THIOPHEN, an organic sulphur compound present in small quantities in coal-tar benzene, from which it is separated only with considerable difficulty, owing to the close similarity of the physical and chemical properties of the two substances. Thiophen is a colourless liquid with the odour of benzene ; the former boils at 84° C, the latter at 80.5° C. Both readily undergo bromination, nitration and sulphonation. Coal-tar toluene is similarly accom panied by small amounts of a- and (3-methylthiophens (thiotolens) and the xylenes contain traces of dimethylthiophens (thioxens). In each case the sulphur compound simulates closely the boiling point and other properties of the hydrocarbon. Hence in their relationship to the coal-tar hydrocarbons, thiophen and its homo logues constitute a remarkable case of chemical "protective mimicry." Thiophen (formula I.) was first recognised in coal-tar benzene and isolated therefrom by V. Meyer (1883), who availed himself of the fact that thiophen is more readily sulphonated than benzene at the ordinary temperature. Benzene containing thiophen was shaken with cold, concentrated sulphuric acid, the impure thio phensulphonic acid converted into lead salt and the latter distilled with ammonium chloride. Impure thiophen was thus obtained
and the process was repeated until the product after rectification was pure. An alternative isolation due to 0. Dimroth (1899) con sists in boiling the benzene–thiophen mixture with mercuric ace tate; a mercury derivative of thiophen which separates is decom posed by hydrochloric acid, regenerating pure thiophen.
Thiophen is obtainable (I) by passing ethyl sulphide through a red hot tube, or (2) by heating succinic anhydride, succindialde hyde or metallic succinates with sulphides of phosphorus. It is readily recognized by the intense blue coloration developed when a trace of thiophen or coal-tar benzene is added to isatin in con centrated sulphuric acid (the indophenin reaction).