Pyridines ionization constants

Pyridine, 3-(dimethylamino)-amination, 2, 236 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342 Pyridine, 4-(dimethylamino)-in acylation, 2, 180 alkyl derivatives pK, 2, 171 amination, 2, 234 Arrhenius parameters, 2, 172 as base catalysts, 1, 475 hydrogen-deuterium exchange, 2, 286 ionization constants, 2, 172 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342... 

Nelson et al.7I have defined a pyridine AT-oxide substituent constant, difference between the ionization constants of pyNOH+ and substituted pyNOH+ acids. The value of 2.09 has been chosen to give approximate numerical comparability with other sets of substituent constants in common use. Fairly consistent correlations between experimental parameters related to donor strength have been observed, e.g. the reduction of v(V=0) in [VO(acac)2] when 4-substituted pyNO ligands are added. 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

Rochow " (1952-1957) laboratories the dissociation of alkylchlorostannanes in water and in organic solvents was studied extensively. The solutions of ethyl- and methylhalostannanes in water, in lower alcohols, in acetone and in pyridine displayed comparatively efficient electrolytic conduction, but their conductivity in ether, nitrobenzene and nitromethane was insignificant . The ionization constant of Me3SnCl in EtOH was 10 at 25 Rochow and coworkers found that Me2SnCl2 was... 

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. 

The ionization constants (pK ) of 4-substituted pyridines have been measured, as have the AH of ionization at 25° C. Calculate AS for each ionization. Compare the contribution of AH and AS to the free energy of ionization. Test the data for linear free-energy correlations. Are the LFER dominated by the AH or AS term ... 

In this sequence, the pKa of IbP 1 is placed almost exactly between those for purine 3 and benzimidazole 4, and IcP 2 is somewhat closer to purine. These ionization constants give some insight into the influence of the fused rings on the acidity of the imidazole NH. Imidazo[4,5-b]pyridines, and imidazo[4,5-c]-pyridines have more in common with purine in terms of molecular structure and chemical properties than with benzimidazole. 

The apparent ionization constants and ultraviolet absorption spectra of the three isomeric pyridine monocarboxylic acids were determined at 25 in aqueous KCI. As expected, it was confirmed that these acids exist in aqueous isoelectric solutions mainly in the dipolar ion form. Similar data for monocarboxylic and monosulfonic acids and polyvalent acids were obtained more recently. The dissociation constants and electrophoretic mobilities of fifteen substituted alkyl-, nitro-, and chloropyiidine carboxylic acids were also determined. The ultraviolet absorption spectra of a series of dihydropyridine-3, S-dicarboxylic acids were correlated with electronic and steric factors. ... 

According to previous reviews, ammonia and amines also retard hydro-formylation. This statement has to be corrected according to more recent work [7, 130, 131, 1042]. Contrary to the statement mentioned, the hydro-formylation reaction is accelerated by small amounts of certain amines having an ionization constant of 10 [130], as e.g. pyridine, chinoUne, picoline, lutidine, aniline, toluidine, xylididine, methyl aniline and aliphatic and aromatic amides such as N,N-dimethyl formamide, N-methyl pyrroli-done and acetanilide. If larger amounts of these amines are used with HCo(CO)4 the effect becomes smaller. Very large amounts hinder the reaction (see table 3). 

Table 5.10. Ionization Constants of Pyridine and Pyridine 1-Oxide and their Derivatives...

The individual ionization constants for pairs of tautomers and the constant for the equilibrium between them can be deduced from the experimentally determined ionization constants in another way. It will be seen (p. 156) that the effect of substituents on the basic strength of the pyridine nitrogen atom can be represented by the Hammett equation. Accordingly, the ionization constant for the NM form (T b) can be estimated from the Hammett substituent constant of the group — A H. Combined with the experimentally determined Ki this leads to estimates of the remaining quantities. In this way Jaffe 2 deduced that nicotinic and isonicotinic acids exist predominantly as zwitterions in aqueous solution. Similarly, Bryson s obtained the values pi B 5-0 and Kt = 0 78 for 3-hydroxypyridine (cf. Tables 5,10 and 5.12), Hammett correlations have been used in evaluating the tautomeric equilibria in a number of 2-arylsulphonamidopyridines . ... 

By the use of ionization constants it has been concluded that 2- and 4-aminopyridine 1-oxide exist predominantly as such in aqueous solution, whilst 4-hydroxypyridine 1-oxide is in equilibrium with a comparable amount of l-hydroxypyrid-4-one. Application of the method to the pyridine 1-oxides is more complicated than in the pyridine series, and it fails with 2-hydroxypyridine 1-oxide . ... 

To k] Ionization constants have been reported for a number of amino-, diamino- and hydroxy-pyridines, and nitro- and halogeno-derivatives of these 233 Ionization constant measurements show that, in aqueous solution, phenyl 2-, 3- and 4-picolyl sulphones exist predominantly as such Kt = 107 6 109 6 in their favour). 3- and 4-Phenacylpyridine are chiefly ketonic, but 2-phenacylpyridine contains appreciable amounts of the chelated enol in non-polar media and in the solid state 234a ... 

The applicability of the Hammett acidity function to the protonation of pyridines and pyridine 1-oxides in sulphuric acid has been confirmed. Ionization constants for several polyhalogeno-pyridines and a number of negatively substituted pyridine 1-oxides were determined ss ... 

The simpler properties of a number of pyridine aldehydes are recorded in Table 6A. Spectra and ionization constants have been mentioned before (pp. 129, 142, 146 see also below). 

The dipole moments (p. 123), spectra (pp. 137, 143), ionization constants (p. 153) and tautomerism (p. 154) of amino-, acylamino-, arenesulphonyl-amino- and nitramino-pyridines have already been discussed. 

Table 6.12 lists the cyanopyridines and a selection of substituted cyano-pyridines. The dipole moments (p. 125), spectra (p. 130) and ionization constants (p. 148) of the cyanopyridines have been mentioned. 

The dipole moments (p. 126), spectra (p. 132) ionization constants (p. 149) and ability to complex with metals (p. 160) have been discussed. Some pyridine 1-oxides are recorded in Table 6,17. 

To a) The ionization constants of pyridine-2,6-dialdehyde dioxime, and the effect upon these of coordination with ferrous ions have been reported 3. Studies of the cupric complex of 2-benzoylpyridine oxime have been published" (the anti- yridyl oxime, but not its stereoisomer, gives a complex). 

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