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 .