Hydrazobenzenes, formation rearrangement

When hydrazobenzene, or 5ym-diphenylhydrazine, is warmed with acids, it undergoes an intramolecular rearrangement, with the formation of benzidine,... 

The conversion of a hydrazobenzene into a diaminodiphenyl upon treatment with acid is termed the benzidine rearrangement. The following mechanism for the formation of benzidine from hydrazobenzene appears reasonable ... 

The reaction of hydrazobenzene (1) refers to reaction via the doubly protonated form. The mechanism for the rearrangement via a mono protonated form was examined17 using 2,2 -dimethoxyhydrazobenezene (7). Again the KIE results for formation of the benzidine derivative 8 show that reaction is also concerted. It appears that there is no major difference between the one- and two-proton reactions. 

The most recent addition to Shine s extensive study of the benzidine-type rearrangements41 involved remeasuring the nitrogen and the carbon-13 and carbon-14 kinetic isotope effects at the 4- and at the 4- and 4 -carbons as well as determining the carbon-13 and carbon-14 isotope effects at the 1- and at the 1- and l -carbons in the benzidine rearrangement of hydrazobenzene (equation 30). The reaction, which was carried out in 75% aqueous ethanol that was 0.1 M in hydrochloric acid and 0.3 M in lithium chloride at 0°C, gave an 86% yield of benzidine (11) and a 14% yield of diphenyline (12). The kinetic isotope effects found for the formation of benzidine and diphenyline under these reaction conditions are presented in Table 5. 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

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. 

Benzidine.—That nitrobenzene, by electrolytical reduction in acid solution, can directly yield benzidine, was first proved by Hiiussermann,1 who used sulphuric acid. Lob 2 later proved the same to be true for hydrochloric-, acetic- and formic-acid electrolytes. However, several reactions predominate in this direct acid reduction, which prevent the carrying out of the reaction up to hydrazobenzene, or the formation of benzidine. Phenylhydroxylamine may particularly be mentioned in this connection. In alcoholic-acid solution it is partly rearranged to amidophenol or its ethers, and partly reduced to aniline. Azoxybenzene, in acid solution, is the starting-point in the benzidine formation however, in this case, the combining velocity of nitrosobenzene and phenylhydroxylamine is not very great, so that the latter is to a very considerable extent subject to the more rapidly acting influence of the acid. 

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