Pyridine basic behavior

This bathochromic shift is typical of 77 —> tt transitions. The behavior of the water solution when acidified was attributed by Albert (175) absorption by the thiazolium cation, by analogy with pyridine. However, allowance is made for the very weak basicity of thiazole (pK = 2.52) compared with that of pyridine (pK = 5.2), Ellis and Griffiths (176) consider the differences between the spectrum of thiazole in water and in... 

Copolymers can be used to introduce a mixture of chemical functionalities into a polymer. Acidic and basic substituents can be introduced, for example, through comonomers like acrylic acid and vinyl pyridine. The resulting copolymers show interesting amphoteric behavior, reversing their charge in solution with changes of pH. 

In their acidity, basicity, and the directive influence exerted on electrophilic substitution reactions in benzenoid nuclei, acylamino groups show properties which are intermediate between those of free amino and hydroxyl groups, and, therefore, it is at first surprising to find that the tautomeric behavior of acylaminopyridines closely resembles that of the aminopyridines instead of being intermediate between that of the amino- and hydroxy-pyridines. The basicities of the acylaminopyridines are, indeed, closer to those of the methoxy-pyridines than to those of the aminopyridines, the position of the tautomeric equilibrium being determined by the fact that the acyl-iminopyridones are strong bases like the iminopyridones and unlike the pyridones themselves. Thus, relative to the conversion of an... 

The pH-independence of Tdhpz at Pt indicates that the driving force for coordination of the nitrogen heteroatom to the Pt surface is much larger than that for protonation even in molar acid. This behavior is in contrast to that of pyridine, where protonation of the nitrogen heteroatom in molar acid hinders N-coordination to the surface (H). Such a difference in chemisorption characteristics may be related to the fact that the basicity of the nitrogen heteroatom in pyridine (pKb =8.8) is much greater than that of the nitrogens in pyridazine (pKb = 11.8) (23.). ... 

At the 145th ACS meeting in New York, September 1963, Dr. Fraser and coworkers reported observing an increase in rate with increasing pH for the iV-methyl derivative as well as for the pyridine-4-carboxylato complex as shown in Figure C. We recently have found similar behavior for the pyridine-2-carboxylato complex and its iV-methyl derivative. In view of the acid dependency found for the N-alkyl pyridinium derivatives of these complexes, it does not seem likely that the acid dependency of the reaction can be explained merely by acidic and basic forms of the complex since the iV-alkyl pyridinium derivatives do not possess such a basic form. 

The study of the relationships between the acidity (N—H) and the basicity ( N ) of azoles has shown that both properties display essentially the same sensitivity to substituent effects (58JA148 69JOC3315). Analogous behavior has been reported for tetrazoles (81KGS559), and, more generally, for all azoles protonated on a pyridine nitrogen (80BSF30). 

The acid-base properties of a mixed solvent is also an important factor influencing the behavior of solutes. Thus, the parameters of the acidity and basicity of mixed solvents have been studied to some extent [35], Figure 2.10 shows the donor numbers of mixtures of nitromethane and other organic solvents. Because ni-tromethane has very weak basicity (DN= 2.7), the addition of small amounts of basic solvents (HMPA, DMSO, pyridine) increase the donor number remarkably. 

Dehydration of hetisine diacetate (126) with phosphorus oxychloride and pyridine followed by basic hydrolysis afforded a mixture of olefins 136 and 137, whose structures were analyzed by H-NMR spectroscopy. These olefins can only be derived from hetisine diacetate if its structure is 126, a fact requiring the structure of hetisinone to be 128. Furthermore, hetisinone is stable to bases, behavior that is consistent with the assigned structure but less likely for either of the alternative /J-ketoalcohols 134 and 135. When hetisinone was heated in D2O-CFI3OD containing sodium deuteroxide, a mixture of deuterated hetisinones was obtained. Mass spectral analysis revealed the presence of 14% d, 53% d2, 24% d3, and 6% dA species. These data further confirmed that hetisinone is correctly represented as 128. 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

In acidic solutions, organic amines protonate to form cations with hydrophobic tails. These ions will seek out and blanket cathodic surfaces, much as carboxylate anions seek out and cover anodic spots in neutral or basic media. The usual choices include amylamine (C5H11NH2), cyclohexyl-amine (C6H11NH2), pyridine (C5H5N), and morpholine [0(CH2CH2)2NH]. Metallic iron behaves as a soft acid in terms of the HSAB classification (Section 2.9), despite the indubitable hard behavior of its trivaJent ion, and so molecules with soft donor atoms adsorb more strongly than hard bases (S > N > 0). This principle can be applied to the design of inhibitors. 

Changing the pK of an acidic or basic group in a molecule so that more of the compound exists in the ionized form at physiological pH lowers log D (at about pH 7) and, in general, should improve aqueous solubility. The improvement in solubility is limited, however, if the solubility of the neutral form of the compound (the inherent solubility) is very low. The situation is worsened if the starting pK is far from 7. We find this to be a particular problem with weak bases. Weakly basic pyridines, quinolines, quinazo-lines and thiazoles seem to be frequent members of combinatorial libraries. Understanding the ionization behavior of drugs and how this property relates to oral absorption is extremely complex and likely beyond the capability (and interest) of many medicinal chemists. The reader is referred to an excellent recent review in this complex area. ... 

Rotundifoline had earlier [41) been assigned the structure 3 although without firm evidence for the stereochemistry at C-15 and C-20. Rotundifoline and isorotundifoline can be equilibrated in pyridine or in acetic acid solution and no other isomer is formed. These observations, together with the low basicity and nonphenolic behavior of rotundifoline, are consistent only with a normal A (3) or (less probably) alio A... 

The quaternization of (benzo)pyridazines by alkyl halides (these systems are not readily susceptible to arylation) was reviewed in CHEC-I <84CHEC-l(3B)l>. Monoquaternization of pyridazines occurs more readily than other diazines but less readily than pyridine, reflecting the intermediate basicity/nucleophilicity of pyridazine. Diquaternization of pyridazine can only be achieved with oxonium salts, particularly Me30 BF4 . As with protonation and A-oxidation, mixtures of products are often obtained on quaternization of unsymmetrical pyridazines and have been the subject of theoretical studies. A number of 2-(ribofuranosyl)-3(2//)-pyridazinones have been prepared by stannic chloride catalyzed alkylation of 3-(trimethylsilyloxy)pyridazines with protected 1-0-ace-tylribofuranose <83JHC369>. The quaternization behavior of phthalazines is similar to that of pyridazines, but with cinnolines alkylation usually occurs at N-2, unless there is a particularly bulky substituent at C-3. 

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