pyridine (pĭrˈĭdēn) [key] or azine ăzˈēn, 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.