Editing Reductive dechlorination

{{Short description|Chemical reaction which breaks carbon-chlorine bonds via a reductant}}

In [[Organochlorine compound|organochlorine chemistry]], '''reductive dechlorination''' describes any [[chemical reaction]] which [[Bond cleavage|cleaves]] the [[covalent bond]] between [[carbon]] and [[chlorine]] via [[reductant]]s, to release [[chloride]] ions. Many modalities have been implemented, depending on the application.  Reductive dechlorination is often applied to remediation of [[chlorinated]] [[Pesticide|pesticides]] or [[dry cleaning solvent]]s. It is also used occasionally in the [[organic synthesis|synthesis of organic compound]]s, e.g. as pharmaceuticals.

==Chemical==
Dechlorination is a well-researched reaction in organic [[Organic synthesis|synthesis]], although it is not often used. Usually stoichiometric amounts of dechlorinating agent are required. In one  classic application, the [[Ullmann reaction]], chloroarenes are coupled to [[biphenyl]]s.  For example, the activated substrate [[2-Chloronitrobenzene|2-chloronitrobenzene]] is converted into 2,2'-dinitro[[biphenyl]] with a copper - bronze [[alloy]].<ref>{{cite journal|title=2,2'-Dinitrobiphenyl
|author=Reynold C. Fuson |author2=E. A. Cleveland
|journal=Org. Synth.|year=1940|volume=20|page=45
|doi=10.15227/orgsyn.020.0045}}</ref><ref>{{cite journal | last1 = Fanta | first1 = P.E. | year = 1974 | title = The Ullmann Synthesis of Biaryls | journal = Synthesis | volume = 1974 | pages = 9–21 | doi = 10.1055/s-1974-23219 | pmid = 21016995 }}</ref>
[[Image:UllmannReaction.svg|center|Ullmann reaction.]]

[[Zerovalent iron]] effects similar reactions. [[Organophosphorus compound|Organophosphorus(III) compounds]] effect gentle dechlorinations. The products are alkenes and phosphorus(V).<ref>{{cite journal |doi=10.15227/orgsyn.046.0093|author=V. Mark|title=Perchlorofulvalene|journal=Organic Syntheses|year=1966|volume=46|page=93}}</ref>  
*Alkaline earth metals and zinc are used for more difficult dechlorinations.  The side product is zinc chloride.<ref>{{cite journal |first1=Rick L. |last1=Danheiser|first2=Selvaraj|last2=Savariar|first3=Don D.|last3= Chad|doi=10.15227/orgsyn.068.0032|title=3-Butylcyclobutenone|journal=Organic Syntheses|year=1990|volume=68|page=32}}</ref>

== Biological ==
Vicinal reduction involves the removal of two halogen atoms that are adjacent on the same [[alkane]] or [[alkene]], leading to the formation of an additional carbon-carbon bond.<ref>{{cite journal | year = 1992 | title = Microbial reductive dehalogenation | journal = Microbiol Rev | volume = 56 | issue = 3| pages = 482–507 | pmid = 1406492 | pmc = 372880  | last1 = Mohn | first1 = WW | last2 = Tiedje | first2 = JM | doi = 10.1128/mmbr.56.3.482-507.1992}}</ref>

Biological reductive dechlorination is often effected by certain species of [[bacteria]]. Sometimes the bacterial species are highly specialized for organochlorine respiration and even a particular electron donor, as in the case of ''[[Dehalococcoides]]'' and ''[[Dehalobacter]]''. In other examples, such as ''[[Anaeromyxobacter]]'', bacteria have been isolated that are capable of using a variety of electron donors and acceptors, with a subset of possible electron acceptors being organochlorines.<ref>{{cite journal | year = 2004 | title = Anaerobic microbial dehalogenation | journal = Annu Rev Microbiol | volume = 58 | pages = 43–73 | pmid = 15487929 | doi = 10.1146/annurev.micro.58.030603.123600  | last1 = Smidt | first1 = H | last2 = de Vos | first2 = WM}}</ref> These reactions depend on a molecule which tends to be [https://www.sciencedaily.com/releases/2017/01/170131080152.htm very aggressively sought after] by some microbes, [[vitamin B12]].<ref>{{cite web | url=https://www.vice.com/en/article/with-help-from-bacteria-biochemists-learn-how-to-break-up-environmental-toxins/ | title=The Secret to Degrading PCBs and Dioxins is in How Bacteria Breathe| date=2014-10-19}}</ref>

