Tetraphenylhydrazine, dissociation

Tetraphenylhydrazine is a while solid, soluble in chloroform, acetone, benzene, or toluene, and upon standing is changed into triphenylaminc plus azobenzene. In solution, tetraphenylhydrazine dissociates into nitrogen diphenyl. (Ct,Hs) N-. free radical, which in toluene at 90cC reads with nitric oxide, NO, Tetraphenylhydrazine is formed by oxidation of diphenylamine,. by lead dioxide. 

The reversible dissociation of colorless tetraphenylhydrazine into the green diphenylaminyl (15) at 100°C was discovered by Wieland in 1911 (equation 7), and extended the free radical concept to nitrogen. These radicals are trapped by nitric oxide to give diphenylnitrosoamine (16), and the rate of this reaction was used to show the rate of dissociation of the hydrazine. ... 

Kavun and Buchachenko (247) have found that the "primary" amino radicals derived from thioamines in the presence of hydroperoxide are very stable and are not readily converted into "secondary" nitroxide radicals. Kommandeur and Wiersma (107) studied the photodecomposition of tetraphenylhydrazine in rigid solution at 77°K, and the observed ESR spectrum was assigned to the dimer of the diphenyl amino radical (248). Shida et al. (249) have also studied the photodimer of tetraphenylhydrazine and the photochromic dimer of triphenylimidazolyl at low temperature by optical and ESR techniques. These authors noted that photolysis of these compounds induces the homolysis while y-irradiation leads to heterolytic dissociation. Blinder et al. (250) reported the ESR spectrum of the monomer tetraphenylpyrryl radical in... 

A photometric method of analysis was used to determine the rate of thermal dissociation of tetraphenylhydrazine" in benzene at 65 to 85 °C, based on the fast reaction between the (CgH5)2N radical and l,l-diphenyl-2-picrylhydrazine forming the intensely coloured l,l-diphenyl-2-picrylhydrazyl radical. The first order rate constant for decomposition hk = 6.15x 10 exp(—28,040/Jir)sec" . In an independent study of the same reaction the activation energy and heat of dissociation for the above dissociation have been determined by an optical method to be 29.2 kcal.mole and 23.5+2 kcal respectively. 

An additional but very important type of quenching process involves the photodissociation of weak bonds, such as found for tetraphenylhydrazine (Scheme 1) where the rate of dissociation into diphenylaminyl radicals shows a strong viscosity dependence. This reaction has been considered in terms of the high damping regime of the Kramers-Smoluchowski theory and it appears that, in addition to the activated process, there is an activationless dissociative pathway that favours formation of free radicals at infinite viscosity. 

The N-centred radical 42a participates in a reversible N-N coupling/dissoci-ation process [3], Tetraphenylhydrazine 43 was formed in the yield of 40% from 42a generated from DPA with a, a-diphenyl-/ -picrylhydrazyl. Various dimeric C-N and C-C coupling products, e.g. 44, 45 were isolated [3-5,53]. 

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