Photosensitised decompositions

The mercury photosensitised decomposition of halogen compounds has been reviewed by Gunning and Strausz and early work was also summarised in a number of papers by Gunning et 

The mercury resonance line at X 2537 A, corresponding to the transition 6(3Pi) 6(1 So), has been used in most studies and monoisotopic photosensitisation has been specifically employed to examine the possible isotopic enrichment of the mercury compounds produced in these reactions. Thus, in the Hg monoisotopic photosensitised decomposition of methyl chloride - the main products are methane, ethane, dichloroethane and hydrogen chloride together with calomel enriched with the ° Hg isotope. The following mechanism has been proposed for the decomposition 

The quantum yields for the formation of methane, calomel and dichloroethane are respectively 0.81, 0.91 and 0.45. 

Ethyl chloride and other alkyl chlorides behave in a manner similar to methyl chloride, but in the case of vinyl chloride a third primary route has been proposed to account for the low efficiency of calomel formation 

Fluorine substituted chloro-compounds have been studied as well - and 

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Mercury-photosensitised decompositions have been reported for fluorinated methanes , fluorinated ethanes , fluorinated ethylenes , perfluorocyclobu-tene and perfluorobuta-1,3-diene , chlorofluoromethane , dichloromethane (ref. 939), and octafluorobut-2-ene . Acetone has been used for sensitising the decomposition of 1,4-dichlorobutane , chlorobutenes , and allylic halides (the last to cyclopropyl halides) other studies have involved the use of hexafluoroacetone and benzene as sensitisers. 

Studies of the mercury-photosensitised decomposition of 2-azetidinone and 4,4-dimethyl-2-azetidinone have been described the major products in the latter case were carbon monoxide, 2-methylpropene and 2,2-dimethylaziridine. Examples of photo-chemically induced N-dealkylation have again been reported. 

Cyclobutadiene is decomposed photolytically into acetylene [56], but 1,3-dimethylcyclobutadiene is photochemically stable [57], It is thought that this may result from the absorbed energy being sufficiently dispersed by the methyl groups to leave not enough available for bond breaking. It appears that alkynes are not formed by photosensitised decomposition of tri-t-butylcyclobutadiene-carboxylic esters [54]. 

A study has been made of mercury-photosensitised decompositions of vinyl fluoride, vinylidene fluoride, and cis- and rra/tr-l,2-difluoroethylene, which proceed by elimination of hydrogen fluoride via an excited molecule mechanism to produce the corresponding acetylenes cis r trans isomerisation... 

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