Phenyl azide azobenzene from

A number of by-products may be isolated from sodium stannite reductions. In addition to benzene, aniline gives phenol, azobenzene, phenyl azide, and phenylhydrazine.107 10 108 109 The danger of hydrolytic cleavage in alkaline media, previously discussed in connection with the... 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

Reports of the products formed from photolysis of phenyl azide in unreactive solvents at or near room temperature presented a confusing picture until it was discovered that the result obtained depends critically on the concentration of the azide and the power of the light source. In essence, only two products are obtained from the irradiation of phenyl azide in oxygen free, inert solvents such as hexane or acetonitrile tarry polymer and azobenzene. Very little, if any, of the anilines observed in high-temperature thermolysis of the azide are obtained. 

Kobayashi et al. [86] studied (4-dimethylamino)phenyl azide by means of laser flash photolysis on a picosecond time scale. They found that the triplet nitrene is formed from an unobserved precursor that has a lifetime of approximately 120ps. By consideration of model compounds, these workers suggested that the precursor is the singlet nitrene, but this conclusion must await confirmation by alternative experiments. However, in this work Kobayashi was able to show that triplet (4-dimethylamino)phenyl nitrene reacts to form the (4-dimethylamino)azobenzene by a kinetically second-order process. 

Comparable results are obtained from the nanosecond time scale flash photolysis study of (4-nitro)phenyl azide reported by Liang and Schuster [45]. Irradiation of this azide leads to rapid formation of the triplet nitrene. This nitrene was identified by comparison of the observed spectrum with that recorded at low temperature, and by its observed dimerization to form the (4-nitro)azobenzene. The rate constant for the triplet nitrene dimerization is 1.0 x 109 M 1 s 1 a value approximately 10 times below the diffusion limit. This relatively small rate constant is consistent with the demands of spin statistics. Combination of two triplets to form a singlet product can be successful only one ninth of the time [64],... 

It is clear now that irradiation of phenyl azide at room temperature gives dehydroazepine. At high concentration of azide, the dehydroazepine polymerizes rapidly in competition with its slow isomerization to triplet phenyl nitrene. The major product formed from photolysis of phenyl azide under conditions where its quantum yield for disappearance is claimed to be greater than unity is poly-1,2-azepine [48], not azobenzene. Of course, the polymer does not elute from an HPLC, and analysis of reaction mixtures by chromatography will show only two components. 

Phenylnitrene and its substituted derivatives tuidergo three principal reactions in the gas-phase [Eq. (53)] (1) dimerization to azobenzene (which can also derive from reaction between phenylnitrene and phenyl azide los.m), (2) H-abstraction giving aniline and (3) ring contraction to 1-cyano-cyclopentadiene Yields of these and other products are given in Table 19. 

The second mechanism worthy of consideration involves initial abstraction of one hydrogen by triplet nitrene to form an anilino radical which may subsequently either dimerize or abstract another hydrogen. This is illustrated for the thermal decomposition of p-anisyl azide in cumene. Indirect support for the hydrazo intermediate 37 comes from the observation that thermolysis of phenyl azide in decalin affords hydrazobenzene and benzidine as well as azobenzene and aniline. Anilino radical intermediates also are implicated in the oxidation... 

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