Pyridine acid-base properties

The 2-methyl-4,9-dioxo-4,9-dihydrothiazolo[4,5-.g]quinoline was first quar-temized with methyl iodide on pyridine nitrogen and then treated with IV-methyl-quinolinium-4-yl salt, affording monomethine cyanine dyes 41 to study solva-tochromism, acid-base properties, and antimicrobial activities (95MI1). 

All of the pyridines presently studied are analogues of benzocycloalkenes. Characterization of the acid-base properties of pyridines which are benzobicy-cloalkene analogues such as (32-37) can provide insight into the possible electronic differences caused by strain-induced through mono versus bicyclic annelations (Figure 5). Cast within this reference framework, these compounds are sorely missing data points. 

Section 2 discusses the syntheses of different classes of concave acids and bases. Convergent synthetic strategies were chosen for an easy structural variation of the reagents (modular assembly). Section 3 characterizes the concave acids and concave bases and checks whether the acid/base properties of the parent compounds benzoic acid, pyridine and 1,10-phenanthroline are conserved in the bimacrocyclic structures. In Section 4, the influence of the concave shielding on the reactivity and selectivity of the concave reagents is measured in model reactions. In principle, the concave shielding should be able to influence inter- and intramolecular competitions as well as chemoselectivity and (dia)stereoselectivity. If the reagent is chiral, enantioselectivity should also be observable. 

Besides the examination of the acid/base properties of the concave acids and bases, the geometry of the new bimacrocyclic acids and bases had to be investigated to show that the concave acids and bases were indeed concave. Therefore crystal structures were elucidated by X-ray analyses. For the pyridine bislactam 3e (s. Table 1) [12b], the pyridine bissulfonamide 13j (s. Scheme 3) [15], the 1,10-phenanthroline cyclophane 21a (Structures 3) [20] and the benzoic acid 3fe (s. Scheme 6) [27a], crystals could be obtained which were suitable for X-ray analyses. The structures are shown as stereoplots in Figs. 3(a)- (d). 

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. 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Effect of nonaqueous solvents on acid-base properties of NGu) (Acidic in dimethyl-formamide, pyridine and acet basic in HAc and formic acid) 26) E. Ripper, Explosivst 17 (7), 145-51 (1969) Bt CA 72, 48454(1970) (a-and /3-Nitroguanidine) (Reinvestigation of both forms by IR UV, Nuclear Magnetic Resonance (NMR), Differential Thermal Analysis (DTA),... 

The surface acid-base properties of polycrystalline MgO surfaces have been assessed by means of thermogravimetry and DSC of desorption of pyridine and CO2 in the room temperature to 400 °C temperature range [44]. The endotherms and corresponding AH of desorption were discussed in relation with results determined previously using differential adsorption calorimetry and taking into account the structure, surface area and defects of the studied surfaces. 

The acid-base properties of V2O5/7-AI2O3 catalysts prepared by the impregnation method have been characterized by ammonia, pyridine and sulfur dioxide adsorption microcalorimetry. Sulfur dioxide adsorption made it possible to differentiate a vanadate layer from free alumina. 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

With the aim of studying the acid-base properties of Mg-Fe mixed oxides we chose the alkylation of m-cresol with methanol as a model reaction. The distribution of products obtained may take account of the different surface properties of the catalysts [3,4]. The catalytic performances were compared with the acid and basic sites distributions, as determined by pyridine and CO2 adsorption and desorption, respectively. 

Oxygen atom in a ring position can be transformed to N or S by use of zeolite catalysts whose acid and base properties are adjusted by ion exchange[34. Hoelderich summarizes the relation between acid-base properties and the selectivity[35]. Increasing the basic properties enhances the activity and selectivity for ring transformation of O into S with H2S. For the reaction of (2), the activity order is CsY > RbY > KY > NaY > LiY, which coincides with the strength of basicity. The reaction is retarded by addition of HCI but not retarded by pyridine, which also... 

Acid-base properties of zeolites are probed by studying their interactions with basic/acidic molecules by appropriate techniques (IR, NMR, calorimetry, TPD). For the external surface region, analogous methods based on XPS detection were developed following the pioneering work of Defosse and Canesson [65]. The probe molecules used are pyridine [55,56,66-72], ammonia [21,43,44,73], and pyrrole and chloroform for basic sites [70,74,75]. Kaliaguine [59] has published a review of the results. 

Fig. 11. Study of acidity and basicity with adsorbed probe molecules. Left column N Is lines of pyridine chemisorbed on Al-ZSM-22 (A),Fe-ZSM-22 (B),Al-ZSM-5 (C),Fe-ZSM-5 (D),B-ZSM-5 ( ) right column N Is lines of pyrrole chemisorbed on alkali-exchanged X. Taken from Borade RB, Huang M, Adnot A, Sayari A, Kaliaguine S (1993) Acid-base properties of zeolites an XPS approach using pyridine and pyrrole probe molecule. In New frontiers in catalysis, Proc 10th Int Congr Catal, Budapest, Hungary 1992, p 1625, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netiierlands...

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

Chemisorption of simple probe molecules provides important information on the number of exposed metal sites, the nature of adsorbate-adsorbent interactions, and the electronic state of the surface. Frequently employed probe molecules include CO, H2, NO, and O2. Simple base molecules such as NH3 and pyridine and acid molecules such as CO2 are used to probe the acid-base properties of the surface. Studies of chemisorption have been concentrated on molybdenum nitrides. In general, chemisorption behavior of metal nitride surfaces is very similar to that of carbide surfaces. 

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