Hydrazobenzene and azobenzene

A round-bottomed flask (capacity 1 1.) is provided with a wellfitting thin-walled double neck attachment (Eig. 30). An ascending Liebig condenser is clamped obliquely and its inner tube is joined by a short wide rubber tube to the oblique neck in such a way that the flask can be vigorously shaken without trouble. The upright neck, which serves for the introduction of the zinc dust required for the reduction, is closed with a cork. 

Sodium hydroxide (50 g.) is dissolved in 150 c.c. of water, and the solution, while still warm, is poured into the flask, along with 50 c.c. of alcohol and 41 g. (0-33 mole) of nitrobenzene. Zinc dust (6-8 g.) is now added with vigorous shaking which is continued until the reaction, at first rather violent, has ceased. The reaction mixture 

The contents of the flask become at first red (azobenzene), but finally turn pale yellow when the necessary amount of reducing agent has had its effect. About 120 to 150 g. of 75 per cent zinc dust are required. If the reaction should cease prematurely, the flask is heated on a vigorously boiling water bath. 

It is essential that the contents of the flask be continually agitated by vigorous shaking in order that the heavy dust may remain in contact with the organic substance. 

The yield of crude product, which can be used directly for further preparations, amounts to 20-25 g. 

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Ahuja S, Thompson G, Smith J. 1988. Trace/ultratrace analyses of unstable compounds -investigations on hydrazobenzene and azobenzene. J Res Natl Bur Stand 93 344-347. 

Tabre H, Hussam-Eddine N, Mandrou B. 1984. Stability-indicating assay for phenylbutazone High-performance liquid-chromatographic determination of hydrazobenzene and azobenzene in degraded aqueous phenylbutazone solutions. J Pharm Sci 73 1706-1709. 

H2S to nitrobenzene, and at low reaction temperatures, measurable amounts of hydrazobenzene and azobenzene are detected. At high temperatures and high conversions, appreciable reaction of product anibne with sulfur is observed. These observations point to a sequential reduction ... 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

Nitrogen Compound Autoxidation. CycHc processes based on the oxidation of hydrazobenzene and dihydrophenazine to give hydrogen peroxide and the corresponding azobenzene—phenazine were developed in the United States and Germany during World War II. However, these processes could not compete economically with the anthrahydroquinone autoxidation process. 

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

They can be isolated in good yields by reduction of the nitrobenzene in aqueous ethanolic sodium acetate under reflux, passing around 10% excess electric charge [103]. Any hydrazobenzene formed is rapidly oxidised back to the azobenzene by air during work-up. Azoxybenzene is formed first and then reduced to azobenzene and finally hydrazobenzene at the cathode. A solution electron transfer reaction between azoxybenzene and the hydrazobenzene reforms azobenzene. 

Aromatic nitro compounds are reduced to arylamines in 60-90% yield. Nitroso-bcnzene, azoxybenzene, and azobenzene are reduced to hydrazobenzene in high yield.2... 

Because organobismuth(V) compounds have found considerable use as oxidizing agents, the oxidizing ability of methyl di-l-naphthylbismuthinate [124066-66-6], C21H17Bi02, was investigated. Benzoin yields benzil, naphthalene, and metallic bismuth hydrazobenzene yields azobenzene, and 1,1,2,2-tetraphenylethanediol yields benzophenone. 1,2-Diphenyl-1,2-ethanedione dihydrazone gives diphenylacetylene in 50% yield. Cyclohexane-1,2-diol and 1-phenylethane-l,2-diol, however, were unaffected. 

It has been known for some time that irradiation of azobenzene (324) in either 22 N sulfuric acid350 351 or acetic acid with added ferric chloride 352 yields benzo[c]cinnoline (325). This is accompanied by the formation of an almost equal quantity of benzidine (326), undoubtedly arising by rearrangement of hydrazobenzene (327). The mechanism of this reaction differs, therefore, from that of the stilbene cyclodehydrogenation, and azobenzene itself functions as the hydrogen acceptor. Yields of not more than 50% of benzo[c]cinnoline are generally observed. 

Since this reaction is inhibited by the presence of strongly complexing diphosphines, it seems likely that oxidation proceeds through the prior complexation of the phosphine to the metal followed by the transfer of an oxo oxygen atom to the coordinated phosphine (equation 107).12 These complexes (96) also oxidize hydrazobenzene to azobenzene,307 but are weak oxidants and do not react with alkenes.56... 

When reduced by electrolysis, nitrobenzene and its homologues yield the same products as may be obtained by the various chemical methods of reduction. Aniline, azobenzene, azoxybenzene, hydrazobenzene, and -amino-phenol, as well as phenylhydroxylamine, can thus be obtained from nitrobenzene, and most, if not all, of these products could be prepared satisfactorily on an industrial scale by electrolysis, by adjusting the manner of working so that economy of energy is combined with maximum yields. Many of these products demand a comparatively high price, so that low power cost is not so important in this class of manufacture as high percentage yields. 

