Pressure effects on Friedel-Crafts alkylation reactions in supercritical difluoromethane

Dielectrometry is used as a novel technique for following the rate of a Friedel-Crafts alkylation reaction in supercritical (sc) difluoromethane. Although the process was not optimized, good product yields were obtained and reaction rates were found to be larger than in CO2 at comparable conditions. The reaction order is determined using the initial rate method and the reaction is shown to be first-order with respect to both anisole and t-butyl chloride. The reaction rate constant is unaffected by pressures above 120 bar but close to the critical pressure the value decreases with increasing pressure. It is also shown that the product distribution for the alkylation of anisole shows significant pressure dependence with substitution at the ortho-position being favored at lower pressures, which is ascribed to hydrogen bonding. This pressure dependency is not observed in sc CO2 or using toluene as a substrate, which supports the idea that hydrogen bonding may be important in the reaction mechanism. The effect of the different reagents and temperature on the rate of the alkylation reaction was also determined.