BENZENE. 253 



\ 

 FIRST GROUP. 



A. HYDROCARBONS, C 71 !! 2 *" 6 . 



In the preparation of coal-gas by the destructive dis- 

 tillation of coal, a secondary product is obtained in the 

 form of a tar (coal-tar), which, subjected to distillation, 

 yields a large number of bodies of various character. 

 At first an oil distills over, which is lighter than water 

 (light oil), and which consists mainly of benzene, toluene, 

 dimethylbenzene, and trimethylbenzene. At a later 

 stage of the process an oil passes over, which sinks 

 under water (creosote oil, dead oil). This contains par- 

 ticularly two alcoholic bodies, phenol and cresol ; and, 

 besides these, volatile bases, anilin, pyridine bases (p. 

 130), and several hydrocarbons, partially liquid, partially 

 solid. 



In order to prepare the hydrocarbons from the light 

 oil, this is first shaken successively with sulphuric acid, 

 alkalies, and water, for the purpose of removing foreign 

 substances, and then that portion of the oil, which 

 remains undissolved, separated into its constituents by 

 means of long-continued partial distillation. The 

 different isomeric modifications of dimethyl- and tri- 

 methylbenzene, however, cannot be separated from each 

 other in this manner. 



1. Benzene (Benzol). 

 C 6 H 6 . 



Preparation. That portion of the purified light oil that 

 boils at 80-85, congeals almost completely when cooled 

 down to 5 to 10. That which remains liquid is 

 poured off and the crystals pressed between layers of 

 filtering-paper below 0. Can be most readily prepared 

 in a pure condition by the distillation of an intimate 

 mixture of 1 part benzoic acid with 3 parts quicklime. 



It is also produced, together with other hydrocarbons 

 ,of higher boiling points, when acetylene (p. 131) is 

 heated to a temperature at which glass begins to soften. 

 Three molecules of acetylene combine to form one 

 molecule of benzene. 

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