Defined mechanochemical reductive dechlorination of 1,3,5-trichlorobenzene at room temperature in a ball mill

1,3,5-Trichlorobenzene (TCB) is completely dechlorinated to benzene when treated with magnesium and n-butylamine at room temperature in a ball mill. A full mass balance is observed for its stepwise reductive dechlorination via the intermediates 1,3-dichlorobenzene (DCB) and monochlorobenzene (MCB). This mechanochemical (MC) reaction occurs due to ongoing, simultaneous comminution and mechanical activation of small particle size magnesium during milling. The base metal is utilized as the actual dechlorinating agent whereas n-butylamine as a hydrogen donor. A kinetic study reveals that consecutive and coupled one-pot Grignard-Zerewitinoff reactions occur: MC activated formations of intermediary Grignard components from 1,3,5-TCB, 1,3-DCB and MCB, take place each first, and then stepwise their protonation to DCB, MCB and benzene, respectively, by the amine, which reacts as a (weak) acid towards strong bases such as Grignard reagents. An appropriate reaction model comprising a corresponding set of rate equations (ordinary differential equations (ODEs) representing an initial value problem) could successfully be fitted to two data sets recorded for a representative model degradation reaction of 1,3,5-TCB at two different initial concentrations. The rate constants for the formation of the postulated intermediary Grignard components were computed, hence this method can readily be extended to determine Grignard formation rate constants in general as well. Results are in very good accordance to data recently reported for an analogous MC reductive dechlorination reaction of 1,3-DCB and correspond well, too, to other data previously reported elsewhere. In conclusion, MC reductive dechlorination employing base metals in combination with hydrogen donors can be applied as a promising non-combustion method to the environmentally friendly, defined and well characterized destruction of persistent organic pollutants (POPs) at room temperature.