THIOPHEN, C4H4S, a compound occurring in small quantities in crude coal-tar benzene, from which it was first isolated in 1883 by V. Meyer (Ber., 1883, 16, p. 1465). The method adopted by Meyer to recover the thiophen was as follows. Ten volumes of the purest coal-tar benzene were shaken for four hours with one volume of sulphuric acid, the acid layer was removed and neutralized with lead carbonate, and the lead thiophen sulphonate obtained was distilled with an equivalent quantity of ammonium chloride. The distillate obtained was diluted with one hundred volumes of ligroin (previously purified by shaking with fuming sulphuric acid) and then shaken for one or two hours with sulphuric acid (using ten volumes of acid to one volume of the distillate), the acid layer diluted with water, neutralized by lead carbonate and the lead salt again distilled with an equivalent quantity of ammonium chloride. The distillate is finally rectified. It may be obtained in small quantity by passing ethylene or acetylene into boiling sulphur; by passing ethyl sulphide through a red-hot tube; by heating crotonic acid, butyric acid or erythrite with phosphorus pentasulphide; by heating succinic anhydride with phosphorus pentasulphide or sodium succinate with phosphorus trisulphide (J. Volhard and H. Erdmann, Ber., 1885, 18, p. 454); or by heating succindialdehyde with two parts of phosphorus trisulphide (C. Harries, Ber., 1901, 34, p. 1496).
It is a colourless liquid having a faint smell resembling that of benzene and boiling at 84° C. In its chief properties it very much resembles benzene, being readily brominated, sulphonated, and nitrated; also, the side chains in the alkyl thiophens are readily oxidized to carboxyl groups. On passing its vapour through a red-hot tube it yields di-thiënyl, C8H6S2. It is completely decomposed by hydriodic acid at 140° C. It condenses with aldehydes (in chloroform solution) in the presence of phosphorus pentoxide to give dithienyl hydrocarbons (A. Nahke, Ber., 1897, 30, p. 2037). It can be readily recognized by the blue colour produced when a trace of thiophen is added to isatin dissolved in concentrated sulphuric acid (the indophenin reaction). The thiophen ketones may be prepared by the interaction of thiophen and its homologues with acid chlorides in the presence of anhydrous aluminium chloride. The thiophen homologues are best prepared by heating the 1·4 diketones with phosphorus pentasulphide, the diketones reacting in the enolic form:
R · C 
|
CH–CH |  C·RP2S5 → R · C
|
CH·CH | C·R.
|
| OHHO | —S— |
Thiolenol, or oxymethyl thiophene, is prepared by heating laevulinic acid with phosphorus pentasulphide (W. Kues and C. Paal, Ber., 1886, 19, p. 555). On this group see also V. Meyer, Die Thiophengruppe.