Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe3+ in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe2+. The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe2+. The photochemistry in the presence of halide ions (X- = Cl-, Br-) generates ethyl halides via halogen atom abstraction from FeXn3-n by ethyl radicals. Near-quantitative yields of C2H5X are obtained at >= 0.05 M X-. Competition experiments with Co(NH3)(5)Br2+ provided kinetic data for the reaction of ethyl radicals with FeCl2+ (k = (4.0 + 0.5) x 10(6) M-1 s(-1)) and with FeBr2+ (k = (3.0 + 0.5) x 10(7) M-1 s(-1)). Photochemical decarboxylation of propionic acid in the presence of Cu2+ generates ethylene and Cu+. Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu+ to Cu2+, and removes gaseous olefins to prevent accumulation of Cu+(olefin) complexes and depletion of Cu2+. The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.