ABSTRACT
Pyridine and its simple derivatives are stable and relatively unreactive liquids, with strong penetrating odours that are unpleasant to some people. Processes for the manufacture of alkylpyridines involve reaction of acetylenes and nitriles over a cobalt catalyst. The reaction is believed to involve initial attack by sulfur at nitrogen, followed by nucleophilic addition of a second pyridine at C-4. Pyridines carrying an activating substituent at C-3 undergo electrophilic substitution at C-2; thus chlorination of 3-aminopyridine affords 3-amino-2-chloropyridine; nitration of 3-hydrox-ypyridine gives 3-hydroxy-2-nitropyridine. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]hexadienes and -hexenones are obtained; in the case of pyridines these are usually unstable and revert thermally to the aromatic heterocycle, but 2-alkylpyridines with an electron-withdrawing group on the alkyl substituent give stable products by base-catalysed proton shift. In reactions which involve bond formation using the lone pair of electrons on the ring nitrogen, such as protonation and quaternisation, pyridines behave just like tertiary aliphatic or aromatic amines.
