Azomethane photolysis

The light intensity was estimated in several experiments using azomethane photolysis as an actinometer. The quantum yield of nitrogen was taken as unity (13). Azomethane was prepared by a modification of the method of Renaud and Leitch (18). 

An analogous reaction of low efficiency has been postulated by Rebbert and Ausloos to explain their results from azomethane photolysis (17). 

In the gas phase however decomposition can occur with unit efficiency Here electronically as well as vibrationally excited molecules are involved in the decomposition step. The azomethane photolysis in the gas phase leads to decomposition with s2 = 1 at lower pressures 55b>. With increasing pressure the quantum yield < N2 drops from 1 to lower values. This is attributed to partial intersystem crossing of the excited azoalkane to the triplet state which gives only dsjtrans isomerisation. 

The cage effect affects the product composition products of the intracage recombination of radicals are formed. For example, the decomposition of azomethane in isooctane gives 65% ethane calculated per decomposed azomethane, whereas in the gas phase only about 3% ethane are formed among its decomposition products. Such a great difference is the result of an intense intracage recombination of methyl radicals. However, when the yield of ethane in azomethane photolysis in a solution is extrapolated to 0, it is close to unity. 

Photolysis in the gas phase leads to the quantitative production of nitrogen and methyl radicals. Photolysis in solution, however, results in a shift in the absorption spectrum to longer wavelengths due to the production of a new species, which is identified as the cw-azomethane (the trcms configuration is the normal isomer). Similarly, irradiation of tro/u-azoisopropane<3) results in trans-cis isomerization to the cis isomer ... 

Free radicals formed from an initiator in the gas phase take part in other reactions and recombine with a very low probability (0.1-2%). The decomposition of the initiator in the liquid phase leads to the formation of radical pairs, and the probability of recombination of formed radicals in the liquid phase is high. For example, the photolysis of azomethane in the gas phase in the presence of propane (RH) gives the ratio [C2H6]/[N2] = 0.015 [76]. This ratio is low due to the fast reactions of the formed methyl radicals with propane ... 

No spin correlation effect was seen in photolysis of azocumene,ls9b perfluoroazomethane,196 or in azomethane itself.160,193 These results are again explained by the lack of any real triplet sensitized decomposition. 

Recently, an experimental reappraisal of this rearrangement was reported, in which cubyl phenyl ketone tosylhydrazone (1) was thermolyzed in an ethanolic sodium ethoxide solution to give a good yield of a 1.5 1 mixture of 9-ethoxy-9-phenylhomocubane (3) and 9-ethoxy-l-phenylhomocubane (4).3 In addition, photolysis (mercury arc, Pyrex filter) of cubylphenyldi-azomethane (2) in ethanol also produced a similar result.3... 

Indeed, determination of the distribution of the label in the ethers shows convincingly that l-phenylhomocub-9-ylidene (6) is formed via the C —C bond migration from 9-phenylho-mocub-l(9)-ene (5).4 Nonetheless, unequivocal evidence for the presence of 1 -phenylhomocub-9-ylidene (6) was also obtained from a trapping experiment. Photolysis of cubylphenyldi-azomethane (2) in neat (Z)-but-2-ene at —78 C afforded the two cyclopropane adducts 7 and 8. As expected, a single adduct 9 was obtained when cubylphenyldiazomethane (2) was irradiated in ( >but-2-ene.3... 

Fig. 8-5. Time dependence of the concentration of monomeric CH3NO in the photolysis of approximately equimolar mixtures of azomethane and nitric oxide [Me2N2] = 5, 11, 20, 40, and 80 torr for runs 1-5, respectively the break in each curve represents the point at which the arc was turned off in that experiment (from Calvert, Thomas, and Hanst82 with permission of the American Chemical Society).

