Abstract
Methods for the syntheses of 4,4'-dichlorobenzophenone either directly, or indirectly via bis-(4-chlorophenyl) methane or 1,1-bis-(4-chlorophenyl)-ethane, have been examined. There are no less than ten direct and twenty-five indirect methods that may be used for the syntheses. Of these, two direct methods from phosgene and 4-chlorobenzoyl chloride and five indirect methods from 4-chlorobenzyl chloride, methylene chloride, acetylene, vinyl chloride and 1,1-dichloroethane were chosen for a detailed examination. All of these involve Friedel-Crafts reactions with the same substrate, chlorobenzene. Up to six possible isomers of di-(chlorophenyl) compounds can be expected from these reactions. In order to identify the isomers and to estimate the isomer ratios obtained in the above methods, all six isomers of dichlorobenzophenone, di-(chlorophenyl) methane and 1,1-di-(chlorophenyl)ethane were first prepared by unambiguous procedures; several of these are new compounds, not previously reported in the literature. It was thus found that either gas-liquid chromatography or 'its combination with nuclear magnetic reasonance enabled the isomer ratios to be determined in mixtures containing all six isomers. It was thus found that the maximum percentages of the required 4,4'-isomer obtained in the various reactions studied experimentally were 43 (phosgene), 100 (4-chlorobenzoyl chloride), 61 (4-chlorobenzyl chloride), 11 (methylene chloride), 34 (vinyl chloride), 36 (1,1-dichloroethane). Previous workers obtained 50% of the 4,4'-isomer from acetylene. On the basis of the present experimental work and the previous work reported in the literature, estimates were made of the costs involved in the various processes. It was concluded that on a laboratory scale, the synthesis from 4-chlorobenzoyl chloride was both the most convenient and the cheapest method available. On a larger scale, the synthesis from, vinyl chloride and phosgene were the cheapest, but the latter was the more convenient of the two. Since similar isomer ratios of 1,1-di-(chlorophenyl) ethanes were obtained in reactions with vinyl chloride and 1,1-dichloroethane, the mechanism of these reactions were investigated and it was found that the reaction with vinyl chloride proceeds via 1,1-dichloroethane.