BENZENE, a hydrocarbon discovered ' in 1825 by Faraday in the liquid produced in the compression of the illuminating gas obtained by distilling certain oils and fats. Its formula is C6H6. E. Mitscherlich prepared it in 1834 by distilling benzoic acid with lime; and in 1845 Hofmann discovered it in coal-tar. It was named "benzin" or "benzine" by Mitscherlich, but J. v. Lie big proposed "benzol" (the termination of being suggested by the Lat. oleum, oil) ; the form "benzene" was due to A. W. Hofmann. The word "benzine" is sometimes used in commerce for the coal tar product, but also for the light petroleum better known as petroleum-benzine; a similar ambiguity is presented by the word "benzoline," which is applied to the same substances as the word "benzine." "Benzene" is the term used by English chemists, "benzol" is favoured in Germany, and "benzole" in France. There is increasing adoption in England of "benzole" for the name of the fraction in which the hydrocarbon benzene is the chief con stituent.
Benzene is manufactured from the low-boiling fractions of the coal-tar distillate (see COAL-TAR). The first successful fraction ations of coal-tar naphtha were devised by C. B. Mansfield who separated a benzol distilling below Ioo° C. from a less vola tile naphtha by using simple dephlegmators. First, the oil was manufactured principally for combustion in the Read Holliday lamp and for dissolving rubber, but the development of the coal tar colour industry occasioned a demand for benzols of definite purity. In the earlier stages 30%, 50% and 90% benzols were re quired, the 30% being mainly used for the manufacture of "aniline for red," and the 90% for "aniline for blue." (The term "30% benzol" means that 30% by volume distils below zoo°.) A purer benzol was subsequently required for the manufacture of aniline black and other dye-stuffs. The process originally suggested by Mansfield is generally followed, the success of the operation being principally conditioned by the efficiency of the dephlegmator, in which various improvements have been made. The light oil fraction of the coal-tar distillate, which comes over below 140° and consists chiefly of benzene, toluene and the xylenes (q.v.) yields on fractionation (I) various volatile impurities such as carbon disulphide, (2) the benzene fraction boiling at about 80° C., (3) the toluene fraction boiling at zoo°, (4) the xylene fraction boiling at 140°. The fractions are agitated with strong sulphuric acid to separate bases and thiophenes, and then washed with caustic soda solution to remove phenolic substances. The washed products are then refractionated.
Benzene is a colourless, limpid, highly refracting liquid, having a pleasing and characteristic odour. It may be solidified to rhombic crystals which melt at 5.4° C. Mansfield obtained per fectly pure benzene by freezing a carefully fractionated sample.
It boils at 80.4°, and the vapour is highly inflammable, the flame being extremely smoky. Its specific gravity is o•899 at o° C. It is very slightly soluble in water, more soluble in alcohol and com pletely miscible with ether, acetic acid and carbon disulphide. It is an excellent solvent for gums, resins, fats and many other organic materials; sulphur, phosphorus and iodine also dissolve in it. It sometimes separates with crystals of a solute as "ben zene of crystallization," as for example with triphenylmethane, thio-p-tolyl urea, tropine, etc.
Benzene is of exceptional importance commercially on account of the many compounds derivable from it, which are exceedingly valuable in the arts. Chemically it is one of the most interesting substances known, since it is the parent of the enormous number of substances belonging to the "aromatic" or "benzenoid" series. The constitution of the benzene ring, the isomerism of its deriva tives, and their syntheses from aliphatic or open-chain compounds, are treated in the article CHEMISTRY : Organic, HOMOCYCLIC. A summary of its chemical transformations may be given here, but reference should be made to the articles on the separate compounds for further details.
Passed through a red-hot tube, benzene vapour yields hydro gen, diphenyl, diphenylbenzenes and acetylene ; the formation of the last compound is an instance of a reversible reaction, since Berthelot found that acetylene passed through a red-hot tube gave some benzene. Benzene unites with ozone to form the triozonide, which is decomposed by water with the production of glyoxal and hydrogen peroxide. When its vapour mixed with air is passed over vanadium pentoxide benzene is oxidized to p-benzo quinone and maleic anhydride. Hexamethylene (cyclo-hexane or hexahydrobenzene) is produced on passing hydrogen and benzene vapours over reduced nickel at 180°. The reverse change to benzene and hydrogen occurs when hexamethylene is passed over nickel at 280°. Chlorine and bromine form additional products with moist benzene in sunlight. Substitution products are formed with the halogens, but only slowly unless a catalyst, such as iodine, molybdenum chloride or ferric chloride for chlorine, and aluminium bromide for bromine, be present. Benzene is readily nitrated to nitro-benzene, two and even three nitro groups being introduced if some dehydrator such as concentrated sulphuric acid be present. Sulphuric acid gives with benzene mono-, di- and even tri-sulphonic acids.