid having an ionization constant about 109 times that of benzenol ... 

Table 2 Ionization Constants of Hydroxy Acids in Aqueous Solutions...

Schmitt, R, Poiger, T., Simon, R., Garrison, A. W., Freitag, D., and Kettrup, A. (1997b). Simultaneous ionization constants and isoelectric points determination of 12 hydroxy-s-triazines by capillary zone electrophoresis (CZE) and capillary electrophoresis isoelectric focusing (CIEF). Anal. Chem. 69,2559-2566. 

From an overwhelming mass of data on 7t-deficient nitrogenous heterocycles, it is usually considered axiomatic that (wherever possible) hydroxy-1,5-naphthyridines will exist predominantly as their respective 1,5-naphthyridinone tautomers for example, l,5-naphthyridin-2-ol (2) will exist as l,5-naphthyridin-2( 1 //)-one (3). This postulate has been strengthened by ionization constant, infrared spectral, and ultraviolet spectral measurements on simple 1,5-naphthyridinones 887,1026,1027,1035,1040 calculations have been less meaningful.1295... 

Ionization Constants of Heterocyclic Substances. Part II. Hydroxy-derivatives of six-membered rings. 

The ionization of hydroxy-substituted benzoic acids is also affected by chelation. If mtramoleculeir H bonding is possible, the carboxylic acid ionization constant is raised due to the stabilization of the anion (265, 114, 273). This is Ulustrated in Table 5-VII. 

Examination of the basic ionization constants (820) and ultraviolet spectra (821, 1081) of 2-hydroxypyrazine and its TV- and D-methyl derivatives (Table VI.1) (820, 821,1082) has revealed that the neutral species of2-hydroxypyrazine exists in aqueous solution as the amide form (46) but, in the absence of a common cation, tautomeric ratios could not be determined (821, 1083). The infrared spectra of 2-hydroxypyrazine in the solid state and in chloroform solution led to a similar conclusion (1084, 1085). Infrared spectra (680) for 2-hydroxy-3-methyIpyrazine and pjn.r. spectra (979,1086) of 2-hydroxy-and 2-methoxypyrazines also indicate that the hydroxy compounds exist in the amide form, and are further supported by MO[Pg.172]

Dinitro-2-hydroxy thiophene and 3-nitro-5-acety 1-2-hydroxy thiophene have been obtained from nitrochlorothiophenes through reaction with sodium formate in methanol. These compounds were colorless crystalline substances which decomposed with evolution of nitrogen oxides and formation of a dark resin even at —20°C. They gave, however, stable, colored, sodium salts, with ionization constants of the... 

C-Hydroxy-8-azapurines do not exist as such but as equilibrium mixtures (e.g., 8 9) in which the cyclic amide tautomers greatly preponderate over the hydroxy tautomer. This behavior parallels what was found in the pyridine, pyrimidine, and purine series on evidence from ionization constants and the UV spectra of C- and A -methyl derivatives. A formal name for9 is 1,6-dihydro-8-azapurin-6-one but such specification of the hydrogen atom s position is, in the absence of data, risky for example, pyrimidin-4-one is an equilibrium mixture in which tautomers with mobile hydrogen on N-3 and N-1 preponderate in a 5 2 ratio, respectively. Hence the simpler names, such as 8-azapurin-6-one, will be used in this review. 

Ionization constants of hydroxy- and mercaptopyridazines, ° of amino- and diaminopyridazines and their quaternization products,of methylsulfinylpyridazines," and of pyridazinium ylides have been recorded. The basicities of a series of pyridazines have been determined and correlated with substituent constants using the Hammett free-energy relationship. The magnetic susceptibility of pyridazine was measured, and the rate constant for the reaction of hydrogen atoms with pyridazine was determined. The experimental dipole moment, Kerr constant, and molar Cotton-Mouton constant, obtained at 298 K and 633 nm, are reported. The magnetic circular dichroism spectrum of pyridazine has been measured. ... 

Studies on the tautomerism of pyridazines with potential tautomeric groups have continued. On the basis of recorded ionization constants and UV spectra of 3,4,5-trimercaptopyridazine and its derivatives, it is concluded that the compound exists as 3,4-dimercaptopyridazine-5(2f/)-thione. Tautomerism of hydroxy- and mercaptopyridazines was investigated with the aid of the corresponding anhydro bases as reference compounds. They exist predominantly in the -one or -thione forms. Similarly, the 3-mercapto-pyridazine-6(l/f)-thione structure in aqueous solution has been redeter-... 

---