Coal, distillation Colours

The recovery and utilisation of these tar products in the manner indicated is a great achievement, The first known coal tar colour mauveine was made from aniline by Perkin in 1856, aniline having been discovered by Unverdorben, thirty years earlier, by distilling indigo. 

Aromatic hydrocarbon resins. The polymerization procedure and variables in the reactions of the aromatic hydrocarbon resins are similar to those for the coumarone-indene resins. However, the Cg feedstreams used in the polymerization of the aromatic hydrocarbon resins do not contain significant amounts of phenols or pyridine bases, so they are submitted directly to fractional distillation. Distillation produced more byproducts than light coal-tar oils. The aromatic hydrocarbon resins obtained have softening points between liquid and 125°C and Gardner colour of 6 to 11. By changing distillation conditions, aromatic hydrocarbon resins with softening points between 65 and 170°C and Gardner colour of 5 to 10 can also be obtained. 

Pure Commercial Benzene, obtained from coal-tai naphtha, should distil w lthin one degiee (80—Si ), and solidify completely when cooled to 0°. Other tests are as follow s shaken with concentrated sulphuric acid for a few minutes, the acid should not darken, and a drop of bromine water should not be immediately decolourised. A single distillation over a few small pieces of sodium, which absorb any traces of water, is usually a sufficient purification. If the benzene impart a brown or black colour to the sulphuric acid, it must be repeatedly shaken with about 20 per cent, of the acid until the lattev becomes only slightly yellow on standing. This is done in a stoppered separating funnel, and after shaking fora few minutes the mixture is allow ed to settle, and the low er layer of acid diawn off. The benzene is then shaken tw o 01 three times with water to free it from acid, carefully separated from the aqueous layer, and left in contact with fused calcium chloride until the liquid becomes clear. It is then decanted, frozen in ice, and any liquid (carbon bisulphide, paraffins) carefully drained off, and die benzene finally distilled over sodium. 

The honour of preparing the first colouring-matters from products of dry distillation belongs to von Reichenbach (pittacal from wood-tar, 1832), and to Runge (rosolic acid from coal-tar, 1834) [1]. 

Hofmann found only small amounts of aniline in coal-tar, but when he found benzene in the light oil distilled from the tar he prepared nitrobenzene and reduced this to aniline with nascent hydrogen from zinc and dilute acid, using as a test for benzene its conversion into nitrobenzene, the reduction of this to aniline, and the formation of a blue or purple colour with this and bleaching powder (see p. 184). Bechamp found that nitrobenzene is reduced to aniline by ferrous acetate but not by other ferrous salts, and also by iron filings and acetic acid. This was the foundation of the aniline dye industry, iron and hydrochloric acid being generally used. 

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