A different mechanism for the reductive dechlorination of chlorinated ethenes: Kinetic and spectroscopic evidence

Reductive dechlorination is the most common reaction in the remediation of groundwater and soils contaminated with chlorinated compounds. The reaction that occurs in anaerobic bacteria can also be catalyzed by vitamin B12 and titanium citrate. Reductive dechlorination without the release of chlorinated ethene intermediates from the chloroalkylcobalamin complexes is proposed as an alternate reaction pathway fur the reductive dechlorination of chlorinated ethenes. The revised scheme is supported by (a) the identification of several chloroalkylcobalamin intermediates by direct liquid injection of the reaction mixtures into an electrospray mass spectrometer, (b) the simultaneous presence of all the dechlorination intermediates in the mixtures, and (c) gas chromatographic data showing rapid formation of ethene and acetylene in the presence of a large excess of the primary substrates. Homolytic cleavage and titanium-catalyzed elimination are presented as competing mechanisms for the formation of the products from the alkylcobalamin intermediates. The distribution of dechlorination products was dependent on the availability of titanium from different chelating agents. This means that it may be possible to favor the formation of the fully dechlorinated products and to reduce the release of undesirable intermediates such as vinyl chloride by adjusting the amount and type of titanium chelate used.