According to more recent research hydrazobenzene and hydrazotoluene can also be used as hydrogen donors to oxygen. The azobenzene or azotoluene resulting from oxidation are reduced to the initial hydrazo- compounds by sodium amalgam. 

Similar to the cases with platinum metals, the rate of hydrogenolysis of hydrazobenzene appears to be smaller than that of azobenzene over Raney Ni as well,234 although Wisniak and Klein observed the formation of only azoxy- and azobenzenes, and no hydrazobenzene, during the course of hydrogenation of nitrobenzene over Raney Ni at 170°C and 1.38 MPa H2.235... 

Diphenylpicrylhydrazyl, which is prepared from MA -diphenyl-A -(2,4,6-trinitrophenyl)hydrazine by oxidation with lead dioxide [982], is used for the dehydrogenation of 1,4-dihydronaphthalene to naphthalene and of hydrazobenzene to azobenzene [983]. Simpler and cheaper compounds are, however, available for such purposes (equation 21). 

Azoxybenzenes are reduced in a four-electron reduction to hydrazobenzenes and are generally in protic solvents more difficult to reduce than the azobenzenes in this respect the system azoxybenzene-azobenzene is similar to that of nitrobenzene-nitrosobenzene. The analogy can be extended Further reduction of azobenzene yields hydrazobenzene, which can be reoxidized to azobenzene and which easily rearranges in acid solution analogously, phenylhydroxylamine can be reoxidized to nitrosobenzene and can be rearranged to p-aminophenol or similar products in acid solution. 

Cinnolines are generally reducible [101] in two or more steps. The first step in the reduction of aryl- or alkyl-substituted cinnolines is the formation of 1,4-dihydrocinnolines. This reduction is not reversible. Only in the case of benzo[c]cinnoline is the reduction, a saturation of the nitrogen-nitrogen double bond the reaction resembles the reduction of azobenzene to hydrazobenzene and, like this system, is nearly polarographically reversible. 

According to a preliminary report, diimide can be generated at 0° by the action of sodium hydroxide on chloroacetylhydrazide hydrochloride (which see). Another precursor is hydroxylamine-O-sulfonic acid (which see). Azobenzene has been reduced quantitatively to hydrazobenzene and tdlyl alcohol to propanol (70%). ClCHjCONHNHi HCl + 2 NaOH ----------> CH2=C=0 + HjO + HN=NH + 2 NaCff... 

NaBHi-Pd-C. Aromatic nitro compounds are not normally reduced by NaBHi but can be reduced smoothly in solution in water or methanol with sodium borohydride catalyzed by 10% palladium charcoal. Azobenzene is reduced by this method to hydrazobenzene and not to aniline. 

A primary amine having an a-methylene group is oxidized by the reagent in methylene chloride in part to the nitrile and in part to the aldehyde, both in very low yield. The reagent oxidizes hydrazobenzene to azobenzene and effects Beckmann rearrangement of oximes. The most significant use is for oxidation of /-butylamine... 

In a series of papers Lewis and co-workers have reported a comprehensive study of the photochemical reactions of azobenzenes in sulfuric acid. In the case of azobenzene itself, the products obtained were benzo[c]cinnoline (48%) together with benzidine (35%), and other azobenzenes gave products derived from the corresponding hydrazo compounds. The mechanism proposed is outlined in Scheme 2. There is an initial rapid establishment of a cis-trans equilibrium of monoprotonated azobenzene, with cyclization involving an excited form (14) of the cis-isomer, the lowest transition now being n-n. In the final step, dehydrogenation of the photocyclization product, 5,6-dihydrobenzo[c]cinnoline, is brought about by a second molecule of azobenzene, which is itself reduced to hydrazobenzene and... 

Benzoin, tetraphenylethanediol, hydrazobenzene and 1,2-diphenyl-1,2-ethanedione bishydrazone are oxidized by methyl bis(l-naphthyl)bismuthi-nate to benzil, benzophenone, azobenzene and diphenylacetylene, respectively [88CL2021] (Section 5.2.4). Amides, thioamides, ureas and thioureas are A-acylated in good yield with acetic acid in the presence of the bismuthi-nate at room temperature [90CL1651] (Scheme 3.11). 

Blaisdell B.E. (1949) investigated the photoreductive processes of azobenzene and 4-amino-4-nitroazobenzene in isopropanol and isooctane, he concluded that the main products were hydrazobenzene and aniline derivatives after photodisproportionation. 

A. fourth example of a radical-type reaction of Grignard reagents is the reaction with azoben-zenc (97. It was found that azobenzene in the reaction with methylmagncsium bromide produces hydrazobenzene and ethane (98). To be able to reduce azobenzene the reagent has to act as an electron donor in an induced homolysis ... 

Oxidation. Bismuthines accept oxygen from iodosylbenzene under ultrasonic irradiation. These AraBiO are mild oxidants, which convert benzylic, allylic, and secondary alcohols to carbonyl compounds, hydrazobenzenes to azobenzenes. By contrast, oxides of lower pnictogen elements are devoid of oxidizing power for organic substances. 

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