Pyrrole black

The only products obtained by carrying out oxidation of pyrroles with the above reagents were pyrrole blacks (see Section V). 

Angeli38,112 first observed that pyrrole with hydrogen peroxide in acetic acid developed a dark color and later a black precipitate. This product, which contains oxygen, was named pyrrole black . Its physical characteristics were very similar to those of natural melanins and this fact stimulated research on the structure of this product, considered an interesting model for natural pigments. 

Pyrrole blacks are obtained by other oxidizing reagents, which were tested mainly on unsubstituted pyrrole. Among these are nitrous acid, lead dioxide,112 ferric chloride,101,102 quinones,89, 61,113 115 diazonium salts,116 and ozone.107 

Mention might be made of some recent work, in which the reaction between pyrrole and hydrogen peroxide in acetic acid was studied by 

Another ESR study118 was concerned with the pyrrole blacks obtained by anodic oxidation of pyrrole in dilute sulfuric acid.119 This substance showed a high conductivity, and an ESR signal, again due to trapped electrons and free from hyperfine structure, but with an exceptionally low g factor (2.0026 + 0.0001). 

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In 1968 DairOlio et al. published the first report of analogous electrosyntheses in other systems. They had observed the formation of brittle, filmlike pyrrole black on a Pt-electrode during the anodic oxidation of pyrrole in dilute sulphuric acid. Conductivity measurements carried out on the isolated solid state materials gave a value of 8 Scm . In addition, a strong ESR signal was evidence of a high number of unpaired spins. Earlier, in 1961, H. Lund had reported — in a virtually unobtainable publication — that PPy can be produced by electrochemical polymerization. 

Pyrrole was first polymerised in 1916 [239, 240] by the oxidation of pyrrole with H202 to give an amorphous, powdery product known as pyrrole black, which was... 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

The oxidation of indoles and pyrroles by Fe(III) ions is less predictable than other chemical oxidations. 2-Methyl- and 3-methyl-indoles, respectively, yield (187) and (188), and whilst pyrroles may form pyrrole black , the rate of oxidation of pyrrole and of 1-methylpyrrole appears to be relatively slow. C-Alkyl and electron-donating substituents enhance the formation of oligimers, e.g. (189) -> (190) and (191) -> (192), and although electron-withdrawing substituents reduce the susceptibility of the pyrrole ring to oxidation, acyl- and alkoxycarbonyl-pyrroles of the type (193) are readily oxidized to the thermochromic dimer (194), which is in equilibrium with the dimer (195) via the monomeric pyrrolyl radical (72BCJ3584). 

The first experiments with hydrogen peroxide have been carried out in acetic solution, varying the concentration of pyrrole and the amount of hydrogen peroxide. Pyrrole blacks were obtained in these conditions (Section V) and other oxidation products were isolated as trace components.38-41 Their structures were recently elucidated as 4342 and 44.43... 

Compounds 43 and 44 are not intermediates in the formation of pyrrole blacks. During oxidation with hydrogen peroxide in pyridine, small quantities of succinimide and maleimide were identified by paper chromatography.4 4... 

Recently, the reaction has been reinvestigated, in buffers at various pH (1.1-9.0), showing that pyrrole blacks are formed only in strongly acidic medium. In neutral media, compound 45 is formed always together with its isomer (46) in the ratio 10 1,45-47 Tautomerism between the isomers was studied by NMR.48 It was proved that 45 and 46 are intermediates, giving 43 and 44 when reacted with pyrrole in acidic medium.46... 

Several alkylpyrroles have been reacted with hydrogen peroxide. The a,a -unsubstituted compounds gave pyrrole blacks in acidic medium, while in neutral media (water or organic solvents) appreciable amounts of liquid or crystalline derivatives could be isolated, along with unidentified syrups. 

Ferric chloride oxidizes a,a -unsubstituted pyrroles to pyrrole blacks 101,102 if substituted with electron-withdrawing groups, pyrroles withstand oxidation quite well.102... 

The first chemical oxidation of pyrrole 106 (scheme 26) was achieved as early as 1916 by using hydrogen peroxide to obtain an amorphous powder known as pyrrole black [158]. The room temperature conductivity of PPy 20 prepared with acid or peroxide are in the range of 10"to 10 " S/cm, which can be inereased by halogen doping to 10 S/cm [159]. This low conductivity is due to the high degree of saturation of the pyrrole monomer units caused by defects. In the last... 

Mengoli, G., Musiani, M. M., Fleishman, M., and Pletcher, D., Studies of pyrrole black electrodes as possible battery cathodes. J. Appl. Electrochem., 14, 285-292 (1984). 

Polypyrrole was one of the earliest conducting polymers produced. In 1968 DalT-Olio et al. [2] oxidized pyrrole in aqueous sulfuric acid to obtain a powdery precipitate on a platinum electrode. Pyrrole black, as this material became known, had a conductivity of 8 S cm". Chemical analysis determined the presence of 75% poly pyrrole and 25% sulfate anions. An intense electron paramagnetic resonance signal indicated that the polymer possessed a large number of free spins and gave a g value of 2.0026 (comparable to that of a free electron, g = 2.0023). 

Polypyrroles. The first reported polymerization of pyrrole was in 1916 (97). Polypyrrole was prepared by the chemical oxidation of pyrrole using hydrogen peroxide. An amorphous black powder known as pyrrole black was obtained, which... 

Oxidative Polymerization of Pyrroles. The synthesis of pyrrole blacks was performed by chemical oxidative polymerization with a variety of oxidizing agents such as hydrogen peroxide, lead dioxide, quinines, ferric chloride, and persulfates (244) before the electrical method was employed (236). The catalytic oxidative polymerization under dioxygen to give polypyrrole (PPY) has also been developed (245-247). 

Like aniline black , pyrrole black has also been reported as a black intractable solid resulting from the oxidation of pyrrole and much of the early work focused... 

Pyrrole was first polymerized in 1916 [1,2] by the oxidation of pyrrole with H2O2 to give an amorphous powdery product known as pyrrole black. However, little further interest was shown in this material until it was electrochemically prepared in the form of continuous films. The electrochemical synthesis of polypyrrole dates to the early work of DalTOllio [3], who also obtained pyrrole blacks by electrochemical oxidation of pyrrole in aqueous sulfuric acid on a platinum electrode. In 1979 [4] electrochemical techniques to synthesize polypyrroles become a useful way to obtain highly conductive free-standing materials. Chemical and electrochemical methods of synthesis have since then been improved in order to optimize the physical and chemical properties of those materials. 

The preparation of poly(pyrrole) (pyrrole red and pyrrole black) by oxidation of pyrrole dates back to 1888 [50] and by electrochemical polymerization to 1957 [51]. A fairly long period elapsed before this organic Ti-system attracted general interest and was found to be electrically conductive [5] in 1963. 

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