Buffer Salts Enhance Peroxide-Assisted Defluorination of the Sulfuryl Fluoride Fumigant by Both pH Control and General Base Catalysis: Implications for Hydrogen Peroxide as a Nucleophilic Reagent

Sulfuryl fluoride (SF, SO2F2) is an effective and widely used fumigant but poses a threat as a potent greenhouse gas. We recently proposed a simple and effective way to remove spent SF by passing exhaust fumes through a scrubber liquid containing alkaline hydrogen peroxide (HP). Such a solution rapidly and quantitatively defluorinates SF by nucleophilic displacement of fluoride by the perhydroxyl anion (HO2–). In this paper, we show that HP-assisted defluorination of SF is aided by including a buffer. At high pH (>∼11), the buffer (phosphate or carbonate) helps maintain pH, which is driven downward by the consumption of hydroxide, thereby prolonging the life of the scrubber solution. At lower pH (6.5 or 8), we found that the buffer ion (formate, acetate, bicarbonate, or phosphate) participates as a general base catalyst that assists in the removal of a proton in the rate-determining step. The general base-catalyzed rate depends on buffer ion concentration and on basicity (with qualifications) and is many times faster than the reaction with HP alone. We propose that general base catalysis occurs by reaction of SF with a reversibly formed base–HP hydrogen-bonded complex, a mechanism that circumvents the statistical improbability of a three-body collision in dilute solution. General base catalysis operates on undissociated HP at all pH. Thus, the reaction at any pH is a combination of specific nucleophilic displacement by HO2– and general base catalysis operating on the neutral form of H2O2. The finding of general base catalysis at circumneutral pH points to the feasibility of scrubber formulations that avoid high alkalinity and suggests a novel pathway for contaminant removal in reactions in which HP is employed in a nucleophilic role.