===Bioremediation using reductive dechlorination===

Reductive dechlorination of chlorinated organic molecules is relevant to [[bioremediation]] of polluted groundwater.<ref>{{Cite journal|date=2015-10-01|title=Reductive Dehalogenases Come of Age in Biological Destruction of Organohalides|journal=Trends in Biotechnology|language=en|volume=33|issue=10|pages=595–610|doi=10.1016/j.tibtech.2015.07.004|pmid=26409778|issn=0167-7799|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Lee|first3=Matthew|last4=Manefield|first4=Michael|last5=Marquis|first5=Christopher P.}}</ref><ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Bohl|first3=Susanne|last4=Lee|first4=Matthew|last5=Marquis|first5=Christopher P.|last6=Manefield|first6=Michael|date=2016|title=Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation|journal=Frontiers in Microbiology|language=en|volume=7|page=249|doi=10.3389/fmicb.2016.00249|pmid=26973626|pmc=4771760|issn=1664-302X|doi-access=free}}</ref> One example<ref>{{cite journal | display-authors = etal | year = 2000 | title = still a cause for concern | journal = Environ Health Perspect | volume = 108 | issue = 7| pages = 579–88 | pmid = 10905993 | pmc = 1638183  | last1 = Kielhorn | first1 = J | last2 = Melber | first2 = C | last3 = Wahnschaffe | first3 = U | last4 = Aitio | first4 = A | last5 = Mangelsdorf | first5 = I | doi = 10.1289/ehp.00108579}}</ref> is the organochloride respiration of the dry-cleaning solvent, [[tetrachloroethylene]], and the engine degreasing solvent [[trichloroethylene]] by [[anaerobic bacteria]], often members of the candidate genera ''Dehalococcoides''. Bioremediation of these [[chloroethene]]s can occur when other microorganisms at the contaminated site provide H<sub>2</sub> as a natural byproduct of various [[Fermentation (biochemistry)|fermentation]] reactions. The dechlorinating bacteria use this H<sub>2</sub> as their electron donor, ultimately replacing [[chlorine]] atoms in the chloroethenes with hydrogen atoms via hydrogenolytic reductive dechlorination. This process can proceed in the soil provided the availability of organic electron donors and the appropriate strains of ''Dehalococcoides''. Trichloroethylene is dechlorinated via [[dichloroethene]] and [[vinyl chloride]] to [[ethylene]].<ref>{{cite journal | year = 1997 | title = Breathing with chlorinated solvents | journal = Science | volume = 276 | issue = 5318| pages = 1521–2 | pmid = 9190688  | last1 = McCarty | first1 = PL | doi = 10.1126/science.276.5318.1521| s2cid = 29183906 }}</ref>

A [[chloroform]]-degrading reductive dehalogenase enzyme has been reported in a ''[[Dehalobacter]]'' member. The chloroform reductive dehalogenase, termed TmrA, was found to be transcriptional up-regulated in response to chloroform respiration<ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Wong|first3=Yie Kuan|last4=Braidy|first4=Nady|last5=Manefield|first5=Michael|last6=Marquis|first6=Christopher P.|last7=Lee|first7=Matthew|date=2016-08-10|title=Genomic, transcriptomic and proteomic analyses ofDehalobacterUNSWDHB in response to chloroform|journal=Environmental Microbiology Reports|language=en|volume=8|issue=5|pages=814–824|doi=10.1111/1758-2229.12444|pmid=27452500|bibcode=2016EnvMR...8..814J |issn=1758-2229}}</ref> and the enzyme can be obtained both in native<ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Bohl|first2=Susanne|last3=Lebhar|first3=Helene|last4=Healey|first4=Robert D.|last5=Manefield|first5=Mike|last6=Marquis|first6=Christopher P.|last7=Lee|first7=Matthew|date=2017-06-20|title=A bacterial chloroform reductive dehalogenase: purification and biochemical characterization|journal=Microbial Biotechnology|language=en|volume=10|issue=6|pages=1640–1648|doi=10.1111/1751-7915.12745|issn=1751-7915|pmc=5658581|pmid=28631300}}</ref> and [[Recombinant protein production|recombinant]] forms.<ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Payne|first2=Karl A. P.|last3=Fisher|first3=Karl|last4=Bohl|first4=Susanne|last5=Lebhar|first5=Helene|last6=Manefield|first6=Mike|last7=Lee|first7=Matthew|last8=Leys|first8=David|last9=Marquis|first9=Christopher P.|date=2018-01-24|title=Heterologous Production and Purification of a Functional Chloroform Reductive Dehalogenase|journal=ACS Chemical Biology|language=en|volume=13|issue=3|pages=548–552|doi=10.1021/acschembio.7b00846|pmid=29363941|issn=1554-8929}}</ref>

