Acetone-photosensitized reduction of carbon tetrachloride by 2-propanol in aqueous solution

Simple carbonyl chromophores can be used to facilitate the solar- promoted reductive dehalogenation of halo-organics. Here we describe the kinetics and mechanism of the reductive dehalogenation of carbon tetrachloride (CT) to chloroform (CF) in the presence of aqueous acetone and 2-propanol, which is added as a hydrogen donor. The reaction is rapid in sunlight, stoichiometrically converting 3 mM CT to CF in approximately 2 min. Under typical conditions (0.69 M acetone, 5.7 M 2-propanol, 3 mM CT) using a 75 W Xe lamp, the observed zero-order reaction occurs at a rate of 2.7 x 10^{- 6} M s^-1. The mechanism appears to become first order in CT near the end of the reaction. The zero-order reaction rate increased with both acetone- and 2-propanol concentration and also with absorbed light intensity. Other combinations of carbonyls, alcohols, and halogenated organics are also effective.