D. Ginsburg
1967
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Abstract
This chapter discusses the effect of the lone pair on nitrogen on the basicity of heterocyclic amines. Piperidine is a cyclic secondary amine whose basicity constant is 2 x 10 −3 , comparable to that of a strongly basic aliphatic amine. The dehydrogenated analog of piperidine contains six less hydrogen atoms called pyridine. Pyridine may be protonated; it easily forms a methiodide or other quaternary salts. Its basicity constant is only 2.3 x 10 −9 , comparable to that of aromatic amines but much lower than that of aliphatic amines. Pyridine is an aromatic substance because of the sextet of electrons within the ring. It undergoes electrophilic as well as nucleophilic substitutions. The lone pair on nitrogen permits pyridine to behave as a nucleophile. In electrophilic reactions, pyridine behaves like a highly deactivated benzenoid compound. Vigorous experimental conditions are required for nitration, sulphonation, and halogenation. Electrophilic substitution occurs at the β -carbon atom in the ring, while the nucleophilic attack occurs at the α and γ-positions of the nucleus.