Mercury, photosensitized decomposition

The mercury-photosensitized decomposition of N20 was studied first by Manning and Noyes160 and later in greater detail and more quantitatively by Cveta-novic161,162. The primary reaction step is... 

Cvetanovic67 was concerned with oxygen atom reactions with unsaturated hydrocarbons. The oxygen atoms were obtained in his experiments by mercury-photosensitized decomposition of N20. Cvetanovi6 came to the conclusion that the reaction of oxygen atoms with ethylene proceeded essentially with scission of the hydrocarbon bond, while with higher olefins this was not observed. Corresponding oxides (epoxides) and carbonyl compounds were formed in the course of the reaction. 

Gunning66 has shown that disproportionation takes place when CF2C1 radicals are generated by the mercury-photosensitized decomposition of CF2CI2 and in recent studies confirming his results47 it was found that the disproportionation/combination ratio was again 0.04. 

The mercury-photosensitized decomposition of ketene27 has recently been shown to yield methylene that is believed to be in a triplet state by the rule of spin conservation. 

Duncan and Cvetanovic27 studied the reaction with isobutene of methylene generated by the mercury photosensitized decomposition of CH2CO, which is believed to produce triplet methylene. Product ratios reached high-pressure limiting ratios at 200 mm. The observed yield of... 

Methylene produced by mercury photosensitized decomposition of 011200 was found to add to cfs-butene-2 to give product ratios similar to those obtained by Frey under conditions of high dilution with argon. Addition of this type of methylene to (rans-butene-2 gave similar product ratios, with the exception that the trans isomers of 1,2-dimethylcyclo-propane and pentene-2 were favored over the cis. These results were taken as proof of the nonstereospecific addition of triplet CH2 to olefins. 

Wright and Winkler456 postulated a sequence of reactions based on a mechanism suggested by Strausz and Gunning405 for the mercury-photosensitized decomposition of NO. Strausz and Gunning proposed... 

The mercury-photosensitized decomposition of 1,1-difluoroethene, and of ( )- and (Z)-1.2-difluoroethene, proceeds by elimination of hydrogen fluoride via an excited molecule mechanism to produce fluoroacetylene (12).55... 

Basseches, H., The Mercury Photosensitized Decomposition of Water Vapor, Ph.D. Dissertation, The Ohio State University, 1951. 

Before mercury-photosensitized decomposition of nitrous oxide could be used as a technique for generation of oxygen atoms, it was necessary to establish unambiguously the primary step in this reaction. Early investigation (70) could not distinguish with certainty between the two possible primary steps... 

Falconer and Cvetanovic (40) attempted to obtain a more quantitative value for the fraction of nonterminal addition in the case of propylene. They produced hydrogen atoms by mercury photosensitized decomposition of H2, using at least 100 times as much H2 as C3H6 and total pressures of 40 and of 250 mm. Under these conditions the reactions of importance were the combination and disproportionation of the iso- and n-propyl radicals and their cross reactions, the combination of the two radicals with H atoms (assumed to be equally probable), and a very small amount of decomposition of hot n-propyl radicals. Disproportionation to combination ratios were taken as 1.64 for two iso-propyl, 1.14 for two w-propyl, and hence 1.39 was taken as the mean of the two values for one iso- and one n-propyl radical. Using these values and the analysis of the products, the nonterminal addition of H atoms to C3H6 and C3D6 was found to amount to 6 1%. 

Chesick (75), in a study of the mercury-photosensitized decomposition of C3H8 and CH3CD2CH3, in the presence of ethylene, has also observed a large isotope effect, attributable to the primary process. 

Fig. 20. Yield of oxime aa a function of the flow rate in the mercury-photosensitized decomposition of propane, in the presence of NO. —2 mole- %NO O—25 mole-% NO.

Fig. 21. Yield of oxime as a function of the nitric oxide concentration in the mercury-photosensitized decomposition of various propanes. O—C3H3 S—C3Da C —CH CD2CH3 -CDaCH-CIU...

Fig. 22. The yield of w-propyl radicals from the mercury-photosensitized decomposition of various propanes as a function of nitric oxide concentration. O—CjH8 ...

