Protonation of pyridine

In another study (40) we found that protonation of pyridine is diffusion-controlled with a one-to-one solute-methanol complex as the reactive species. Thus, while methanol plays essentially no role in the proton transfer to dipicrylamine in the first study, it is indeed intimately involved in the proton transfer to pyridine. 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

Fig. 28. 36-MHz l5N CP/MAS spectra of pyridine-15N on zeolite HY. The experimental conditions were all the same for (a) and (b), except that sample b was extensively dealuminated by increasing the activation temperature to SSITC (400°C for sample a). Both spectra were acquired at 77 K to prevent chemical exchanges on the NMR time scale, (a) The single resonance at —176 ppm as well as its associated sidebands indicates protonation of pyridine by the Brpnsted sites, (b) In addition to the protonated pyridine, four additional resonances at -68, -88, —116, and -140 ppm are also seen, indicating complexation of pyridine with different extraframework Lewis sites.

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

Phillips and Lee calculated the 15N isotope effect for the decarboxylation of 1-methyl orotate (lb) via 2-protonation (4b) and via 4-protonation (6b). They found that in both cases, the calculated isotope effect is normal 1.0043 for 2-protonation, and 1.0054 for 4-protonation. An examination of the optimized structures showed clearly that very little bond order change occurs at Nl, regardless of which oxygen is protonated. Phillips and Lee also benchmarked their calculations by computing the IEs for protonation of pyridine and for decarboxylation of picolinic acid (17) and A-methyl picolinic acid (18) the results of these calculations are in agreement with the experimental values mentioned above. Therefore, Philips and Lee asserted that... 

Cations derived from azole structures by protonation of pyridine-type nitrogen atoms, e.g. [31], show an appreciable low-frequency shift... 

The particular acidity of the protons of pyridine a- and y-alkyl groups is echoed by quinoline-2- and... 

The protonation of pyridine Af-nitroimine may occur at nitrogen or at oxygen. CNDO/2 calculations suggest the possibility of the latter.120... 

Explain why protonation of aniline has a dramatic effect on the compound s UV spectrum, whereas protonation of pyridine has only a small effect on that compound s UV spectmm... 

Standard Thermodynamic Quantities for the Protonation of Pyridine N-Oxide and its Methyl Substituted Derivatives in Aqueous Media0... 

The particular acidity of the protons of pyridine a- and 7-alkyl groups is echoed by quinoline-2- and 4-alkyl groups and by alkyl at the isoquinoline 1-position, but to a much lesser extent by alkyl at isoquinoline C-3. Condensation reactions with alkyl groups at these activated positions can be achieved in either basic or acidic media the key nucleophilic species in the latter cases is probably an enamine, or enamide, and in the former, a side-chain carbanion. ... 

Figure 6. Plot of the dipolar shifts of the and y protons of pyridine vs. concentration of Co HBpzs 2...

The pK associated for protonation of piperidine, a typical secondary amine, is about 13. The equivalent pK for protonation of pyridine—a compound with a similar heterocyclic structure, but with its lone pair in an sp rather than an sp orbital, is only 5.5 pyridine is a weaker base than piperidine (its conjugate acid is a stronger acid). Nitriles, whose lone pair is sp hybridized, are not basic at all. Lone pairs with more p character (sp orbitals are 3/4 p, while sp orbitals are 1/2 p) are higher in energy—they spend more time further from the nucleus—and are therefore more basic. 

Deprotonation of a pyridinylmethylenic proton of pyridine- and bipyridine-based pincer complexes can lead to dearomatization. The dearomatized complexes can then activate a chemical bond (H-Y, Y = H, OH, OR, NH2, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Figure 1.1). The overall process does not involve a change in the metal s oxidation state [6-8]. In this chapter, we describe the novel, environmentally benign catalytic synthesis of esters, amides, and peptides that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

The first type of perturbation, a monocentric perturbation, is seen in the protonation of pyridine (1) to pyridinium ion (2) ... 

This then is the first-order perturbation due to the change in atom k. It is possible to calculate higher-order perturbations, but in the simple treatment that concerns us here there will be no need for these. In the case of the protonation of pyridine... 

Single-chain pyridinium surfactants can be prepared by quaternization or protonation of pyridine derivatives [38]. Phase transitions of pyridinium salts are affected by the nature of the headgroup and counterions. Compounds with an N-... 

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 ... 

In accordance with theoretical predictions, protonation of pyridine enhances the proportioni2586 of radical phenylation which occurs at C(2). 

---