Rearrangement benzidine

Good agitation is essential, and a solvent may be used in some cases. This, however, is not always necessary if the agitation is efficient. The compounds are isolated by dissolving out the zinc with ice-cold hydrochloric acid. The hydrazo compounds, when heated with mineral acids, undergo a rearrangement (benzidine conversion). 

In the presence of acids, hydrazobenzene rearranges to give a mixture containing about 70 per cent of benzidine (4,4 -diaminobiphenyl) and about 30 per cent of 2,4 -diaminobiphenyl (diphenyline), the benzidine rearrangement. Benzidine is carcinogenic and its preparation and storage is under strict control. 

The rearrangement of hydrazobenzene to benzidine under the influence of strong acids is the prototype of numerous similar reactions which are therefore called benzidine rearrangements. Benzidines are intermediates in the production of substantive wool dyes. 

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 rearranging entity has been shown to be the bivalent cation the adjacent charges may so weaken the N—N link that charges of nearly integral size may be built up in the 4 and 4 positions. In the bent, but strainless, cation the minimum separation of the two p-positions would suffice for the establishment of a lai ely electrostatic bond, which could pass smoothly into the covalent rearrangement product (benzidine). 

The benzidine-like rearrangement of 2-hydrazinothiazoles has been observed by the Beyer group (523, 546). This rearrangement may be very fast (539). but does not occur with 2.2 -hydrazothiazole (300). where the... 

Although manufacture of benzidine itself has virtually ceased in Europe and the United States, similar rearrangement processes are operated for 3,3 -dichlotobenzidine [91-94-1J, o-dianisidine (3,3 -dimethoxybenzidine), and benzidine-2,2 -disulfonic acid. 

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. 

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 ... 

The benzidine rearrangement is of interest for mechanistic considerations. The preparative applicability may be limited because of the many side products, together with low yields. Furthermore benzidine is a carcinogenic compound. ... 

Heating of an aryl hydrazone 1 in the presence of a catalyst leads to elimination of ammonia and formation of an indole 2. This reaction is known as the Fischer indole synthesisy and is somewhat related to the Benzidine rearrangement. 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration... 

Secondly, it has been found that the benzidine rearrangement is subject to a solvent isotope effect d2o/ h2o > 1- If a proton is transferred from the solvent to the substrate in a rate-determining step the substitution of protium by deuterium will lead to a retardation in the rate of reaction (primary isotope effect) whereas if a proton is transferred in a fast equilibrium step preceeding the rate-determining step as in... 

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