Reductive dechlorination has been investigated for bioremediation of [[Polychlorinated biphenyl|polychlorinated biphenyls]] (PCB) and  [[Chlorofluorocarbon|chlorofluorocarbons]] (CFC). The reductive dechlorination of PCBs is performed by anaerobic microorganisms that utilize the PCB as an electron sink. The result of this is the reduction of the "meta" site, followed by the "para" site, and finally the "ortho" site, leading to a dechlorinated product.<ref name=":7">{{Cite journal|last1=Payne|first1=Rayford B.|last2=May|first2=Harold D.|last3=Sowers|first3=Kevin R.|date=2011-10-15|title=Enhanced Reductive Dechlorination of Polychlorinated Biphenyl Impacted Sediment by Bioaugmentation with a Dehalorespiring Bacterium|journal=Environmental Science & Technology|volume=45|issue=20|pages=8772–8779|doi=10.1021/es201553c|pmid=21902247|issn=0013-936X|pmc=3210572|bibcode=2011EnST...45.8772P}}</ref><ref name=":6">{{cite journal|last1=Tiedje|first1=James M.|last2=Quensen|first2=John F.|last3=Chee-Sanford|first3=Joann|last4=Schimel|first4=Joshua P.|last5=Boyd|first5=Stephen A.|title=Microbial reductive dechlorination of PCBs|journal=Biodegradation|date=1994|volume=4|issue=4|pages=231–240|doi=10.1007/BF00695971|pmid=7764920|s2cid=2596703}}</ref><ref name=":5">{{cite journal|last1=Quensen|first1=J. F.|last2=Tiedje|first2=J. M.|last3=Boyd|first3=S. A.|title=Reductive Dechlorination of Polychlorinated Biphenyls by Anaerobic Microorganisms from Sediments|journal=Science|date=4 November 1988|volume=242|issue=4879|pages=752–754|doi=10.1126/science.242.4879.752|pmid=17751997|bibcode=1988Sci...242..752Q|s2cid=35371230}}</ref> In the Hudson River, microorganisms effect dechlorination over the course of weeks. The resulting monochlorobiphenyls and dichlorobiphenyls are less toxic and more easily degradable by [[aerobic organism]]s compared to their chlorinated counterparts.<ref name=":5" /> The prominent drawback that has prevented the widespread use of reductive dechlorination for PCB detoxification and has decreased its feasibility is the issue of the slower than desired dechlorination rates.<ref name=":6" /> It has been suggested that [[bioaugmentation]] with DF-1 can lead to enhanced reductive dechlorination rates of PCBs through stimulation of dechlorination. Additionally, high inorganic carbon levels do not affect dechlorination rates in low PCB concentration environments.<ref name=":7" />

The reductive dechlorination applies to CFCs.<ref>{{Cite journal|last1=Lovley|first1=Derek R.|last2=Woodward|first2=Joan C.|date=1992-05-01|title=Consumption of Freons CFC-11 and CFC-12 by anaerobic sediments and soils|journal=Environmental Science & Technology|volume=26|issue=5|pages=925–929|doi=10.1021/es00029a009|issn=0013-936X|bibcode=1992EnST...26..925L}}</ref> Reductive dechlorination of CFCs including CFC-11, CFC-113, chlorotrifluoroethene, CFC-12, HCFC-141b, and tetrachloroethene occur through [[hydrogenolysis]]. Reduction rates of CFC mirror theoretical rates calculated based on the [[Marcus theory]] of electron transfer rate.<ref>{{Cite journal|last1=Balsiger|first1=Christian|last2=Holliger|first2=Christof|last3=Höhener|first3=Patrick|title=Reductive dechlorination of chlorofluorocarbons and hydrochlorofluorocarbons in sewage sludge and aquifer sediment microcosms|journal=Chemosphere|volume=61|issue=3|pages=361–373|doi=10.1016/j.chemosphere.2005.02.087|pmid=16182853|bibcode=2005Chmsp..61..361B|year=2005}}</ref>

