An Efficient Electrochemical Carboxylation of Polychloromethanes at Zinc Cathode in Acetonitrile

Abstract

An efficient and selective electrochemical carboxylation process of some polychloromethanes was developed to synthesize chloroacetic acids using a zinc cathode and sacrificial anodes under a galvanostatic regime. The electrochemical behavior of carbon tetrachloride ͑CT͒, chloroform ͑CF͒, and its carboxylated products was studied by cyclic voltammetry in acetonitrile using tetrabutylammonium perchlorate as a supporting electrolyte in the presence and absence of CO 2 . Galvanostatic electrolyses of CT and CF in the presence of CO 2 were carried out in an undivided cell. The influence of some operative parameters on the performance of the process was studied. The electrochemical carboxylation of CT resulted in high faradaic yields and in the selective formation of trichloroacetate ͑TCA͒. The electrocarboxylation of CF led to high current efficiencies and a good dichloroacetate yield. TCA and dichloromethane ͑DCM͒ were obtained as by-products in this process. The formation of TCA and DCM suggests that the electrocarboxylation reaction of CF competes with the self-protonation of the carbanion CHCl 2 − generated in its reduction process.The polychloromethanes are considered to be among the most harmful pollutants due to their high toxicity and carcinogenic character. In fact, the Montreal Protocol, now signed into a treaty, recommended a complete ban on chlorofluorocarbon refrigerants that are synthesized using carbon tetrachloride ͑CT͒ and chloroform ͑CF͒ once their role in ozone layer depletion was recognized. Furthermore, the existing regulation ͑European Community͒ no. 1907/ 2006 of the European Parliament and Council considers CT and CF to be hazardous substances, and the U.S. Environmental Protection Agency classifies these chemicals as possible human carcinogens. Thus, the use of CT and CF has been restricted or at least reduced significantly, their main application being as organic solvents. Despite these restrictions, CT and CF are still being indirectly produced mainly in two processes: in the industrial synthesis of chloromethanes and in the disinfection of drinking water with chlorine. 1,2 A wide range of techniques aimed at the elimination of CT is under development. 3-7 However, all of them have in common the formation of unexploitable substances and pollutants.In previous work, one of the methods proposed for the elimination of CT was based on its electrochemical reduction under mild conditions, 8,9 the main product of this reduction being CF. However, this does not constitute a solution because the use of CF itself is being drastically limited for the reasons given above.However, carbon dioxide is one of the main greenhouse gases. In light of these considerations, we propose the electrochemical carboxylation of polychloromethanes as a synthetic process toward carboxylic acids and as a "green" process for the conversion of these pollutants.Many studies have been devoted to the electrocarboxylation of aromatic organic halides as a way to prepare carboxylated products. 10-21 However, studies of electrocarb...

Electrocarboxylation: towards sustainable and efficient synthesis of valuable carboxylic acids

“…The electrocarboxylation of organic halides can also be considered as an alternative dechlorination pathway for chlorobenzenes [ 122 ] and polychloromethanes [ 142 ]. While the synthesis of halogenated reagents is rather hazardous, the electrocarboxylation of organic halides is an interesting method to revalue waste products, like for example carbon tetrachloride, a toxic liquid, causing ozone depletion.…”

Sustainable Generation of a Homogeneous Ni(I) Catalyst in the Cathodic Compartment of a Divided Flow Electrolytic Cell for the Degradation of Gaseous Carbon Tetrachloride by Electroscrubbing

“…The majority of electrochemical degradation studies on CVOCs have targeted liquid CVOC removal due to groundwater contamination. − Two electrocatalytic processes have been devised for this purpose; that is, direct electrocatalytic oxidation (DEO) using an undivided electrolytic cell and boron doped diamond (BDD) and PbO 2 electrodes, which achieved complete mineralization of CVOCs, , and direct electrocatalytic reduction (DER) using a similar undivided electrolytic cell with Ni or Ag cathodes, which achieved good degradation efficiencies for CVOCs present in surface and groundwater aquifers. , Several studies have shown cathodic DER is more favorable for CVOC degradation, − and to enhance this process, researchers have utilized a packed bed multiphase iron oxide column, a concentric 3D copper column, and a granular graphite reactor . Furthermore, the DER process has been carried out in several aprotic media, such as, acetonitrile (AN) and dimethylformamide (DMF) and in ethanol/water mixtures using different Zn, Ag, and carbon cathodes to synthesize useful compounds from CCl 4 . ,, On the other hand, it has been proposed that secondary products generated by reduction directly on an electrode’s surface by hydrogen adsorption or water splitting could be utilized to degrade target organic pollutants, considered as indirect reduction processes, as shown below: …”

“…21 Furthermore, the DER process has been carried out in several aprotic media, such as, acetonitrile (AN) and dimethylformamide (DMF) and in ethanol/water mixtures using different Zn, Ag, and carbon cathodes to synthesize useful compounds from CCl 4 . 9,10,22 On the other hand, it has been proposed that secondary products generated by reduction directly on an electrode's surface by hydrogen adsorption 23 or water splitting 13 could be utilized to degrade target organic pollutants, considered as indirect reduction processes, as shown below:…”