j3-Pyridinesulphonic acid, is formed when pyri dine is treated with fuming sulphuric acid. The sodium salt on distillation with potassium cyanide yields 0-cyanopyridine, which on hydrolysis gives nicotinic acid. On fusing the sulphonic acid with potash, /3-hydroxypyridine is formed.
Nitration of the pyridine nucleus is only possible when amino-, hydroxy- or similar groups are present. Aminopyridines are ob tainable by the action of ammonia upon the corresponding chloro pyridine. The a-amino- and aa'-diamino-derivatives are formed by the action of sodamide on pyridine and subsequent treatment with water. The aminopyridines can be diazotised in the same way as aniline and can form azo-dyes.
Hydroxypyridines correspond to aminophenols in that they form salts with strong bases and acids. They are feebly basic in character and are readily soluble in water. They also exhibit keto-enol tautomerism (q.v.), derivatives of keto-dihydropyridine, or pyridone, being obtainable.
The carboxylic acids of pyridine are usually prepared by oxidizing the homologues of the base with potassium perman ganate. Many of the alkaloids yield these acids upon energetic oxidation. When heated with strong hydrochloric acid the higher polycarboxylic acids give rise to the lower acids by loss of carbon dioxide. By heating with lime all the carboxyl groups are eliminated and pyridine is formed. The most important pyridine carboxylic acids are described in the following table : The resemblance in chemical behaviour found between the de rivatives of benzene and pyridine indicates that these two sub stances must have a similar ring constitution. The above formulae also allow for the existence of all the known isomeric substitution products. Thus, when substituent groups are identical, six di derivatives, six tri-, three tetra-, and one penta-derivative have been isolated. The five hydrogen atoms which can be replaced by a substituting group are represented by the Greek letters a, 0, y, Homologues.—The homologues of pyridine constitute that fraction of the crude bases from bone or coal-tar oil boiling be tween 120-230° C. They can be synthesized in various ways.
One of the most important is A. Hantzsch's "collidine" synthesis, which consists in the condensation of two molecules of acetoacetic ester with one of aldehyde and one of ammonia: The resulting dihydro-compound is oxidized with nitrous acid, the ester hydrolysed, and the acid heated with lime, when the tri methylpyridine is obtained. The a- and 7-alkyl-pyridines are also produced by heating the pyridinium alkyl iodides, a molecular rearrangement taking place. These homologues are of importance because of their relation to the alkaloids. A simple alkyl-pyridine is often the main product of distillation of an alkaloid with zinc dust or lime. They are liquids of an unpleasant odour resembling pyridine. The chief alkyl-pyridines are summarized in the fol lowing table: tainable in large amount from animal and vegetable nucleins; it has also been synthesised by E. Fischer; it crystallises from water in colourless leaflets melting at 358-322° C.
Piperidine is often used as a catalyst in synthetic organic chemistry. It was one of the first organic accelerators used in the vulcanisation of both synthetic and natural rubber, and certain of its derivatives are still used for this purpose. The hydrogen tartrate and other salts have been employed in medicine for urinary calculi and as uric acid solvents, of doubtful efficacy.
(J. RD.)