Fig. 23. Yield of n-propyl radicals as a function of added 1-butene in the mercury-photosensitized decomposition of propane. —C3D8 O—C3Ha (2 mole-% NO).

We can therefore write the detailed reaction path for the mercury-photosensitized decomposition of the propane molecule as follows ... 

Recent studies of the photolysis of bicyclic ketones have led to the preparation of unusually strained systems by the eUmination of carbon monoxide from excited ketone molecules. Irradiation of Formula 286 gives bicydo [2.2.0[hexane (Formula 287) in low yield (118). Mercury-photosensitized decomposition of bicyclo [2.2.1 ]heptan-2-one (Formula 258) gives bicyclo [2.2.1 [hexane (Formula 289) in 20% yield (119). Camphor (Formula 290) undergoes a similar mercury photosensitized decomposition to 1,5,5-trimethylbicyclo [2.2.1 [hexane (Formula 291) (10%) (119). 

Radical yields are usually below unity for mercury photosensitized decompositions. 

Monogermane was first shown to undergo mercury photosensitized decomposition at low pressures and at 298 K through the overall equation (31). 

The formation of cyclopropane as a minor primary product in the mercury photosensitized decomposition of propylene has been reported/ and the formation of methylcyclopropane from but-l-ene. (This may be evidence for the formation of some triplet propylene.)... 

The possibility that two states for carbenes may be of importance in their chemistry has received attention in the case of methylene itself, and there is much experimental evidence on this subject. In the case of ethylidene two such states have been postulated to account for the results from the mercury-photosensitized decomposition of ethylene. We will, however, defer discussion of this point until later in this chapter. The occurrence of methyl migration in carbenes [reaction (48) ] is a reaction that has already been noted in other systems. ... 

The mercury-photosensitized decomposition of ethane has been studied by Bywater and Steacie , and by Back who worked at very low conversions in order to avoid back reactions. Back s results, otained at 240 °C, are consistent with the following simple mechanism... 

The mercury-photosensitized decomposition of propane has been investigated by Steacie and Dewar , Bywater and Steacie , Woodall and Gunning , Chesick Back , Back and Fakamuki and Gunning et At temperatures below 300 °C and at low conversions the results can be explained by the simple mechanism... 

The mercury-photosensitized decomposition of -butane has been investigated by Hay and Winkler , Bywater and Steacie and by Cvetanovic et a/. . At lower temperatures the main reactions are... 

At higher temperatures thermal decomposition of the C4H9 radicals occurs, leading to chain processes. The mercury-photosensitized decomposition of isobutane has also been investigated by several workers ". ... 

A number of investigations have been made of the mercury-photosensitized decomposition of cyclopropane (Gunning and Steacie Ivin Rabinovitch et Strausz et There has been some difficulty in interpreting the re-... 

Studies by Gunning et al. of the mercury-photosensitized decompositions of cyclopentane and cyclohexane indicate that the main primary steps are now the hydrogen-atom elimination reactions... 

The mercury-photosensitized decomposition of benzene has been investigated by Scott and Steacie . The main initiation step appears to involve energy transfer, viz. 

Recent work on the mercury photosensitized decomposition of aziridine has shown that initial rupture of the N-H bond leading to the ethylenimino radical is followed by dimerization, viz. 

Relatively few rate constants are available for the alkyl homolysis reactions mainly because clean sources of the alkyl radical have proved difficult to find. Consequently, the data are not always reliable, but some check is available [64, 65] from thermochemical and kinetic data for the reverse reaction. Direct photolysis of azo-compounds and mercury-photosensitized decomposition of alkanes have so far provided the most reliable (although old) data [64]. For good results, the method depended on precise product analysis in the early stages of reaction, with equation (1.9) used to determine where Rabs and Rr r are the initial rates of formation... 

Pottie, R.F..A.G. Harrison, and F. P. Lossing Free Radicals by Mass Spectrometry XXII. Primary Decomposition Steps in the Mercury-Photosensitized Decomposition of Methanol and Dimethyl Ether. Can. J. Chem. 39, 102 (1961). 

---