PYRIDINE AND ITS DERIVATIVES 307 



PYRIDINE AND ITS DERIVATIVES. 



The six-membered heterocyclic ring compounds, containing 5 

 atoms of carbon and I atom of nitrogen, of which pyridine is the 

 simplest member and from which all the other compounds of the group 

 can be derived, resemble the benzene compounds very closely. The 

 simpler members are present in coal tar and bone oil and are formed 

 by the oxidation of the complex alkaloids which occur in plants. 



The Structure of Pyridine. 



The empirical formula of pyridine, C 5 H 5 N, points to its not being an open 

 chain compound. For reasons similar to those which led to the adoption of 

 a closed ring structure for the constitution of benzene and from the great 

 similarity which pyridine has to benzene in its reactions the following ring, 

 structure has been assigned to pyridine : 



N 



This structure shows that pyridine is a tertiary base, that three isomeric 

 monosubstitution and six disubstitution derivatives can be derived from it ; 

 in general it expresses all the facts known about pyridine and its derivatives, 



Pyridine. 



Pyridine was first obtained from bone oil, but is contained in coal 

 tar from which it is usually prepared 



Preparation. 



The acid solution, or liquor, which results in the purification of benzene 

 and its homologues from coal tar is treated with sodium hydroxide; the 

 basic substances separate out as oils. This oil consists of a mixture of 

 pyridine, its homologues, quinoline and other substances. These constituents 

 can be separated- by repeated fractional distillation, but the pure compounds 

 are finally isolated by the fractional crystallisation of their salts. The bases 

 are liberated from the salt by alkali and purified by distillation. 



Bone oil is extracted with sulphuric acid ; the bases are separated by 

 sodium hydroxide, distilled and purified as described above. 



Properties. 



Pyridine is a colourless liquid which boils at 1 1 5 and has a specific 

 gravity of I -003 at o. It has a pungent characteristic and disagree- 

 able odour and mixes with water in all proportions. 



It is a strong base, which can turn red litmus blue, and forms salts 

 with acids. 



Pyridine is a tertiary amine as shown by its negative behaviour 

 to nitrous acid and the fact that it combines with alkyl halides to 

 form pyridine alkyl halides, such as pyridine methiodide C 5 H 5 N . CH 3 L 



On heating, the methyl group attached to the N atom changes its 

 position and a-alkyl pyridines are formed. 



