Evaluation of phosgenation reactions

An understanding of the sources and ensuing reactions of chlorine-containing compounds in the stratosphere is essential for the evaluation of their impact on the ozone layer. Whilst phosgene should be largely removed in the troposphere, it is anticipated that oniy slow photolysis will occur in the stratosphere. 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

Furthermore, based on specific color reactions, indicator papers for the continuous measurement of ammonia, bromine, chlorine, hydrocyanic acid, hydrofluoric acid, ozone, phosgene, sulfur dioxide, and hydrogen sulfide are commercially available using photometric evaluation. Because of the often extremely intense coloration, detection hmits as low as the lower ppb range can be achieved by this method. Table 6.34 gives an overview of the range of determination and the detec-... 

To reduce the high costs of a linear synthesis a convergent approach was evaluated using trimethylaminoguanidine as intermediate. Following the concept to use a new intermediate in different ways a lot of chemistry was performed, including reaction with phosgene (Scheme 10.4). 

Sakakura et al. (150) have investigated the use of SCCO2 for the selective synthesis of dimethyl carbonate (DMC) by reaction of CO2 with trimethyl orthoacetate, which also produces methyl acetate (AcOMe) as a byproduct and dimethyl ether as a side product (Scheme 5). A primary incentive of this study was to evaluate the use of SCCO2 as an alternative to phosgene for this reaction. Various metal alkoxides with and without promoters were evaluated as catalytic systems, and the reaction was found to be strongly dependent on the alkox-... 

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