The Mechanism of the Benzidine Rearrangement. Part I. The Rearrangement of p-Hydrazotoluene. Part II. The Rearrangement of m-Hydrazoaniline

Citation

Clovis, James Stanley (1963) The Mechanism of the Benzidine Rearrangement. Part I. The Rearrangement of p-Hydrazotoluene. Part II. The Rearrangement of m-Hydrazoaniline. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/E47T-2V77. https://resolver.caltech.edu/CaltechTHESIS:01042013-085236180

Abstract

Part I

A number of unsuccessful experiments were carried out in an attempt to show that the benzidine rearrangement proceeds through intermediate cation radicals. The rearrangement of p-hydrazo- toluene (HzT), which has been reported to give rise to p-toluidine, p-azotoluene, and 2-amino-4, 5'-dimethyldiphenylamine (an o-semidine) in a 2:1:2 weight ratio, was extensively investigated.

Attempts to initiate polymerization by intermediates formed during the rearrangement of HzT (and hydrazobenzene) failed. The product solution that was obtained after all of the HzT had reacted was found to oxidize N,N,N',N' tetramethyl-p-phenylenediamine to Wurster's Blue . The kinetics of this oxidation were investigated.

The composition of the mixture of products obtained from HzT was re-examined by the technique of isotope dilution. The results showed that p-toluidine, p-azotoluene, and the o-semidine account for only 90% of starting material and are actually formed in a 3:2:4 weight ratio. An additional product (or products) must account for 10% of the reac tion of HzT.

Radioactive HzT was rearranged in the presence of a number of added compounds. The presence of hydrazobenzene was found to reduce the yield of p-azotoluene, but not to alter the formation of p-toluidine and the o-semidine. These results show that p-azotoluene is formed by an intermolecular process, while the remaining products probably originate from intramolecular paths.

The rearrangement of p,p'-ditrideuteromethylhydrazobenzene-4,4'-C^(14) was investigated and found to be unaffected by the presence of the deuterium.

An unknown compound was isolated in very small quantities from the product mixture obtained from the rearrangement of HzT. This compound was in the form of an oil and was only partially characterized.

A possible explanation for the formation of the 10% of unknown product is that it arises from coupling of the para carbon atoms of HzT.

Part II

The rearrangement of m-hydrazoaniline (HzA) to 2,2'-di - aminobenzidine has been investigated. The reaction rate exhibits an inverse dependence upon hydrogen ion. As HzA is a diacidic base, the observed rate law requires that the transition state of the rearrangement involve a monoconjugate acid of HzA.

HzA-N^(15)H-N^(15)H- was prepared and rearranged at four acid concentrations, three of which were within the concentration range used to determine the acid dependence of the rearrangement. The N^(15) content of the o-amino groups of the resulting benzidine was found to increase as the acidity of the rearrangement solutions was increased. In terms of coupling reactions the results showed that the extent of ortho coupling that occurred in the formation of the benzidine varied from approximately 35% at the lowest acid concentration to 44% at the highest. As only one hydrogen is involved in the transition state of the rearrangement, a metastable intermediate must be formed which can either collapse to product or be attacked by a second proton. The results are consistent with a mechanism which involves initial heterolysis of the N-N bond to give Dewar's "n-complex." This complex can either collapse to product or can be attacked by proton to form a pair of cation radicals which will also collapse to product.

Starting material was recovered after a rearrangement had proceeded to 50% completion and the N^(15) content of the hydrazo linkage was found to be unchanged.

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