Mechanistic Considerations on the Hydrodechlorination Process of Polychloroarenes

Abstract

Defunctionalization of organochlorines through reductive dechlorination (also known as hydrodechlorination-replacement of chlorine atoms by hydrogen-is one of the main methodologies used in the detoxification of these harmful compounds. Most of the published papers on this particular matter focused on specific reagents, reaction conditions, and mainly result efficiency. Some of the authors were also concerned with reaction pathways (e.g., the order in which chlorine atoms were removed from a polychlorinated aromatic substrate-polychlorinated biphenyls, PCBs; polychlorinated dibenzo-p-dioxins, PCDDs; or polychlorinated dibenzofurans, PCDFs). However, the papers that dealt with the investigation of reaction mechanism were rather scarce. This chapter presents the advances made by researchers in understanding, from a mechanistic point of view, the hydrodechlorination process, along with our own assumptions. In doing so, it would be easier to predict the behavior of such compounds in a specific environment, showing more clearly the scope and limitations of each process, depending on the reaction conditions and reagents.

Studies of hydrogen isotope scrambling during the dehalogenation of aromatic chloro‐compounds with deuterium gas over palladium catalysts

“…The mechanism of dechlorination of variously substituted aliphatic chlorides on palladium has been investigated in detail and has been shown to be relatively insensitive to substituent effects. 59 However, our findings relate to aromatic dehalogenation, an area for which mechanistic information has been reviewed recently 60 and are broadly consistent with a published tentative mechanism for the palladium-catalysed nonisotopic aromatic dechlorination reaction. 9 This mechanism involves a single electron transfer (SET) from the tertiary amine to the complexed chloroaromatic to give a trialkylaminium radical cation and a chloroaromatic radical anion.…”

“…The literature provides many examples of proposed dehalogenation reaction mechanisms 60 and single electron, ionic or oxidative‐addition/reductive‐elimination processes are all represented. Moreover, the heterogeneous nature of the catalyst has not been definitely confirmed in many, perhaps most, cases 81 .…”