Indophenines

Properties.—Mobile, colourless liquid m. p. 5 4 b. p. So 4 sp. gr. o 874 at 20°. Coal-tar benzene usually contains a little thiophene, C4H4S, which may be detected by dissolving a few crystals of isatin (see p. 229) in concentrated sulphuiicacid and shaking up with the benzene. If thiophene is present, a blue colour is produced (indophenin reaction). 

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

Thienothiophenes undergo the indophenine reaction with isatin (see Section III,I). Oster reported that thieno[2,3-6]thiophene (1) gave a green product with a ratio of isatin to thienothiophene 1 1, and a blue substance with the ratio of the initial substances 2 1. Steinkopf and HempeP could not obtain this blue substance instead they isolated a brown material with the isatin to thienothiophene 1 ratio 2 3 at +50° they isolated a substance with the ratio 1 1 and at +70°, with ratio 1 2. Steinkopf and PetersdorP found that a reaction of 2-acetylthieno[2,3-6]thiophene with isatin produces 2- or 3-(2-thieno[2,3-6]thienyl) cinchoninic acid. The acid was decarboxylated to 2- or 3 2-thieno[2,3-6]thienyl)quinoline (the site of quinoline group in thienothiophene 1 molecule was not established). 

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 . 

In a manner similar to that noted above, pyrrole gave polypyrrole-indophenines (222, X = 0, CH2, and a bond),631 and o-diamines gave polyindoloquinoxalines 223,424 2 24,424,632 2 25,633 2 26,423 2 27,423 and 228.423 These latter compounds showed good thermal stability. Polymer... 

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. 

However, it was not until 1939 that the correct structure of indophenine was established as (359). Other ortho -quinones also condense with thiophene in the presence of acid to form dyes having the general structure (360). Although the indophenine dyes were not as commercially important as the thioindigo dyes, many compounds were prepared and evaluated (54HC(7)l). 

Yield.—30% theoretical (15 gms.). Colourless liquid faint smell resembling that of benzene B.P. 84°. Gives blue colour (indophenine) with isatin and cone, sulphuric acid. (B., 18, 454 0. S., XII., 72.)... 

Under acidic catalysis, isatin condenses with thiophene or pyrrole to give indophenine dyes. These compounds are formed as a mixture of geometric isomers450, and may possess one or two thiophene units the latter being the major product451 (Scheme 108). 

The reaction of isatin with thiophene in an acidic medium, containing ferrous ion, gives rise to an intense violet color, due to the formation of indophenine dyes. Due to this phenomenon, it was proposed that isatin could be used as a revealing agent for the presence of thiophene in water-soluble organic solvents where it is used as a denaturating agent637. 

The well-known indophenin has now been obtained from isatin and thiophene using polyphosphoric acid. ... 

Isatin forms a blue condensation-product (indophenine) with thiophene. On reduction with ammonium sulphide, isatide C16H12N2O4 [16] is formed. With zinc powder in acetic-acid solution, isatin yields bydroisatinj by more energetic reducing-agents oxy- and dioxy-indol are formed. Chlorine and bromine react with isatin, forming chlorine and bromine derivatives respectively. Acetic anhydride produces an acetyl isatiu [15],... 

A reaction of special historical interest, mentioned in the introduction to this chapter, is the condensation of thiophene with isatin in concentrated sulfuric acid, to give the deep blue indophenine as a mixture of geometrical isomers. ... 

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. 

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