BENZENE, C6H6, 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. 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 in 1833, but in the following year Liebig proposed “benzol” (the termination ol 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 used in Germany, and “benzole” in France.
Benzene is manufactured from the low-boiling fractions of the coal-tar distillate (see Coal-Tar). The first successful fractionation of coal-tar naphtha was devised by C. B. Mansfield (1819-1855), who separated a benzol distilling below 100° from a less volatile naphtha by using a simple dephlegmator. At 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 required, 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 100°.) 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 principally of benzene, toluene and the xylenes, yields on fractionation (1) various volatile impurities such as carbon disulphide, (2) the benzene fraction boiling at about 80° C., (3) the toluene fraction boiling at 100°, (4) the xylene fraction boiling at 140°. The fractions are agitated with strong sulphuric acid, and then washed with a caustic soda solution. The washed products are then refractionated. The toluene fraction requires a more thorough washing with sulphuric acid in order to eliminate the thiotolene, which is sulphonated much less readily than thiophene.
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 perfectly 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 0.899 at 0° C. It is very slightly soluble in water, more soluble in alcohol, and completely miscible with ether, acetic acid and carbon disulphide. It is an excellent solvent for gums, resins, fats, &c.; sulphur, phosphorus and iodine also dissolve in it. It sometimes separates with crystals of a solute as “benzene of crystallization,” as for example with triphenylmethane, thio-p-tolyl urea, tropine, &c.
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 compounds styled the “aromatic” or “benzenoid” compounds. The constitution of the benzene ring, the isomerism of its derivatives, and their syntheses from aliphatic or open-chain compounds, are treated in the article Chemistry. A summary of its chemical transformations may be given here, and reference should be made to the articles on the separate compounds for further details.
Passed through a red-hot tube, benzene vapour yields hydrogen, 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 is very stable to oxidants, in fact resistance to oxidation is a strong characteristic of the benzene ring. Manganese dioxide and sulphuric acid oxidize it to benzoic and o-phthalic acid; potassium chlorate and sulphuric acid breaks the ring; and ozone oxidizes it to the highly explosive white solid named ozo-benzene, C6H6O6. Hydriodic acid reduces it to hexamethylene (cyclo-hexane or hexa-hydro-benzene); chlorine and bromine form substitution and addition products, but the action is slow unless some carrier such as iodine, molybdenum chloride or ferric chloride for chlorine, and aluminium bromide for bromine, be present. It is readily nitrated to nitrobenzene, two, and even three nitro groups being introduced if some dehydrator such as concentrated sulphuric acid be present. Sulphuric acid gives a benzene sulphonic acid.