Pyridine base strength

Materials of these types have T s of some 290-300°C and some grades are claimed to be stable to about 400°C. Whilst resistant to hydrocarbons, halogenated hydrocarbons, ethers and acids the polymers are soluble in such materials as dimethylformamide, N-methylpyrrolidone and pyridine. Bases can cause stress cracking. These non-crystalline polymers are tough at temperatures as low as -46°C whilst at 260°C they have the strength shown by PTFE at room temperature. The polymers also exhibit excellent electrical insulation properties. 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

Table 24.1 lists pXa values of some ammonium ions and indicates that there is a substantial range of amine basicities. Most simple alkylainines are similar in their base strength, with p/q/s for their ammonium ions in the narrow range 10 to 11. Aiylctmines, however, are considerably less basic than alkylamines, as are the heterocyclic amines pyridine and pyurole. 

Small amounts of pyridine bases increase the corrosion resistance of cement stone without any associated loss of strength [1016]. The use of nitrilo-trimethyl phosphonic acid and an adduct between hexamethylene tetramine and chlorinated propene or butene improves the adhesion to the metal, hardening times, mobility, and strength [ 1770]. The latter adducts are further claimed to be useful as additives in cementing oil and gas wells in salt-bearing strata [1768]. 

Pyridine has practically the same ionization potential as benzene it does not, however, necessarily follow that the flrst ionization potentials (Table 9) relate to an electron of the highest occupied 7r-orbital. These compounds are bases, though weaker than aliphatic amines, and parallels have been noted between base strength and ease of ionization (Nakajima and Pullman, 1958 Krishna and Chowdhury, 1963), which might indicate that the flrst ionization potential is that of a lone-pair... 

Thieno[2,3-. ]pyridines, 115, form via a condensation reaction with halogenated carbonyl compounds, as shown in Scheme 35. Depending on the base strength of the reagent and reaction conditions, intermediate 114 may be isolated <2005CCA63>. 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

Substantial differences in base strength are found between alkanamines and unsaturated amines that have the group C=N —. An example is azabenzene (pyridine, C3H5N), which is a nitrogen analog of benzene ... 

Detection and Determination of the Pyridine Bases. (A) Qi ai.itativK lV.sf. 1. From 5 to f> c.c. of the alcoholic distillate, which should have- a strength of at least Hjj% (by vol.), is shaken with a- c.c. of a 5% solution of anhydrous cadmium chloride, in alcohol. In presence of pyridine a white precipitate forms either immediately or after some time. 

Fig. 1. Effect of alkyl groups in the 2,6-positions on the strength of pyridine bases (in 50% ethanol).

Two equivalents of amine or ammonia are needed to replace one halogen atom the amine also functions as the hydrogen halide acceptor. In some cases the aminophosphazene formed in the reaction has a base strength comparable to that of the parent amine, and it is subsequently isolated as a hydrohalide adduct (see Section IV, A,2). A tertiary base, such as triethylamine or pyridine, can also be employed as a hydrogen halide acceptor. 

The technique has been fruitfully used to characterize acid and basic sites in many catalysts, in particular for zeoHtes and metal oxides [143]. It has also been applied for POMs [144]. It consists of measuring the differential heats of adsorption when adsorbing successive increments of a basic probe molecule such as ammonia or pyridine for acidity characterization or of an acid probe molecule such as GO2 or SO2 to characterize basicity. The technique produces a histogram of the acid-base strength as a function of coverage, in particular when heterogeneity in strength exists. The data should then be compared with ammonia or pyridine desorption data from IR and thermal desorption experiments (see above). 

Reactions of a series of substituted pyridines with hydrogen ions show the order of base strengths to be... 

The phosphonitrilic halides are weaker bases than are some related nonaromatic compounds, just as pyridine is a weaker base than piperidine. The bond type in the phosphonitrilic series is probably the same as in the phosphine-imines RaPrNR (80, 81) the stability of these compounds, too, increases with the electronegativity of the substituents on the phosphorus. Tetraphenylphosphine-imine is one of the most stable of these compounds, and forms a hydrochloride. Its base strength is therefore greater than those of the phosphonitrilic halides, presumably because the opportunities for electronic delocalization are less. The Af-ethyl ester of the trimeric phosphonitrilic acid, which has a cyclic structure, but in which resonance of the same type as in the phosphonitrilic halides cannot occur, also forms a hydrochloride ( 1). It is, therefore, a comparatively strong base, in spite of the inductive effect of the two oxygen atoms on the phosphorus. The extreme weakness of the phosphonitrilic halides as bases cannot therefore be wholly due to the inductive effect of the halogens. 

During oxidation in a nonaqueous solvent, the removal of a proton or an attack of a nucleophile on the oxidized substrate may be a chemical step in an ECE reaction. To explore which reaction is operating one can, in parallel voltammetric experiments, use two compounds that are equal in base strength but differ widely in nucleophilicity such a pair is pyridine and 2,6-dimethylpyridine. 

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