Pyridine

PYRIDINE (Pyridin), a six-membered heterocyclic ring found in wood and Scotch oils, and in coal tar. It was discovered by Anderson in animal oil. The similarity of the pyridine ring to the benzene ring is manifested in its stability to high temperatures, oxidizing agents, and acids. The ring is easily cleaved in partially hydrogenated pyridine bodies; sulfonation, nitration, and bromination proceed much more difficultly than in benzene. Pyridine is a weak base and tertiary amine. By maximal hydrogenation, it is converted into the secondary amine piperidine. Pyridine is a transparent, colorless liquid with a sharp odor: boiling point 115.1°, d²⁰₄ = 0.978; soluble in water, hygroscopic, a good solvent for many organic and some inorganic substances (HgCl₂, AgNO₃, Pb(NO₃)₂); electrical conductivity at 25° is 0.96·10^-6. Ring cleavage with the formation of pentane and ammonia occurs only upon boiling at 300° with concentrated hydriodic acid. It gives a precipitate with copper salts; it forms double and complex salts, for example with platinum. It is obtained synthetically from acetylene and hydrocyanic acid at 800°–900°. Homologs of pyridine: picolines (methylpyridine), lutidines (dimethylpyridine), collidines (C₈H₁₁N), parvolines (C₉H₁₃N) are obtained from coal tar, by the dry distillation of animal oil, from cinchonine, brucine, strychnine, peat, and by the dry distillation of wood; they are formed alongside ptomaines during the putrefaction of protein substances. Many alkaloids are derivatives of pyridine or hydrogenated pyridine: nicotine (tobacco), atropine, hyoscyamine, cocaine, piperine (pepper), coniine, coniceine, conhydrine (in hemlock), arecaidine and arecoline (fruits of the areca palm), pelletierine and methylisopelletierine (pomegranate bark), ricinine (castor bean), trigonelline (many plants).

Pyridine is an occupational poison of the tar industry; the poisoning symptoms are similar to those of benzene poisoning. A mixture of pyridine and its homolog picoline is used for the denaturation of alcohol; workers dealing with denatured alcohol exhibit symptoms of chronic poisoning similar to those of nicotine poisoning (excitation followed by paralysis of the respiratory center). Pyridine is excreted in the urine in the form of methylpyridine chloride. It was sometimes used in asthma in the form of inhalation. It is part of Oleum animale foetidum. Pyridine bases are a common impurity in ammonia and amyl alcohol. Pyridine has found application in histological technique (see Bielschowsky's method) and partly in the practice of forensic medical examination of blood.