Basicity pyridine

Problem 20.18 (a) Account for the aromaticity of pyridine, a planar structure with 120° bond angles. (f>) Is pyridine basic Explain, (c) Explain why piperidine (azacyclohexane) is more basic than pyridine, (d) Write the equation for the reaction of pyridine with HCl. 

Ife RJ, Dyke CA, Keeling DJ, Meenan E, Meeson ML, Parsons ME, et al. 2-[[(4-Amino-2-pyridyl)methyl]sulfinyl]-benzimidazole H+/K+-ATPase inhibitors. The relationship between pyridine basicity, stability, and activity. /. Med. Chem., 1989, 32(8), 1970-1977. 

This reaction was initially reported by Diels and Reese in 1934. It is the conjugate addition between hydroazobenzene and dimethyl acetylene-dicarboxylate. The resulting adduct can be transformed into three different heterocyclic compounds under various experimental conditions (i.e pyrazolones with acid, indoles upon heating in xylene, and quinolones with base ). For example, l,2-diphenyl-3-carbomethoxy-5-pyrazolone will be generated from the adduct in acetic acid (acidic condition), whereas dimethyl indole-2,3-dicarboxylate is produced in xylene (neutral condition) and 2-hydroxy-3-anilino-4-carbomethoxy-quinoline is yielded in pyridine (basic condition). The latter can be further converted into 2,3-dihydroquinoline upon decarboxylation and hydrolysis." This reaction has been extended to heat the 1 1 adduct in picoline. ... 

Pyridine is considered as one of the commonest heterocycles among nitrogen-containing heterocychc compounds. Substituent effects in meta-and para-X-substituted pyridines (with X = OMe and Me) on their basicity have been studied in water and ionic hquid (IL, l-buthyl-3-methyl-imidazolium hexafluorophosphate) (2010JOC(75)3912). The equibbrimn constants were evaluated by spectrophotometric titration of pyridine solution with trifluoroacetic acid. The results of the Hammett approach (Eqn (1)), applied to aU studied systems, allow one to state that pyridine basicity is almost twice as less sensitive to the substituent effect in IL than in water (Figure 2). 

The pyridinolysis rates of carbonyi compounds with good leaving groups usually exhibit a biphasic dependence on pyridine basicity. This has now been shown to be the case for the pyridinolysis of thiophenyl 4-nitrobenzoates in MeCN. The reactions between X-pyridines (118 X = 4-MeO, 4-Me, 3-Me, 3-CONH2, 3-MeCO,... 

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