==Electrochemical==

The [[electrochemical reduction]] of chlorinated chemicals such as [[chlorinated hydrocarbons]] and chlorofluorocarbons can be carried out by [[electrolysis]] in appropriate solvents, such as mixtures of water and alcohol. Some of the key components of an electrolytic cell are types of electrodes, electrolyte mediums, and use of mediators. The [[cathode]] transfers electrons to the molecule, which decomposes to produce the corresponding hydrocarbon (hydrogen atoms substitute the original chlorine atoms) and free chloride ions. For instance, the reductive dechlorination of CFCs is complete and produces several [[Hydrofluorocarbon|hydrofluorocarbons]] (HFC) plus chloride.

Hydrodechlorination (HDC) is a type of reductive dechlorination that is useful due to its high reaction rate. It uses H<sub>2</sub> as the reducing agent over a range of potential electrode reactors and [[Catalysis|catalysts]].<ref>{{Cite journal|last1=Hoke|first1=Jeffrey B.|last2=Gramiccioni|first2=Gary A.|last3=Balko|first3=Edward N.|title=Catalytic hydrodechlorination of chlorophenols|journal=Applied Catalysis B: Environmental|volume=1|issue=4|pages=285–296|doi=10.1016/0926-3373(92)80054-4|year=1992|bibcode=1992AppCB...1..285H }}</ref> Amongst the types of catalysts studied such as [[precious metal]]s (platinum, palladium, rhodium), [[transition metal]]s (niobium and molybdenum), and [[Oxide|metal oxides]], a preference for precious metals overrides the others. As an example, palladium often adopts a lattice formation which can easily embed hydrogen gas making it more accessible to be readily oxidized.<ref>{{Cite journal|last1=Cheng|first1=I. Francis|last2=Fernando|first2=Quintus|last3=Korte|first3=Nic|date=1997-04-01|title=Electrochemical Dechlorination of 4-Chlorophenol to Phenol|journal=Environmental Science & Technology|volume=31|issue=4|pages=1074–1078|doi=10.1021/es960602b|issn=0013-936X|bibcode=1997EnST...31.1074C}}</ref> However a common issue for HDC is catalyst deactivation and regeneration. As catalysts are depleted, chlorine poisoning on surfaces can sometimes be observed, and on rare occasions, metal [[sintering]] and [[leaching (chemistry)|leaching]] occurs as a result.<ref name=":0">{{Cite journal|last=Ju|first=Xiumin|date=2005|title=Reductive Dehalogenation of Gas-phase Trichloroethylene using Heterogeneous Catalytic and Electrochemical Methods|url=http://arizona.openrepository.com/arizona/handle/10150/193594|journal=University of Arizona Campus Repository}}</ref>

Electrochemical reduction can be performed at ambient pressure and temperature.<ref>{{Cite book|title=Chemical degradation methods for wastes and pollutants: environmental and industrial applications|date=2003|publisher=M. Dekker|others=Tarr, Matthew A.|isbn=978-0-203-91255-3|location=New York|oclc=54061528}}</ref> This will not disrupt microbial environments or raise extra cost for remediation. The process of dechlorination can be highly controlled to avoid toxic chlorinated intermediates and byproducts such as [[Polychlorinated dibenzodioxins|dioxins]] from [[incineration]]. [[Trichloroethylene]] and perchloroethylene are common targets of treatment which are directly converted to environmentally benign products. Chlorinated alkenes and alkanes are converted to hydrogen chloride which is then neutralized with a base.<ref name=":0" /> However, even though there are many potential benefits to adopting this method, research have mainly been conducted in a laboratory setting with a few cases of field studies making it not yet well established.

== References ==
{{Reflist}}

{{DEFAULTSORT:Reductive Dechlorination}}
[[Category:Environmental chemistry]]
[[Category:Green chemistry]]
[[Category:Biological engineering]]