Hanst and Calvert64 photooxidized azomethane at room temperature proceeding to about 50% decomposition. They put forward the idea that ozone was an intermediate in the oxidation and showed that it was removed rapidly only when photolysis was taking place. However, this merely proves that ozone will react with one of the radicals produced in the system. Similarly, the test which they used for ozone (tetramethyl-ethylene) was not shown to be specific to ozone and, indeed, is not likely to be. They were unable to detect methyl hydroperoxide in these experiments under conditions in which it was shown to be peculiarly stable at 200°C. for several hours. Formic acid was shown to be the major product of the prolonged further oxidation of formaldehyde in the presence of 1 atm. of oxygen (initially). 

The extreme sensitivity of CH3CHO to chain decomposition makes it very susceptible to free radical sensitization. Thus OH3 radicals from the pyrolysis of azomethane can induce the chain decomposition in CIIsCHO at 300°C, the chain length being as great as 500. The photolysis of azomethane at room temperature can also sensitize the decomposition. Letort showed that CH3 radicals from dtBP will decompose as many as 50 molecules of CH3CHO per molecule of dtBP at 160 0. 

Dodd , as well as Ausloos and Steacie applied relation (35) to the experimental results of acetaldehyde photolysis. The data obtained at high temperatures seem to fit the Arrhenius straight line derived from the azomethane-acetaldehyde and di-/-butyl peroxide-acetaldehyde systems. At low temperatures, however, considerable deviation from this line could be observed, the possible result of additional methane producing reactions of some sort. According to Dodd, these processes could be (i) primary process II, (//) disproportionation reaction (26) and (in) the wall reaction of the methyl radicals. 

O, direct photolysis at 3130 A O, CH3 from azomethane 0, CH3 from di-f-butyl peroxide. 

The occurrence of trans cis isomerization was reported by Hutton and Steel . In liquid phase photolysis the quantum yield of nitrogen production falls off markedly (0.17-0.01 depending on solvent) and cw-azomethane builds up in the system to a photostationary level ( 10 %). It was also possible to isolate the cis isomer and obtain it pure. 

The vapor phase y-radiolysis of azomethane " also appears to proceed via an electronically excited molecule formed either by neutralization or direct excitation. The products are similar to those obtained in photolysis, implying thermalized radicals in the reaction. An additional minor primary step is H-atom split, giving rise to H2 formation. Scavenger experiments revealed the occurrence of a small amount of molecular CH4 and CjHg elimination as well. 

Bartlett and Engel investigated the liquid phase direct and sensitized photolysis of 2,2 -azoisobutane. They showed that direct photolysis and singlet photosensitization were efficient to induce decomposition of azoisobutane (at 20 °C) but triplet photosensitization was not. The measured values of quantum yields were surprisingly large, values obtained in the photolysis of azomethane, azoethane or azoisopropane under similar conditions. 

The rate of decay of nitrosomethane (formed in the photolysis of azomethane-nitric oxide mixtures at 25° C) to form the stable dimer, reaction (2), was measured by infrared techniques and the homogeneous second order rate coefficient = 87 l.mole. sec was derived. The cis and trans iomers of nitrosomethane have been characterized and the experimental conditions which favour the formation of each of the dimers and the rearrangement to formaldoxime have been described ... 

Very high pressure (1-200 bar) room temperature data have been obtained by Hippier et al. [61] by following the [CH3] produced by the photolysis of azomethane with UV absorption spectroscopy. The agreement with the data of McPherson et al. [58] is excellent and shows no pressure dependence between 1 and 10 bar. Thereafter there is a slight decrease in the rate due to the onset of diffusion limited kinetics. 

Temperatures of around 1000 K are the upper limit for conventional flash photolysis experiments, higher temperatures require specially designed apparatus or shock tubes. There have been three shock tube studies of reaction (32). Glanzer et al. [62] determined k22 at 1350 K, between 1 and 25 atm, initiated by the rapid thermal decomposition of azomethane with the methyl radical concentrations being monitored by UV absorption. Direct measurements of the absorption coefficient at 1400 K were used to determine absolute methyl radical concentrations. Similar measurements were performed by Hwang et al. [63]... 

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