RADICAL-MEDIATED DEGRADATION MECHANISMS OF TRIBROMO-ACETIC-ACIDS AND OTHER TRIHALOGENATED ACETIC-ACIDS IN OXYGEN-FREE SOLUTIONS AS STUDIED BY RADIATION-CHEMISTRY METHODS

(CBr2CO2-)-C-. and (CCl2CO2-)-C-. radicals, generated upon one electro n reduction of tribromo- and trichloroacetic acids and (CF2CO2-)-C-. r adicals produced from difluoroacetic acid by reaction with (OH)-O-., e xhibit optical absorptions in the UV with lambda(max) at 290 nm (epsil on = 2580 dm(3) mol(-1) cm(-1)), 330 nm (epsilon = 3000 dm(3) mol(-1) cm(-1)) and 310 nm (epsilon = 660 dm(3) mol(-1) cm(-1)), respectively, Mechanistically, the present report focuses on the free-radical-induc ed degradation of tribromoacetic acid. Absolute rate constants have be en determined for the reactions of CBr3CO2- with e(aq)(-), H-., CO2.-, (CH2OH)-C-., (CH3CHOH)-C-., (CH3)(2)(COH)-C-. and (CH3)-C-. radicals to be k = 1.8 x 10(10), 1.5 x 10(10), 2.8 x 10(9), 1.6 x 10(9), 2.3 x 10(9), 3.0 x 10(9) and 3.0 x 10(7) dm(3) mol(-1) s(-1), respectively, The major fate of (CBr2CO2-)-C-. is self-termination to yield tetrabro mosuccinic acid which, however, is unstable and thermally decomposes t o HBr, CO2 and tribromoacrylic acid, Dibromofumaric acid, dibromomalei c acid and carbon monoxide were found as minor secondary products, for mation of which is explained by a small yield of reductive decompositi on of the transient tetrabromosuccinic acid, A complete and mechanisti cally satisfying material balance is provided for several systems in w hich CBr3CO2- has been degraded via a variety of radicals under variou s conditions, (OH)-O-. Radicals do not react directly with CBr3CO2-. T hey have been shown, however, to contribute indirectly to the degradat ion of this acid via their reaction with reductively liberated bromide ions, The Br-. atoms formed in this process are considered to abstrac t a bromine atom from CBr3CO2- or oxidize the carboxyl function in a o ne-electron transfer process. The formation of free Br-. atoms has bee n recognized by pulse radiolysis through their conjugate dimer radical anions Br-2(.-). With respect to the other trihalogenated acids it is noteworthy that CCl3CO2- is efficiently reduced by CO2.- radicals and that CF3CO2- exhibits a high stability toward gamma-irradiation and p ractically resists any reductive attack.