Архивы рубрики ‘Chemistry and Applications of Leuco Dyes’


With few exceptions, by far most applications of tetrazolium salts (e. g., analytical, photographic, and biochemical) utilize their reducibility to the corresponding formazan dyes. Tetrazolium salts, due to their resistance to acid and oxidation and the presence of a positive charge, find use in other applications such as antistatic agents and phase transfer catalysts. Over […]

Metal Complexation

The coordination chemistry of formazan dyes has been extensively reviewed in Venkataraman’s classic treatise on synthetic dyes,374,375 and more recently in Wilkinson’s treatise on coordination compounds.376 1,3,5-Triphenylformazan behaves as a bidentate ligand forming 2:1 complexes (217) with divalent copper, nickel, and cobalt.377 Formazan metal complexes can be compared to complexes of azo dyes or beta […]

Electrochemical Oxidation and Reduction

A few electrochemical reductions of formazans to hydrazidine have been reported.370,372 Ho wever, as discussed in Section, electrochemi­cal techniques have been widely used to study the redox chemistry of tetrazolium salts and formazans. Opinions about the reversibility of the electron transfer step, the number of electrons involved, and the identity of the rate-determining step […]

Reduction of Formazans

As discussed in Section, the reduction of tetrazolium salts to formazans often results in further reduction products. As seen in Scheme 24, reduction of formazans with ammonium sulfide leads to the hydrazidine 161. The reduction can proceed further eliminating an arylamine, yielding an amidrazone, e. g., 162.364 By contrast, alcoholic hydrogen sulfide attacks G […]


As one of the main synthetic routes to tetrazolium salts, the oxidation of formazans has been discussed in Section Strong oxidizing agents such as concentrated nitric acid and singlet oxygen (produced chemically or with a sensitizer of low triplet energy such as methylene blue) can lead to degradation to benzoic acid, phenol, and benzene […]

Action of Light

It has been recognized early that the red and yellow forms of triphenyl — formazan are interconvertible under the influence of light.262,352 Using spectroscopic, flash photolytic, and kinetic methods, the mechanism of the photochemical and thermal interconversion reactions between the various geometric isomers and conformers has been studied (Scheme 33a, b).245,342 However, the exact mechanism […]


The alkylation of formazan anions (166) with methyl iodide yields the A-methylformazan (202) which cannot be prepared by direct methods (Scheme 31).4 However, formazans that contain hydroxyaryl (203) or heterocyclic groups (205) alkylate preferentially on these groups to yield the alkylation products 204349 and 206,346-348 respectively (Eqs. 24,25). Although formazans are resistant to acylation,334,350,351 it […]

Reaction with Bases

Formazans are stable in alkaline solution. Alkaline hydrolysis of functionalities on formazans such as nitriles, esters, and amides leads to the acids (Section The case of 3-nitroformazans (198) is unique. Reac­tion with hydroxide ion gives 3-hydroxy formazan (199) which can be readily oxidized to the tetrazolium betaine. In the presence of hydrosulfide, a reduction […]

Chemical Properties Acid—Base Properties Formazans behave as weak acids as well as weak bases. Salts of formazans have been isolated.26’334’335 The acid dissociation constants of some substituted formazans have been determined from their solution spectra.336 Though formazans can be protonated, there are no reports of isolation of formazan cations. The study of the basicity of formazans […]

Electronic Spectra

Absorption spectra of formazans have been studied in detail. Almost all formazans exhibit UV/visible spectra between 300 and 600 nm.1’2’12’13’40 62’325’326 The absorption maxima are very sensitive to substituent effects. For example, the 1,5-diphenyl formazan 185 when X is hydrogen, methyl, phenyl, cyano, and mercapto shows a band at 420, 410, 470, 504, and 590nm […]