Many benzene derivatives occur in nature, e.g. oil of bitter almonds, benzoic acid, salicylic acid, and hippuric acid, while others are obtained from the destructive distillation of organic substances, especially of coal.
The destructive distillation of coal yields (a) gases (illuminating gas); (b) an aqueous distillate containing ammonia and its salts, (c) coal-tar; and (d) coke. Coal-tar is the chief source of benzene derivatives, and is formed in the manufacture of coal-gas for illuminating, purposes, and in "coke ovens" used for the production of high-grade coke for metallurgical purposes. In both cases coal is distilled from closed retorts at relatively high temperatures, about 1000' C, and the main difference between the two processes is the nature of the coal used. For gas, making a bituminous coal containing 32-40 per cent of volatile matter is used, and in order to obtain the maximum yield of hard coke bituminous coals containing from 18-32 per cent of volatile matter are employed.
The tar from the two processes is much the same. At the present time numerous low-grade coals, e.g. cannel coal, lignite or brown coal, and even bituminous shales, are distilled at comparatively low temperatures (500-600' C.) in order to obtain oils, and, in the case of cannel coals, smokeless fuel for household purposes, coalite. The tar produced in all these cases is essentially different from the coal-tar obtained from gasworks and coke ovens. It consists largely of paraffin hydrocarbons, and is valueless for the manufacture of dyestuffs, explosives, etc., but yields valuable illuminating and fuel oils.
When coal-gas was first generally used for illuminating purposes (1813) the tar was regarded as a waste product, and could only be used as fuel, and its value as the source from which important synthetic dyes, perfumes, explosives, medicinal drugs, and photographic developers could be manufactured was only gradually recognized. For many years after the introduction of coke ovens for the manufacture of metallurgical coke, the ammonia and tar formed at the same time were not collected (so-called bee-hive ovens), but at the present time the great majority of the ovens are of the closed type, and are provided with by-product recovery plant. Still more recently, as the demand for benzene and toluene has increased, it has become customary to recover the benzene and toluene contained in the gas from the coke ovens, and even from the gas from gasworks, although this removal appreciably diminishes the illuminating power of the gas. The benzene hydrocarbons are usually removed by passing the gas through scrubbers containing creosote oil, which absorbs the hydrocarbons, and these can be afterwards isolated by heating the creosote oil or subjecting it to steam distillation. The amount of benzene and toluene in coal-gas is, roughly, About 15 times as much as that contained in the tar formed at the same time. In coke-oven gases the amount is only about half this. By this method of extracting benzene and toluene from the gases the amounts of these materials for the manufacture of explosives etc. has been increased enormously.
The following figures will give some idea of the importance of the coal-tar industry: In 1914, in Great Britain, about 14.5 million tons of coal were coked in by-product coking plants and in the USA about 20 million tons were treated in recovery coke ovens, and about 20 million tons in gasworks. In the USA the output of crude benzol was about 14.5 million gallons in 1914 and this was increased to 40 million gallons in 1917. Coal-tar contains as many as 200 different chemical substances these are not present in the coal itself, but are formed during the distillation. During the past thirty years Investigators have attempted to isolate compounds from coal itself by extraction with solvents, such as chloroform (Keinsch, 1910), pyridine (Bedson, 1908), benzene (Pictel and Ram, 1911), but so far few relationships have been established between the, different materials present in coal and the chemical compounds present in tar (cf. Tideswell and Wheeler, J. C. S. 1919, 115, 619). The most important compounds present in coal-tar are benzene, toluene, xylenes, phenol, cresols, naphthalene, and anthracene. Among the other compounds present are homologues of benzene, especially the methyl homologues; complex hydrocarbons, such as diphenyl, phenanthrene, fluorene, acenaphthene, chrysene and retene, indene and its homologues, and homologues of naphthalene; thiophene, aniline, pyridine and its homologues; quinoline, euorene, quinoline, pyrrole, indole, carbazole, and acridine. Most these are of little commercial importance, as the amounts present are small and their isolation from the tar is difficult. Many of the hydrocarbons present in the tar are probably formed by the pyrogenic polymerization of acetylene, as this hydrocarbon when heated yields many of the products present coal-tar (R. Meyer and H. Frieke, B. 1914, 47, 2765). The crude tar contains appreciable amounts of water, and as to be dehydrated before it can be distilled. Numerous methods are adopted, e.g. centrifuging the warm tar; heating the tar, allowing the water to rise to the surface, and removing it by a draw-off cock, or allowing the wet tar to come in contact with the hot vapour from another lot of boiling dehydrated tar. The actual distillation is carried out in iron stills directly fire-heated. In many tar distilleries continuous stills are employed; in others intermittent distillation is used, the pitch being removed from time to time and a fresh charge of tar introduced. The fractions collected vary in different distilleries, but, as a rule, in the first distillation the following are collected: (1) First runnings up to 105’ or 110'; this contains water, ammonia, and some light oil (2) Light oils up to 210' (3) Middle oil or carbolic oil up to 240' (4) Creosote oil up to 270' (5) Anthracene oil above 270' (6) Residue in the still=pitch. The relative amounts of the different fractions vary considerably in different countries and different districts, but the following are fairly typical values for 1 ton of tar -Light oils, 12 gall; carbolic oil, 20 gal; creosote oil, 17 gal; anthracene oil, 38 gal; and pitch, 11 cwt. Calculated on 1 ton of tar, the yields of important products are:-Benzene and toluene, 25 lb., or 1.1 per cent; phenol, 11 lb., or 0.5 per cent; cresols, 50 lb., or 2.2 per cent; naphthalene, 180 lb., or 8 per cent; creosote, 200 lb., or 8.8 per cent; and anthracene, 6 lb., or 0.27 per cent.
The light oils, including those from the first runnings, give rise to 60-65 per cent of benzene hydrocarbons, 12-15 per cent of naphthalene, 8-10 per cent of phenols, and 1-3 per cent of pyridine bases. The phenols are readily removed by treatment with caustic soda solution, and bv treatment with dilute mineral acids.the pyridine bases The neutral substances, on further fractionation under varying conditions, yield 90 per cent benzol, 50 per cent benzol, 30 per cent benzol, and solvent naphtha. The numbers 90, 50, and 30 denote the percentage of the oil which passes over below 100' C., and not the actual benzene content of the oil. 90 per cent benzol contains 81 per cent of benzene, 15 per cent of toluene, 2 per cent of xylenes, and 2 per cent of impurities; and 30 per cent benzol contains respectively 13.5, 73.4, 11.7, and 11.7 per cent. From these crude benzols, by careful fractionation, pure benzene, toluene, and xylenes can be isolated.
In addition to the compounds, such as benzene, toluene, naphthalene, phenol, and anthrace, which are actually isolated and form important articles of commerce, a number of products consisting of complex mixtures are also manufactured. The most important of these are (1) solvent naphtha, which is used as a solvent for rubber in preparing waterproof fabrics and also for burning purposes, and (2) creosote oil, which is used in enormous quantities for pickling timber for use as railway sleepers, posts, and other purposes. For hydrogenation of coal cf. Bergius, Chem. Age, 1927, 134.
Many methane derivatives, e.g. alcohol, yield a mixture containing a large number of the derivatives of benzene when their vapours are led through red-hot tubes. Acetylene, C2H2, polymerizes at a low red heat to benzene, C6H6, (Berthelot)
Benzene is formed when benzoic acid is distilled with soda-lime: C6H5COOH = C6H6 + CO2
