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Originally appearing in Volume V22, Page 690 of the 1911 Encyclopedia Britannica.
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PYRIDINE, C51-15N, an organic base, discovered by T. Anderson (Trans. Roy. Soc. Edin., 1851, 20, p. 251) in bone oil. It is also found among the distillation products of bituminous coal, lignite, and various shales, and has been detected in fusel oil and crude petroleum. It is a decomposition product of various alkaloids (nicotine, sparteine, cinchonine, &c.), being formed when they are strongly heated either alone or with zinc dust. It may be synthetically prepared by distilling allyl ethylamine over heated lead oxide (W. Konigs, Ber., 1879, 12, p. 2341) by passing a mixture of acetylene and hydrocyanic acid through a red-hot tube (W. Ramsay, Ber., 1877, 10, p. 736); by heating pyrrol with sodium methylate and methylene iodide to 2o0° C. (M. Dennstedt and J. Zimmermann, Ber., 1885, 18, p. 3316); by heating isoamyl nitrate with phosphorus pentoxide (E. T. Chapman and M. H. Smith, Ann., 1868, Sup p1. 6, p. 329); and by heating piperidine in acetic acid solution with silver acetate (J. Tafel, Ber., 1892, 25, p. 1619). The amount of pyridine produced in most of these processes is very small, and the best source for its preparation is the " light-oil " fraction of the coal-tar distillate. The basic constituents are removed by dilute sulphuric acid, the acid layer removed, and the bases liberated by alkali, separated, dried, and fractionally distilled. Pyridine is a colourless liquid of a distinctly unpleasant, penetrating odour. It boils at 114.5° C., and is miscible with water in all proportions. It is a tertiary base, and combines readily with the alkyl halides to form pyridinium salts. Nascent hydrogen reduces it to piperidine, C5H11N (see below), whilst hydriodic acid above 300° C. reduces it to n-pentane (A. W. Hofmann, Ber., 1883, 16, p. 59o). •It is a very stable compound, chromic and nitric acids being without action upon it, whilst the halogens only yield substitution derivatives with difficulty. It reacts with sulphuric acid only at high temperatures, yielding a sulphonic acid. It forms addition compounds with mercuric and auric chlorides. On the constitution of the pyridine nucleus, see Korner, Gior. dell' acad. di Palermo, 1869, and C. Riedel, Ber., 1883, 16, p. 1609. As regards the isomerism of the pyridine substitution products, three mono-derivatives are known, the different positions being indicated by the Greek letters a, /3 and y, as shown in the inset formula. This 4 e formula also allows of the existence of six di-deriva- tives, six tri-derivatives, three tetra- and one penta- derivative, when the substituent groups are identi-°` \N%a cal; all of which are in agreement with known facts. The three monochlorpyridines are known, the a and y compounds resulting from the action of phosphorus pentachloride on the corresponding oxypyridines, and the fl compound from the action of chloroform on potassium pyrrol. a-Aminopyridine, C5H4N•NH2, is formed by heating 5-aminopyridine-2-carboxylic acid. It is a crystalline solid which melts at 56° C. and boils at 204° C. It can only be diazotized in the presence of concentrated sulphuric acid, and even then the free diazonium sulphate is not stable, readily passing in the presence of water to a-oxypyridine. $-Aminopyridine is obtained by heating $-pyridyl urethane with fuming hydrochloric acid until no more carbon dioxide is liberated (T. Curtius and E. Mohr, Ber., 1898, 31, p. 2493), or by the action of bromine and caustic soda on the amide of nicotinic acid (F. Pollak, Monats., 1895, 16, p. 54). It melts at 64° C. and boils at 250-252° C. The aminopyridines are readily soluble in water, and resemble the aliphatic amines in their general chemical properties. The oxypyridines may be prepared by distilling the corresponding oxypyridine carboxylic acids with lime, or by fusing the pyridine carboxylic acids with caustic potash. The mono-oxypyridines are easily soluble in water and possess only feeble basic properties. The compound is hydroxylic in character, whilst the a and y derivatives behave frequently as if they possess the tautomeric ketostructure, yielding according to the conditions of the experiment either N- or 0-ethers (H. v. Pechmann, Ber., 1895, 28, p._1624), thus corresponding to the formulae CH CH C.OH CO HC/\CH HC" CH HC/\CH HC CH HC\ jC•OH HC\/CO HC'\ CH HC/I CH N NH N~ NH a-oxypyridine a-pyridone y-oxypyridine 7-pyridone The homologues of pyridine may be synthesized in various ways. One of the most important is the so-called " collidine " synthesis of A. Hantzsch (Ann., 1882, 215, p. I ; Ber., 1882, 15, p. 2914) which consists in the condensation of two molecules of aceto-acetic ester with one of an aldehyde and one of ammonia: R02C•CH2 R'•CHO CJ-12•CO2R _~ R02C•C•CHR'• C•CO2R
End of Article: PYRIDINE

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