Benzene

Benzene, also called Benzol, a hydro-carbon. The name is derived from the gum benzoin (q.v.). It is contained in coal-tar, which forms the chief source of all the benzene compounds. When coal is distilled for the production of illuminating gas, tar and ammoniacal liquors are also obtained. The coal-tar contains a large number of solid and liquid substances, amongst which are benzene and certain of its derivatives. This tar is then distilled. The portion of the distillate which comes over below 160° is known as Light Oil, the part distilling over between 160° and 250° is known as Intermediate Oil, and the distillate above 250° is called Heavy Oil. The light oil consists chiefly of benzene and some derived products. It is washed first with caustic soda, and then with sulphuric acid in order to remove certain acid and basic substances, viz. phenol and pyridine. It is then distilled in a suitable form of apparatus, and the part distilling over first consists of benzene. Benzene thus obtained is a colourless liquid which boils at 80-5°, and has a sp. gr. of .899. It has a peculiar odour, and the vapour when inhaled produces giddiness. It burns with a bright flame. It is very extensively used for the manufacture of the aniline colours, and as a solvent for many organic compounds. It is also of very great theoretical importance, as it is the starting-point of an exceedingly large number of compounds known as the benzene derivatives or the aromatic compounds. On this account its constitution has been, at different times, the source of much speculation, and it is now generally accepted that the carbon atoms are all arranged in the form of a closed chain, each being united with one hydrogen atom and two other carbon atoms. By replacement of one or more hydrogen atoms by other elements or radicals a large number of derivatives can be obtained.