Issue 19, 2012

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

Abstract

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)5Br2+ provided kinetic data for the reaction of ethyl radicals with FeCl2+ (k = (4.0 ± 0.5) × 106 M−1 s−1) and with FeBr2+ (k = (3.0 ± 0.5) × 107 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.

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

Supplementary files

Article information

Article type
Paper
Submitted
27 Jan 2012
Accepted
09 Mar 2012
First published
14 Mar 2012

Dalton Trans., 2012,41, 5974-5980

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

J. M. Carraher, O. Pestovsky and A. Bakac, Dalton Trans., 2012, 41, 5974 DOI: 10.1039/C2DT30210A

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