C5H5N = 79.05.
Pyridine, C5H5N, is a base which occurs with its homologues in the products resulting from the dry distillation of nitrogenous organic matter (Dippel's oil), also in tobacco smoke, coal tar naphtha, etc. From bone oil it is prepared by treatment with sulphuric acid, separation of the sulphuric compounds, and decomposition with sodium hydroxide. The mixture of bases set free (pyridine, aniline, etc.) is carefully fractionated, the distillate treated with acid oxidising agents which attack only the aniline bases, neutralised with sodium hydroxide, and again fractionated. The pyridine may be purified by treating the crude bases dissolved in to per cent. solution of hydrochloric acid with mercuric chloride dissolved in hot water. The crystalline double salt of pyridine with mercuric chloride thus formed is distilled with sodium hydroxide. The base may be prepared synthetically by the action of phosphorus pentoxide on amyl nitrite, and also by hearing piperidine with sulphuric acid at 300°. It is of special interest because of having been recognised as one of the parent substances of many alkaloids. Pyridine occurs as a colourless, limpid, volatile liquid, having a powerful, persistent, empyreumatic odour, and causing a bitter taste in the mouth and back of the throat; it is hygroscopic and difficult to obtain absolutely dry. The aqueous solution has a strong alkaline reaction, and is precipitated by the fixed alkali hydroxides; it does not affect phenolphthalein. Specific gravity, 0.980. Boiling-point, 117°. With water it unites to form a hydrate, C5H5N, 3H2O, Which boils at 93°. Heated with hydriodic acid to 300° it yields normal pentane, C5H12. It combines with acids to form addition salts which are crystalline, hygroscopic, and readily soluble in water. As in the case of ammonia, it gives rise to white fumes in contact with hydrochloric acid vapour. With copper chloride it gives a precipitate soluble in excess of the base; the solution is blue, and leaves on evaporation crystals of a double salt of the same colour. It precipitates ferric, chromium, and aluminium hydroxides from solutions of the salts of these metals; and has the property of forming crystalline double salts with the metals, especially with the chlorides of zinc, cadmium, copper, and mercury. A half per cent. solution should give with an equal volume of saturated solution of picric acid a crystalline precipitate, becoming almost solid. Nascent hydrogen converts it into piperidine. It is not attacked by boiling nitric acid or chromic acid, being highly resistant to oxidation. It should remain unaltered on exposure to the air (absence of furfurol), and should not be affected by phenolphthalein (absence of ammonia). If 5 mils of a 60 per cent. aqueous solution be mixed with 1 decimil (0.1 milliliters) of a 1 in 1000 solution of potassium permanganate, the red colouration should not disappear within an hour (absence of readily oxidisable impurities). One mil dissolved in 20 mils of water should require for neutralisation 12.4 mils of normal hydrochloric acid solution, using tincture of cochineal as indicator.
Soluble in all proportions of water, alcohol, ether, chloroform, benzene, fatty oils, etc.
Action and Uses.—Pyridine and similar compounds are amongst the products of the dry distillation or slow combustion of most leaves, and to this substance is ascribed in part the beneficial action of the various burning-powders and cigarettes used in asthma (see Pulvis Stramonii Compositus). Pure pyridine is employed for a similar purpose; 2 to 4 mils (30 to 60 minims) is placed on a flat dish in a small room, and the vapour inhaled for fifteen or twenty minutes. It is also employed, diluted With 20 to 40 parts of liquid paraffin, in a nebulising apparatus. It apparently acts as a depressant to the nerve-endings in the bronchioles. It is much less toxic than either collidine or nicotine, which are given off along with pyridine from burning tobacco. Pyridine has been given internally in angina pectoris and cardiac dyspnoea, but without much benefit.
Dose.—3 to 6 decimils (0.3 to 0.6 milliliters) (5 to 10 minims).