Benzidine protonation

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... 

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. 

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

Micellar rate enhancements of bimolecular, non-solvolytic reactions are due largely to increased reactant concentrations at the micellar surface, and micelles should favor third- over second-order reactions. The benzidine rearrangement typically proceeds through a two-proton transition state (Shine, 1967 Banthorpe, 1979). The first step is a reversible pre-equilibrium and in the second step proton transfer may be concerted with N—N bond breaking (17) (Bunton and Rubin, 1976 Shine et al., 1982). Electron-donating substituents permit incursion of a one-proton mechanism, probably involving a pre-equilibrium step. 

Another well-studied electron transfer reaction is the oxidation of aqueous benzidine in the presence of various clays (2, 40, 43, 50, 51). An electron from the colorless benzidine molecule is abstracted by the clay with formation of a blue monovalent radical cation. Upon drying of the blue clay-benzidine system, a yellow color is produced. There is disagreement in the literature with respect to the chemical identity of the yellow product (2, 40, 52) however, in the case of hectorite, the yellow product has been suggested to be the protonated form of the radical cation (divalent radical cation) (2, 52). There is also disagreement about whether the electron-accepting sites of the clay are ferric iron at the planar surfaces, aluminum ions at the edges, or exchangeable cations <2, I). 

The source of H+ is the dissociation of coordinated water (see section on Br nsted acid). The protonation of benzidine (Equation 8) shifts the oxidation reaction (Equation 7) to the left. 

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 benzidine rearrangements can also be brought about thermally, but very few mechanistic studies have been carried out. One set of heavy-atom KIE measurements has been made in the reaction of 2,2 -hydrazonaphthalene (18)21. Substantial nitrogen (1.0611 for the [15N, 15N ]) and carbon (1.0182 for the [1,1 -13C2]) KIE values were obtained showing that, just as for the acid catalysed reaction, this is a [3,3]-sigmatropic rearrangement, this time presumably of the non-protonated reactant. 

Shine and coworkers36 also investigated the mechanism of the one-proton benzidine rearrangement of 2,2/-dimethoxyhydrazobenzene. The doubly labelled 2,2 -dimethoxy-[15N,15N]hydrazobenzene, the 2,2 -dimethoxy-[4,4 2H2]hydrazobenzene and the 2,2 -dimethoxy-[4,4 13C2]hydrazobenzene required for this study were synthesized using the reactions in Schemes 15, 16 and 17, respectively. 

TABLE 1. The nitrogen, carbon-13 and secondary hydrogen-deuterium kinetic isotope effects found for the one- and two-proton benzidine rearrangements... 

Yin, Y, Fang, J. H., Watari, T., Tanaka, K., Kita, H. and Okamoto, K. 2004. Synthesis and properties of highly sulfonated proton conducting polyimides from bis(3-sulfopropoxy)benzidine diamines. Journal of Materials Chemistry 14 1062-1070. 

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. 

It was concluded that non-protonated 3,3 -dichlorobenzidine is subject to hydrophobic bonding to some extent (Boyd et al. 1984). It is clear from these studies that adsorption constants for 3,3 -dichloro-benzidine cannot be accurately predicted for a given soil based only on a K, value. 

The limited information that is available suggests that 3,3 -dichlorobenzidine may photolyze in water to yield benzidine, which is more photostable yet still toxic. It does not appear that the chemical is susceptible to any other transformations in water except protonation by acid-base reactions. 

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. 

Hydrolysis of Schiff bases derived from benzidine (4,4 -diaminobiphenyl) and from substituted benzaldehydes has been studied in aqueous ethanol 34 attack of water molecules on the protonated substrates is suggested as the rate-determining step. 

It is worth mentioning that movement of the CBPQT4 ring from the benzidine unit to the biphenol unit could also be effected by protonation of the benzidine nitrogen atoms by trifluoroacetic acid, making the unit a much weaker it-electron donor. The recovery stroke of the CBPQT4 ring could be effected by the addition of a stoichiometric amount of pyridine, which deprotonates the protonated benzidine unit. 

The [2]rotaxane 224+ can be switched (Figure 15) by controlling the pH. Upon addition of an excess of trifluoroacetic acid, the benzidine unit becomes protonated, generating the [2]rotaxane [22-2H]6+. The tetracationic cyclophane moves away from this newly generated dicationic unit because of electrostatic repulsion. In this case, the absorption spectrum lacks the 690 nm band, confirming the deprotonation of the benzidine unit and the relocation of the cyclophane to the biphenol unit. The [2]rotaxane [22-2H]6+ can be subsequently de-protonated by the addition of pyridine, regenerating the [2]rotaxane 224+. 

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