Chlorine atoms, from phosgene

Dialkyl carbonates can be made from the reaction of alcohols with phosgene, a highly toxic gas. The second chlorine atom of phosgene increases the electrophilicity of the carbonyl carbon atom. As in the reaction of an alcohol with an acid chloride, a base is required to neutralize the HCl by-product. 

A different experimental approach to the study of chlorine atom reactions with olefins will be mentioned briefly. Wijnen(l06) has studied the photolysis of phosgene as a source of chlorine atoms in the presence of ethylene, and Guercione and Wijnen (49) have carried out similar experiments with propylene. The features of these processes are quite different from those encountered in photochlorination in the presence of molecular chlorine, since the chain propagating reaction (3) cannot occur. Although in the photolysis of phosgene Cl and COC1 are initially formed, it appears that all COC1 radicals further decompose into CO and Cl. 

A thorough work on the photocatalytic oxidation of TCE was performed by Nimlos et al. (1993), who identified, apart from CO2 and HCl, also CI2, dichloroacetyl chloride (DCAC (CHCI2COCI)), CO, and phosgene (COCI2), known to be much more toxic than TCE. The high quantum efficiency observed in this case (50-80%) was attributed to a chain reaction initiated by the formation of chlorine atoms, similar to the scheme proposed for the... 

From its structure it will be seen that phosgene may be considered as carbonic acid in which both the hydroxyl groups are substituted by chlorine atoms. 

Phosgene is the main oxidation product of dichloromethane, resulting from the abstraction of the molecule s two hydrogen atoms. Further photolysis could lead to chlorine atoms being produced in the atmosphere [36a, 1926a], Further research in this area is clearly warranted. 

Since chlorine atoms, alkyl radicals and chloroaikyl radicals do not attack phosgene, no regeneration of chlorine atoms occurs by secondary reactions. Hence, the principal reaction products, 1-chlorobutane and 1,4-dichlorobutane, are formed from the reactions of the... 

Bromine compounds are much more expensive than chlorine compounds and, since the atomic weight of bromine is about twice that of chlorine, the halogen cost incurred in the use of bromine compounds as intermediates is likely to be at least an order of magnitude greater than the cost of the corresponding chlorine compound. Not least for this reason, the synthesis of phosgene from bromine-containing compounds is unlikely to be economically practical and are probably not worthy of development on a purely commercial basis. However, C-Br bonds are more readily cleaved than C-CI bonds, and bromide is readily oxidized back to bromine. 

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