pyridine or azine, C₅H₅N, colorless, flammable, toxic liquid with a putrid odor. It melts at -42°C; and boils at 115.5°C;. Chemically, it is a heterocyclic aromatic tertiary amine (see under amino group). Its molecule resembles that of benzene, one carbon-hydrogen unit in the benzene ring being replaced with a nitrogen molecule. It is miscible with water and with most organic solvents. Its aqueous solution is slightly alkaline. Pyridine is used as a solvent, as a denaturant for alcohol, and as a starting material in the synthesis of other compounds. Compounds that can be derived from pyridine include antihistamines and vitamins. Pyridine is obtained from bone oil or from coal tar by destructive distillation, which decomposes alkaloids that contain it. Alkaloids that contain pyridine include coniine, piperine (the alkaloid in pepper), and nicotine (present in tobacco); free pyridine is present in tobacco smoke.

Any of a class of aromatic compounds with a six-member aromatic ring composed of five carbon atoms and one nitrogen atom, making it a heterocyclic compound. The simplest one is pyridine itself (C₅H₅N). Natural compounds with pyridine rings include niacin and pyridoxine (see vitamin B complex), the tuberculosis drug isoniazid, and several plant products (e.g., nicotine). Pyridine is used as a raw material for various drugs, vitamins, and fungicides and as a solvent. Because it has a nauseating odour and a burning taste, it is added to ethanol and antifreezes to make them undrinkable (see denaturation).

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