Basicity of pyridine

Closely related to the basicity of pyridine is its hydrogen-bonding ability two indices of this property have been measured for a very short series of substituted pyridines and are very well correlated by CT-values. ... 

For coupling with 2-naphthol-6,8-disulphonic-l-isotope effects (kK/kD) varied with the substituent in the benzenediazonium ion as follows 4-C1 (6.55) 3-C1 (5.48) 4-N02 (4.78), i.e. the reactivity of the ion was increased so that i correspondingly decreased. Base catalysis was observed127, 129, and there was a free energy relationship between this catalytic effect and the basicity of pyridine, 3- and 4-picoline. However, for 2-picoline and 2,6-lutidine, the catalysis was 3 times and 10 times less than expected from their basicities showing that, in this particular proton transfer, steric hindrance is important. 

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

Imidazole (p/fa 7.0) is a stronger base than either pyridine (p/fa 5.2) or pyrrole (pA a — 3.8). When we compared the basicity of pyridine with that of the... 

Pyridine was used in the beginning of the development of the method. The reaction was slow and the endpoint unstable because of weak basicity of pyridine. The pyridine system buffers at about pH 4. A stronger base, imidazole, has been used to replace pyridine since it gives a faster response and has the advantages of lower toxicity and decreased odor. The optimal pH range for the SO2 imidazole buffer is at pH 6. It is important that the pH of the Karl Fisher reaction be maintained within the range 5 to 7. Outside this recommended pH range, the endpoint may not be reached. 

Pyridines form stable salts with strong acids. Yellow ionic picrates were used for characterization in the past. Pyridine itself is often used to neutralize acid formed in a reaction and as a basic solvent. The basicity of pyridine (as measured by the dissociation constant of its conjugate acid, pKa 5.2) is less than that of aliphatic amines (cf. NH3, pA"a 9.5 NMe3, pKd 9.8). This reduced basicity is probably due to the changed hybridization of the nitrogen atom in ammonia the lone electron pair is in an sp3-orbital, but in pyridine it is in an s/r-orbital. The higher the s character of an orbital, the more it is concentrated near the nucleus, and the less available for bond formation. Nitriles, where the lone electron pair is in an. vp-orbital, are of even lower basicity. 

Much work has been done on the quantitative correlation of the basicity of pyridines with Hammett substituent constants. The best single parameter correlation for 4-substituents is with ap... 

It is concluded from the foregoing considerations that pyridine may successfully be applied as a specific poison, provided the possible pitfalls are carefully kept in mind. The lower basicity of pyridine as compared to ammonia renders its chemisorption more selective. However, its basicity is in most cases still much higher than that of the commonly used reactants, so that one is usually able to determine an upper limit for the number of active sites by pyridine poisoning (239). On the other hand, the hardness of reactants or reaction products may be comparable with that of pyridine [e.g., dehydration of alcohols (47)] the poison will then be partially displaced. The molecular size of pyridine may bring about difficulties, since it restricts the accessibility of pyridine to narrow pores or even the approach to an adsorption site (214). In favorable cases, however, steric effects may be utilized to improve the specificity of poisoning (35, 36,241). 

Effects of substituents on the basicity of pyridine. The pATa values of representative substituted pyridines as compared with pyridine itself are shown in Table 1. Substituent effects are in line with the discussion in Section... 

The small basicity of pyridine cannot be explained in such a simple way the same resonance is possible in the base and in... 

As shown in Table 24.1, pyridine (pK = 5.25) is a stronger base than pyrrol hut a weaker base than alkylamines. The diminished basicity of pyridine com pared with an alkylaminc is due to the fact that the lone-pair electrons on tin pyridine nitrogen are in an sp orbital, while those on an alkylamine n trogei are in an sp- orbital. Because s orbitals have their maximum electron density a the nucleus but p orbitals have a node at the nucleus, electrons in an orbital wit) more. s character are held more closely to the positively charged nucleus and art less available for bonding. As a result, the hybridized nitrogen atom V character) in pyridine is less basic than the. s/Ahybridized nitrogen in an alkylamine (25% s character). 

The isolation of a mixture of THF and pyridine complexes in this case reflects the stronger Lewis basicity of pyridine versus a-picoline. 

Electron-releasing substituents generally increase the basic strength 2-methyl- (pK 5.97), 3-methyl (5.68) and 4-methylpyridine (6.02) illustrate this. The basicities of pyridines carrying groups that can interact mesomerically as well as inductively vary in more complex ways, for example 2-methoxypyridine (3.3) is a weaker, but 4-methoxypyridine (6.6) a stronger base than pyridine the effect of inductive with-... 

Most unsaturated ring quats of industrial importance are alkylpyridinium halides (I) and substituted imidazolinium compounds (11). To prepare alkylpyridinium halide, pyridine is reacted with the equimolar amounts of alkyl halide for several hours at 100-150 °C without solvent according the reactions given above. The reaction is two orders slower in this case because the basicity of pyridine is much weaker in comparison with aliphatic tertiary amines and heterocyclic aliphatic amines, such as N-alkylpiperidine and N-alkylmorpholine, too. Typical examples of the aromatic quaternaries are given below ... 

C.L. Perrin and P. Karri, Position-specific secondary deuterium isotope effects on basicity of pyridine, J. Am. Chem. Soc. 132 (2010), pp. 12145-12149. 

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