Indophenin reaction

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). 

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). 

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. 

By using oligothiophenes (thienyls) instead of the monomer thiophene in the indophenin reaction, for example (Figures 3.17), the electron donor can be inserted into the indophenin molecule in a controlled manner, changing the physical properties, especially the... 

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. 

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. 

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. ... 

The structure suggested by Steinkopf for the indophenine pigment formed by reaction between thiophen and isatin has been confirmed as (208) by n.m.r. and mass spectra and by chemical degradations such as desulphurization. 

Poly(indophenine)s [17] are prepared by reaction of isatin with thiophene (Figure 14) and have conductivities up to 10 S/cm without additional dopant. The polymers are soluble in dimethyl sulfoxide and can be cast as films the material is thermostable up to 230 °C [129]. 

Keywords Aromaticity Bromination Diels-Alder reaction Electrophilic substitution Furan Indophenine Isatin Metallatimi Palladium(O) catalysis Pyrrole S-oxidation Sulfur lone pair Thiophene numbering Viktor Meyer TT-System... 

Recently, 5,5 -biisatyl has been condensed with thiophene in strong acid to yield the poly(indophenine) [16] in quantitative yield 34, 48). The reaction (1-8) is over in 5 minutes at room temperature. Viscosity data indicate that the product is of low molecular weight ([ ] 0.06). It did not melt up to 500°C and... 

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