Indophenine test

Isomeric thienothiophenes, selenophenoselenophenes, and seleno-phenothiophenes have an odor reminiscent of naphthalene. They all show the indophenin test with isatin and concentrated sulfuric acid. 

Thiophene and its homologues occur in coal-tar benzene, shale oil and crude petroleum. They show the indophenine test (Section 3.3.1.5.7.ii), and the discovery of thiophene followed the observation that pure benzene did not give this test. 

More rarely, ions of type (97) form dimeric products (possibly by initial loss of nuclear protons) thus, thiophenes with two free a-positions, or free adjacent a- and (3-positions, give indophenines (e.g. 103) with isatin (104). This reaction is used as a test for thiophene, the so-called indophenine test . 

The reaction of isatin with thiophene in the presence of sulfuric acid gave a blue color. This indophenine test was reported to be sensitive to 0.025% of thiophene in benzene. 

When thiophene is treated with isatin (667) and sulphuric acid a blue coloration is formed. The production of color in this way, the so-called indophenin reaction, led to the discovery of thiophene by Victor Meyer. Before thiophene was discovered it was thought that the reaction was characteristic of benzene. A certain sample of the hydrocarbon failed to give the test. On investigation it was found that the benzene used had been prepared from benzoic acid. As benzene obtained from coal-tar gave the test, it was evident that the color-reaction was produced by a substance mixed with the hydrocarbon. When benzene containing thiophene is shaken with concentrated sulphuric acid, the thiophene is more rapidly converted into a sulphonic acid than is the benzene. Thiophene is obtained from the sulphonic acid prepared in this way by heating the latter with water under pressure. 

Thiophene, the foundation of this book, had a tricky birth. It masqueraded as benzene from 1879 to 1882, when Maeyer [1] uncovered the subterfuge. It turned out that coal tar-derived benzene, when treated with isatin and sulfuric acid, produced a beautifully deep-blue precipitate, named indophenine. This pigment was claimed by Baeyer in 1879 [2] to be a qualitative test for benzene and was the product of the indophenine reaction . The pigment s structure was eventually shown to consist of a quinoid form of bithiopene, shown below. 

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