Hydrazobenzene, production

Formation of the hydrazobenzene product is strongly favored when platinized zinc or cadmium sulfide is used as the photocatalyst (Table 3). In both cases the rate decreases considerably and hydrazobenzene becomes the major product. It is known that platinum favors multi-electron processes [34]. 

If the methyl alcohol is distilled off before thorough cooling in a freezing mixture, the yield of hydrazobenzene is appreciably increased, but the product is considerably more coloured due to admixture with a trace of azobenzene. About 12 g. of impure hydrazobenzene may be recovered by distilling off the methyl alcohol from the hltrato after the colourless hydrazobenzene has been collected. 

The reduction of the nitro group to yield aniline is the most commercially important reaction of nitrobenzene. Usually the reaction is carried out by the catalytic hydrogenation of nitrobenzene, either in the gas phase or in solution, or by using iron borings and dilute hydrochloric acid (the Bechamp process). Depending on the conditions, the reduction of nitrobenzene can lead to a variety of products. The series of reduction products is shown in Figure 1 (see Amines byreduction). Nitrosobenzene, /V-pbenylbydroxylamine, and aniline are primary reduction products. Azoxybenzene is formed by the condensation of nitrosobenzene and /V-pbenylbydroxylamine in alkaline solutions, and azoxybenzene can be reduced to form azobenzene and hydrazobenzene. The reduction products of nitrobenzene under various conditions ate given in Table 2. 

In the reduction of nitro compounds to amines, several of the iatermediate species are stable and under the right conditions, it is possible to stop the reduction at these iatermediate stages and isolate the products (see Figure 1, where R = CgH ). Nitrosoben2ene [586-96-9] C H NO, can be obtained by electrochemical reduction of nitrobenzene [98-95-3]. Phenylhydroxylamine, C H NHOH, is obtained when nitrobenzene reacts with ziac dust and calcium chloride ia an alcohoHc solution. When a similar reaction is carried out with iron or ziac ia an acidic solution, aniline is the reduction product. Hydrazobenzene [122-66-7] formed when nitrobenzene reacts with ziac dust ia an alkaline solution. Azoxybenzene [495-48-7], C22H2QN2O, is... 

The rearranged dicationic species 4, which has been shown to be an intermediate, leads to the stable benzidine 2 upon deprotonation. It has been demonstrated by crossover experiments that the rearrangement does not proceed via a dissoci-ation/recombination process. From the reaction of hydrazobenzene the benzidine is obtained as the major product (up to 70% yield), together with products from side reactions—2,4 -diaminobiphenyl 5 (up to 30% yield) and small amounts of 2,2 -diaminobiphenyl 6 as well as o- and /j-semidine 7 and 8 ... 

It has been long known23 that hydrazobenzene (VII) is converted in acid solution into benzidine (VIII), which is the main product, and diphenyline (IX), viz. 

THE EFFECT OF RING-DEUTERIUM SUBSTITUTION ON THE RATE AND PRODUCTS OF THE REARRANGEMENT OF HYDRAZOBENZENE IN 90% ETHANOL AT 0 °C... 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

Since azoxybenzene is attacked by more powerful reducing agents, e.g. zinc dust and sodium hydroxide solution or ammonia, the use of such agents converts nitrobenzene to azobenzene and hydrazobenzene, by passing at once beyond the azoxybenzene stage. The three reduction products with paired " nitrogen atoms, therefore, stand in very close genetic relation to each other. 

There continue to be a few examples reported where rearrangement has been used synthetically to develop new products sometimes important in the industrial world. Monomers for polyamides and polyimides (which are used for making moisture sensitive films, fibres and mouldings) have been synthesized24 by the reduction of a nitro compound, followed by a benzidine rearrangement of the resulting hydrazobenzene derivative as outlined in Scheme 5. 

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

The reaction was second order in acid and first order in substrate, so both rearrangements and the disproportionation reaction proceed via the doubly-protonated hydrazobenzene intermediate formed in a rapid pre-equilibrium step. The nitrogen and carbon-13 kinetic isotope effects were measured to learn whether the slow step of each reaction was concerted or stepwise. The nitrogen and carbon-13 kinetic isotope effects were measured using whole-molecule isotope ratio mass spectrometry of the trifluoroacetyl derivatives of the amine products and by isotope ratio mass spectrometry on the nitrogen and carbon dioxide gases produced from the products. The carbon-12/carbon-14 isotope... 

Matsui F, Lovering EG, Curran NM, et al. 1983. Determination of azobenzene and hydrazobenzene in phenylbutazone and sulfinpyrazone products by high-performance liquid chromatography J Pharm Sci 72 1223-1224. 

To a mixture of 50 g (0.4 mol) of nitrobenzene, 180 ml of 30% sodium hydroxide, 20 ml of water and 50 ml of ethanol, 100-125g (1.5-1.9g-atom) of zinc dust is added portionwise with efficient mechanical stirring until the red liquid turns light yellow. After stirring for an additional 15 minutes, 1 liter of cold water is added, the mixture is filtered with suction, the solids on the filter are washed with water, and the hydrazobenzene is extracted from the solids by boiling with 750 ml of ethanol. The mixture is filtered while hot, the filtrate is cooled, the precipitated crystalline hydrazobenzene is filtered with suction, and the mother liquor is used for repeated extraction of the zinc residue. Recrystallization of the crude product from an alcohol-ether mixture gives hydrazobenzene of m.p. 126-127°. 

Azoxybenzene is reducible under polarographic conditions. The final product is hydrazobenzene formed in an irreversible process for which the half-wave potential [105] varies with pH as illustrated in Fig. 11.3. The half-wave potential is close to that of the nitrobenzene. Azobenzene, which is an intermediate in the process, is... 

Aliphatic amines have been oxidized to aliphatic azo compounds with iodine pentafluoride [80-81], In the aromatic series, this reagent evidently gives rise to tarry products when used to oxidize amines, although it has been used to oxidize hydrazobenzene to azobenzene. 

The hydrogen-substrate ratio of 4.3 indicates formation of azobenzene as the main product, as well as some hydrazobenzene. Both products were isolated from such a run in a 2 to 1 ratio, respectively. Addition of less than stoichiometric quantities of azobenzene to CoH (no added alkali) did not result in hydrogen absorption. However, absorption of 0.53 atom of hydrogen per mole of azobenzene was observed upon further injection of small amounts of the substrate after alkali was added a 69% yield of hydrazobenzene was isolated. 

Lund and Iversen first showed that azobenzene was an effective probase it is reduced to radical-anion at a low potential (—0.9 V vi. Ag/AgCl) and the reduced form is sufficiently basic to deprotonate benzylphosphonium salts. Its usefulness as an alternative to conventional bases was illustrated by the near quantitative production of stilbene by electrolysis of azobenzene in the presence of benzaldehyde and benzyl-triphenylphosphonium bromide (Table 2, entry 1). However, the concomitant formation of the carcinogenic benzidene, by acidic work-up of a product mixture which contains hydrazobenzene, is a severe drawback for this system. 

When hydrazobenzene is treated with acids, it rearranges to give about 70% 4,4 -diami-nobiphenyl (132, benzidine) and about 30% 2,4 -diaminobiphenyl (133). This reaction is called the benzidine rearrangement and is general for N,N -diarylhydrazines.541 Usually, the major product is the 4,4 -diaminobiaryl, but four other products may also be produced. These are the 2,4 -diaminobiaryl (133), already referred to, the 2,2 -diaminobiaryl (134), and the o- and p-arylaminoanilines (135 and 136), called semidirtes. The 134 and 136 com-... 

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