Benzimidazole ring dicarboxylic acid

The technical production of poly(benzimidazole) (PBI) is also carried out in two steps. In the first step an aromatic tetramine is condensed with the diphenyl ester of an aromatic dicarboxylic acid at 220-260 °C, yielding a poly(amino amid) with elimination of phenol. Ring closure with elimination of water occurs in the second step (solid-phase polycyclocondensation), conducted at 400 °C and yielding the polybenzimidazole (experimental procedure, see Table 2.3). 

Vigorous oxidation (e.g., with KMn04) usually degrades fused benzene rings in preference to many azole rings, especially under acidic conditions. Thus, benzimidazoles are oxidized by chromic acid or 30% hydrogen peroxide to imidazole-4,5-dicarboxylic acid, and 2,1,3-benzothiadiazole is oxidized by ozone or potassium permanganate to the dicarboxylic acid 553. 

That imidazoles and benzimidazoles have high stability has been known for many years. Resistance to acids, bases, heat and oxidation or reduction are common traits of these compounds, which display considerable aromatic character. Thus treatment of benzimidazole with permanganate leads to imidazole-4,5-dicarboxylic acid imidazoles, in general, are not easily oxidized or reduced (Sections 4.07.1.4.11, 4.07.1.5.6, 4.07.1.7.4). Thermal stability too is evidenced by the resistance of the imidazole nucleus to ring fission... 

Benzimidazole (but not 1-methylbenzimidazole) is oxidized by permanganate, dichromate or hydrogen peroxide to imidazole-4,5-dicarboxylic acid, while napth-[l,2-d]- and -[2,3-d]-imidazoles also form products in which the heterocyclic ring remains intact, hence demonstrating its stability to these conditions. With lead peroxide benzimidazole is subject to an unusual oxidation as it forms (101), also the reaction product of lead dioxide and 2,2 -bibenzimidazolyl. In dioxane, selenium dioxide oxidizes 2-methylbenzimidazole to o-hydroxyacetanilide (66RCR122). 

Electrophilic reagents preferentially attack benzimidazoles in the fused benzene ring, while nucleophiles react at C-2 which has enhanced nucleophilic activity because of the electron-withdrawal effect of the benzene moiety. The fused aryl ring appears to exhibit less aromatic stability than the heteroring as evidenced by the ready oxidation of benzimidazole to imidazole-4,5-dicarboxylic acid, and by its catalytic reduction over platinum... 

Poly(benzimidazoles) are produced from dicarboxylic acids and aromatic tetramines. Commercially, 3,3 -diaminobenzidine tetrahydrochloride and diphenyl isophthalate are preferentially used. The diphenyl ester is used because (a) the free acids decarboxylate under the high reaction temperatures of 250-400 C (b) the acyl chlorides react too fast, making ring closure difficult and (c) the amino groups are partially methylated if the methyl esters are used. The hydrochloride is used because it is more stable to oxidation than the free amine itself. The polycondensation is carried out in two stages. A prepolymer. A, is formed in the first stage with foaming and phenol elimination ... 

N-Condensed benzimidazole ring from dicarboxylic acid anhydrides... 

Poly(benzimidazoles) have become commercially important. Whereas polyimides contain two carboxyl groups per amine group, two amine groups are allowed to react with a carboxyl group in poly(benzimidazole) manufacture. Diphenyl esters are used as dicarboxyl compounds, since (a) the free acids decarboxylate under the reaction conditions (250°C at first, subsequently 400°C), (b) the acyl chlorides react too quickly, so that ring closure becomes difficult, and (c) with methyl esters, the amino groups become partially methylated. Moreover, eliminated phenol can easily be washed out. Typical materials are 3,3 -diaminobenzidine and diphenyl... 

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