Pressure Effects on Friedel-Crafts Alkylation Reactions in Supercritical Difluoromethane

• Published in: Chemphyschem Vol. 6; no. 3; pp. 466 - 472
• Main Authors: Abbott, Andrew P., Corr, Stuart, Durling, Nicola E., Hope, Eric G.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 11.03.2005; WILEY‐VCH Verlag; Wiley
• Subjects: alkylation; Catalysis; Chemistry; dielectrometry; difluoromethane; Exact sciences and technology; General and physical chemistry; solvent effects; supercritical fluids; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; solvent effects; Ether; Dielectric properties; Acid catalysis; difluoromethane; Supercritical state; alkylation dielectrometry; Alkylation; Pressure effect; Friedel Crafts reaction; supercritical fluids; Supercritical solvent; Benzenic compound
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200400363

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. Reagent clustering is shown to affect the reaction dynamics and results in the ability to change the product distribution of a Friedel–Crafts (FC) alkylation reaction with pressure (see graphic). Dielectrometry is used as a novel technique for following the rate of an FC reaction in supercritical difluoromethane.