retour à l'accueil

Laboratoire d'Electrochimie Moleculaire, LEM, Paris

UMR CNRS - Université Paris Diderot - Paris France
   
 
Master Frontiers in Chemistry | UFR de Chimie - Université Paris Diderot - Paris 7 CNRS - Institut de chimie Université de Paris Master Chimie Sorbonne Paris Cité UFR de Chimie - Université Paris Diderot - Paris 7 CNRS - Institut de chimie
 |   Ecole Doctorale 388  |    Master Frontiers in Chemistry   |   C'Nano IdF   |   Respore  |
Université Paris Diderot
Université de Paris CNRS, Centre National de la Recherche Scientifique 		 
 


Le LEM - Publications: Abstracts

Publication 571
J. Am. Chem. Soc., 125 (35), 10729 -10739, 2003
DOI: 10.1021/ja036141t S0002-7863(03)06141-9 		 

 


Successive Removal of Chloride Ions from Organic Polychloride Pollutants. Mechanisms of Reductive Electrochemical Elimination in Aliphatic Gem-Polychlorides, ,-Polychloroalkenes, and ,-Polychloroalkanes in Mildly Protic Medium

Cyrille Costentin, Marc Robert and Jean-Michel Savéant

Contribution from the Laboratoire d'Electrochimie Moléculaire, Université Paris 7, Denis Diderot, associé au CNRS (UMR 7591), 2 place Jussieu, case 7107, 75251, Paris Cedex 05, France


The factors that control the successive reductive expulsion of chloride ions from aliphatic gem-polychlorides are investigated, taking as examples the electrochemical reduction of polychloromethanes and polychloroacetonitriles in N,N-dimethylformamide. At each elimination stage, the reaction involves, as a rate-determining step, the transfer of one electron concerted with the cleavage of the carbon-chloride bond. The second step is an immediate electron transfer to the ensuing radical, taking place at a potential more positive than the potential at which the first electron transfer occurs. The carbanion thus formed is sufficiently basic to be protonated by any trace weak acid present in the reaction medium. The three successive elimination steps require increasingly negative potentials. Application of the "sticky" dissociative electron transfer model allows one to quantitatively unravel the factors that control the energetics of the successive reductive expulsion of chloride ions. The large potential gaps between each stage stem primarily from large differences in the dissociative standard potentials. They are also strongly affected by two cumulative intrinsic activation barrier factors, namely, the bond dissociation energy of the substrate that decreases with the number of chlorine atoms and the interaction between chloride ion and the radical that increases in the same direction. In the case of , -polychloroethanes (Cl3C-CCl3, Cl2HC-CCl3, Cl2HC-CHCl2, ClH2C-CHCl2) too, the first step is a dissociative electron transfer with sizable ion-radical interactions in the product cluster. Likewise, a second electron transfer immediately leads to the carbanion, which however prefers to expel a second chloride ion, leading to the corresponding olefin, than to be protonated to the hydrogenolysis product. The ion-radical interaction in the product cluster plays a major role in the control of the reduction potential. The reduction of the , -polychloroethenes (Cl2C=CCl2, ClHC=CCl2, ClHC=CHCl) follows a similar 2e--2Cl- reaction sequence, leading then to the corresponding alkynes. However, unlike the polychloroethane case, the expulsion of the first chloride ion follows a stepwise electron transfer/bond cleavage mechanism. The reduction potential is thus essentially governed by the thermodynamics of the anion radical formation.
 
   
 
© 2005 LEM CréditsContactVenir au LEM