Methyl iodide photolysis

The absorption spectrum of the methyl radical has been observed by Herzberg11 during the flash photolysis of methyl iodide and bromide, but... 

Unfortunately, it is not possible to determine the maximum vibrational energy content of the methyl radicals produced in the photolysis of methyl iodide. On the basis of the arguments outlined above, strong excitation of the C-H stretching vibration would probably not be anticipated. The methyl radical is planar, however,36 so that its deformation vibration would be excited. On the other hand, the halomethyl radicals are believed to be pyrimidal,37 and unless the grouping has a planar configuration in the excited parent polyhalomethane, deformation would not make any appreciable contribution to the vibrational excitation of the radical. 

Certain divinyl disulfides (a) are readily available by oxidation of dithioic esters in basic solution (Scheme 9). Heating (a R = Me or Ph) in toluene converted it to a mixture of the 3,4-disubstituted 2,5-di(methylthio)thiophene (b) and the corresponding 5-methylthio-2-thiophenethiol (c). Addition of potassium f-butoxide to the toluene resulted in a nearly quantitative yield of the thiol (c), but further addition of methyl iodide in a second step converts (c) to (b) so that the yield of either product can be maximized (74RTC258). The mechanism is the same as that shown in Scheme 7 (Scheme 9). Photolysis of enethiol esters gives divinyl sulfides such as those shown in Scheme 7, and these form thiophenes under the conditions of photolysis (77JOC1142). 

This work was useful in establishing that the low quantum yield in the photolysis of pure methyl iodide was due to back reactions and that the initial decomposition into a methyl radical and iodine atom occurred with approximately 100% efficiency. In 1942, Blaedel et al.22 showed that the quantum yield for the decomposition of methyl iodide was, in fact, somewhat less than unity in the presence of oxygen because Bates and Spence11 had used an incorrect value for the quantum yield of the chloracetic acid actinometer. They suggested the reactions... 

Hot isopropyl radicals are believed to be formed by the photolysis of isopropyl iodide as with methyl iodide.189... 

While a quantum yield in excess of unity is evidence of a chain reaction, it is not true that quantum yields lower than unity indicate the absence of chains. On the contrary, the photolysis of methyl iodide in solutiqn may have a quantum yield of 0.008, while it is known that the first ep is to produce CH3 radicals and I atoms. ... 

Zika R. G., Gidel L. T., and Davis D. D. (1984) A comparison of photolysis and substitution decomposition rates of methyl iodide in the ocean. Geophys. Res. Lett. 11, 353-356. 

Secondary reactions then follow involving these and subsequently formed radicals. The nature of the species formed in the primary photolytic act has been inferred from the product distribution and other kinetic evidence, and also from observations of transient spectra during flash photolysis studies. For example, spectra characteristic of CIO, BrO, and 10 have been observed following the flash photolysis of methyl chloride , methyl bromide and methyl iodide in the presence of oxygen, and the absorption spectrum of the free methyl radical has been ob-served during the flash photolysis of methyl iodide and methyl bromide. 

Early studies on the photolysis of methyl iodide have been summarised by Emschwiller . In the gas phase at 25 °C with A 2537 A the main hydrocarbon products methane, ethane and ethylene, in the approximate... 

The relatively large quantity of methane formed in the gas phase photolysis of methyl iodide is hard to reconcile with step (3) for which the activation energy would be about 9 kcal.mole . Schultz and Taylor showed that methane formation is independent of temperature over the range 40-100 °C and is also reasonably independent of iodine concentration. They therefore proposed that methane formation proceeds via a hot radical mechanism as follows... 

Evidence has also been presented for the importance of hot radical reactions in the liquid and solution phase photolysis of methyl iodide, and in the photolysis of methyl iodide in the presence of hydrogen halides . Majer and Simons , however, have discussed the possibility of the formation of an energised CH3I-CH3 radical complex. 

Benson has suggested that, in the photolysis of methyl iodide, ethylene may be formed via the following steps rather than via steps (7) and (8)... 

The conversion of berberines into spirobenzylisoquinolines has been examined in detail. As described in section 8 photolysis of the betaines (190, R=H) and (190, R=Me) affords the aziridines (198, R=H) and (198, R=Me). Treatment of (198, R= Me) with methyl iodide is accompanied by Hofmann degradation to give the ochotensine analogue (307, R=Me) and treatment with ethyl chloroformate results in a von Braun degredation... 

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