Hydrazobenzenes rearrangement

In the presence of acids, hydrazobenzene rearranges to give about 0 per cent, of benzidine (4 4 -diaminodiphenyl) and about 30 per cent, of diphenyline (2 4 -diaminodiphenyl) ... 

The next benzidine-type rearrangement studied by Shine and coworkers was the p-semidine rearrangement of 4-methoxyhydrazobenzene26. In this reaction, 4-methoxy hydrazobenzene rearranges in acidic medium to form p-semidine (4) and an o-semidine (5) see equation 12. 

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. 

The object of this experiment is to prepare hydrazobenzene, rearrange it to benzidine, and finally eliminate the two amino groups to form biphenyl. 

Zinc reduction in alkaline aqueous or alcoholic medium is especially useful to bring about bimolecular reduction. This kind of reaction is illustrated by the conversion of nitrobenzene to hydrazobenzene. Rearrangement of hydrazobenzene with acid gives benzidine, a formerly important intermediate for azo dyes that now is banned because of its carcinogenic activity. 

M.p. 127 5°C. Dibasic, readily diazotizes. Prepared by treating hydrazobenzene with hydrochloric acid, intramolecular rearrangement taking place. 

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 ... 

ZININ Benzidine (semidine) Rearrangement Acid catatyzed rearrangement of hydrazobenzenes to benzidines and sermdines... 

Benzidine rearrangement of fluonnated hydrazobenzenes proceeds with a high yield in the presence of a strong acid [57] (equation 12) Ring contraction occurs when 2-bromoheptafluoronaphthalene is treated with antimony pentafluoride [52] (equation 13)... 

Hydrazobenzene 1 (1,2-diphenyl hydrazine) is converted to benzidine 2 (4,4 -diaminobiphenyl) under acidic conditions.This unusual reaction is called the benzidine rearrangement, and can be observed with substituted diphenyl hydrazines as well. 

In accord with the experimental findings a mechanism via a [5,5]-sigmatropic rearrangement has been formulated. In a first step the hydrazobenzene is proto-nated to the dicationic species 3, in which the phenyl groups can arrange in such a way to allow for rearrangement ... 

The reaction can be first or second order with respect to the H+ concentration. In weakly acidic solution it is first order in [H+], but second order in strongly acidic solution. This indicates that the monoprotonated as well as the diprotonated hydrazobenzene can undergo rearrangement. 

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 ... 

There is one further piece of kinetic evidence which throws light on an aspect of the benzidine rearrangement mechanism, and this is comparison of the rates of reaction of ring-deuterated substrates with the normal H compounds. If the final proton-loss from the benzene rings is in any way rate-determining then substitution of D for H would result in a primary isotope effect with kD < kH. This aspect has been examined in detail42 for two substrates, hydrazobenzene itself where second-order acid dependence is found and l,l -hydrazonaphthalene where the acid dependence is first-order. The results are given in Tables 2 and 3. 

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

M. O Sullivan, The acid-catalysed rearrangements of hydrazobenzenes. M. Sc. Thesis, London, 1966. 

Diaminodiphenyl derivatives can be obtained from appropriately substituted nitrobenzenes by a rather dated process of alkaline reduction with zinc powder/ sodium hydroxide solution, which affords hydrazobenzene. A more recent method uses a catalytic reduction process with hydrogen and specifically deactivated catalysts of precious metals. Subsequent acid-catalyzed rearrangement with hydrochloric acid yields the hydrochloride of the target diamine ... 

Under the influence of the catalyst (PC15, concentrated H2S04) a compound rich in energy is converted into its stable isomeride the change which occurs is similar to that which was discussed on pp. 186, 187 in connexion with the relationship between hydrazobenzene and benzidine. A comparison with the benzilic acid rearrangement also suggests itself. 

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