Pyridinium ion

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Davidson W R, Sunner J and Kebarle P 1979 Flydrogen bonding of water to onium ions. Flydration of substituted pyridinium ions and related systems J. Am. Chem. Soc. 101 1675-80... 

A more detailed study of the nitration of quinolinium (l) in 80-05 % sulphuric acid at 25 °C, using isotopic dilution analysis, has shown that 3-) 5-) 6-, 7- and 8-nitroquinoline are formed (table 10.3). Combining these results with the kinetic ones, and assuming that no 2- and 4-nitration occurs, gives the partial rate factors listed in table 10.4. Isoquinolinium is 14 times more reactive than quinolinium. The strong deactivation of the 3-position is in accord with an estimated partial rate factor of io for hydrogen isotope exchange at the 3-position in the pyridinium ion. It has been estimated that the reactivity of this ion is at least 10 less than that of the quinolinium ion. Based on this estimate, the partial rate factor for 3-nitration of the pyridinium ion would be less than 5 x io . 

Strauss and Williamst have studied coil dimensions of derivatives of poly(4-vinylpyridine) by light-scattering and viscosity measurements. The derivatives studied were poly(pyridinium) ions quaternized y% with n-dodecyl groups and (1 - y)% with ethyl groups. Experimental coil dimensions extrapolated to 0 conditions and expressed relative to the length of a freely rotating repeat unit are presented here for the molecules in two different environments ... 

An acidimetric quantitative determination is based on treatment of the hydantoia with silver nitrate and pyridine ia aqueous solution. Complexation of the silver ion at N-3 Hberates a proton, and the pyridinium ions thus formed are titrated usiag phenolphthaleia as an iadicator. In a different approach, the acidity of N-3—H is direcdy determined by neutralization with tetrabutylammonium hydroxide or sodium methoxide ia dimethylformarnide. 

Pig. 4. Synthesis of tigemonam where Boc is the /-butoxycarbonyl group PyrH" is the pyridinium ion and the numbers given are percent yield of product. 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

Aromatic six-membered heteroeyeles, isoeleetronie with benzene, are widely distributed in nature, and in the world of synthetie ehemistry. Sinee and C are isoeleetronie, the simplest and most direet hetero-analogue of benzene (1) is the pyridinium ion (2). Further azonia substitution of this kind is theoretieally possible, but knowledge of this type of strueture does not extend beyond the disubstituted speeies (3)-(5). 

It is of interest to compare the stability of the protonated forms of benzo[u], benzo[Z>] and benzo[c] fused pyrroles, i.e. the cations derived from indolizines, indoles and isoindoles. Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. 

Pyridinium iodide, 4,4 (l,3,4-thiadiazole-2,5-diyl)-bis(l-methyl)-reduction, 6, 564 Pyridinium ion, Af-methyl-as metabolite of pyridine, 1, 234 Pyridinium ions hydrogen bonding to water mass spectrometry, 2, 135 magnetic circular dichroism, 2, 129 NMR, 2, 121... 

The stronger base is derived from the weaker conjugate acid. Therefore, add a proton to fflnmonia to give its conjugate acid (anmonium ion) and a proton to pyridine to give its conjugate acid (pyridinium ion), then look up the values for each. 

Ammonium ion is a weaker acid than pyridinium ion therefore, fflnmonia is a stronger base than pyridine. 

One reason for the low reactivity of pyridine is that its nitrogen atom, because it is more electronegative than a CH in benzene, causes the tt electrons to be held more tightly and raises the activation energy for attack by an electrophile. Another is that the nitrogen of pyridine is protonated in sulfuric acid and the resulting pyridinium ion is even more deactivated than pyr idine itself. 

Z values are obtained from Eq. (8-76) for solvents having Z in the approximate range 63-86. In more polar solvents the CT band is obscured by the pyridinium ion ring absorption, and in nonpolar solvents l-ethyl-4-carbomethoxy-pyridinium iodide is insoluble. By using the more soluble pyridine-1-oxide as a secondary standard and obtaining an empirical equation between Z and the transition energy for pyridine-1-oxide, it is possible to measure the Z values of nonpolar solvents. The value for water must be estimated indirectly from correlations with other quantities. Table 8-15 gives Z values for numerous solvents. 

Newkome employed this methodology in his synthesis of halogenated terpyridines. The requisite pyridinium ion 62 was prepared from the corresponding... 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

From this work the deactivation of pyridine to benzene was estimated as about 107. The partial rate factors for nitration of the 3 position of pyridine and the corresponding pyridinium ion were 101,7-2,5 and 1020 respectively. 2-Phenylpyridine was evaluated as 4.9 x 10"5 times less reactive than benzene. 

Pyrylium ions react with ammonia or primary amines to give pyridinium ions (see p. 447). ... 

Additional ionization equilibria involving ortho substituents have been reported by Charton (34) to follow eq. (1). The results of our analysis of the data for aqueous ionization of 2-substituted pyridinium ions, -substituted anilinium ions, and -substituted phenols are given in Table XXVIII. Comparison with the corresponding meta and para data set results is also included. 

While the ortho ionization data are generally fitted with notably poorer precision than for the m- and p- positions, there appear to be characteristic trends indicative of predominant contributions from polar and resonance effects. Thus, for ionization from the ring position (pyridinium ions), p yX" > whereas for ionization from the side-chain position (anilinium ions and phenols),... 

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