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Author: Subject: Is it possible to perform electrolytic organic reductions without using any toxic metals as electrodes?
Melgar
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[*] posted on 25-10-2017 at 05:11
Is it possible to perform electrolytic organic reductions without using any toxic metals as electrodes?


Reading the literature for organic electrolytic reductions (and oxidations, for that matter), a pattern emerges when you look at electrode materials: lead, lead, lead, mercury, lead, lead, lead oxide, lead, lead. Surely, of the 75+ metals on the periodic table, there is some second-best or third-best metal that can be used in place of lead. I know lead is cheap and easy to cast, and there sure is a lot of literature describing its use for this purpose, but does anyone know where a good place to start would be, if I wanted to use a less toxic metal? Are metals so different in activity that a paper that used lead electrodes would be completely useless if other electrode materials were used? I've tried to understand electrolytic reduction before, but the fact that lead always seems to be necessary has so far prevented me from doing any experiments.



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[*] posted on 25-10-2017 at 05:41


The thing about lead is that the metal isn't actually that dangerous because the body can't absorb it, I mean people have been handling it for thousands of years and I have no qualms with doing so as long as I wash my hands afterwards. The problem lies more in the salts which are more efficiently absorbed, especially lead nitrate which is water soluble and can be absorbed transdermally, and volatile forms such as tetraethyllead historically used in petrol as well as the particulate lead produced upon combustion - these are typically the largest exposures people have to lead.



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[*] posted on 25-10-2017 at 06:32


This is true, however if you accidentally switch the electrode polarity for whatever reason, all of a sudden now you have a water-soluble lead salt. I make too many mistakes already to believe that I wouldn't accidentally contaminate anything with lead salts. Also, I'm looking into possible educational experiments to work out the details of, and younger people who would be learning chemistry are the ones that would have the most detrimental effects from lead exposure.



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[*] posted on 26-10-2017 at 01:04


You've got a good point, there's always the possibility when working with lead. Sorry I couldn't help out much with your actual question - maybe zinc and tin are good and somewhat cheap places to start looking? I've seen them involved as generic reducing agents, coupled with HCl for example but they could possibly be used on their own under certain circumstances. Other than that, maybe you could work with aluminium but the passivation layer is an issue, or magnesium rod/ribbon.





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Melgar
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[*] posted on 26-10-2017 at 06:57


Quote: Originally posted by LearnedAmateur  
You've got a good point, there's always the possibility when working with lead. Sorry I couldn't help out much with your actual question - maybe zinc and tin are good and somewhat cheap places to start looking? I've seen them involved as generic reducing agents, coupled with HCl for example but they could possibly be used on their own under certain circumstances. Other than that, maybe you could work with aluminium but the passivation layer is an issue, or magnesium rod/ribbon.


There's the possibility of "hybrid" dissolving metal/electroreduction cells, where a small amount of metal is continually reduced and then oxidized by the substrate. A good candidate for this would be tin, since in the presence of HCl, it will do a lot of organic reductions. I haven't actually seen this seriously proposed before, so maybe I should start a new thread on this subject. It would eliminate the largest detriment to using stannous chloride as a reducing agent, and also would be simple enough chemistry that you don't have to worry about things like hydrogen overpotential of the electrode surface, since you'd only be directly reducing the metal, not the substrate.

I actually have plating solution for copper, nickel, gold, silver, rhodium, and palladium, but obviously I'd prefer not to waste these unless I knew that the resulting electrode would work. I have a platinized titania electrode, and some MMO ones, but those are meant to be used for anodes. Stainless steel is a common material as well, because of the passivation layer, but when you use a passivated metal as a cathode, then bad things happen.

All things considered, graphite might just be the way to go. I've seen it mentioned at least once for electroreduction of oxalic acid to glyoxylic acid, although the yields weren't that good. It was only mentioned briefly in the literature review section, before the paper went on to describe in much detail, the electroreduction of oxalic acid using electrodes made out of... wait for it... LEAD! But oxalic acid is cheap, so who cares?

Really, everyone here seems to be thinking that porous ceramic pots are needed for divided cells, because that's what papers written in the 19th century used. But during the 19th century, plastics weren't invented yet, and so they used what they had. But I'd bet money that I can find some synthetic fabric that works much better than a porous ceramic pot or a sheepskin condom or whatever nonsense people are going to think is needed next.




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[*] posted on 26-10-2017 at 07:14


You've brought up quite few very interesting questions in your post.

One thing that occurs to me is the similarity (at least to me) of using mercury as the electrode in many of the electrolytic reductions and the use of zinc-mercury couples for reductions like the Clemmensen and a few others.

However there are some similar reductions that use a zinc -copper couple so I wonder if copper might be a reasonable replacement for mercury in electrolytic reductions.

There's also the possibility of replacing mercury in these cells with a mercury coated electrode of some less toxic metal. You'd still have mercury, but the actual amount you'd have in the cell would be a lot less, and that would at least be something of an improvement.

Perhaps if the mercury was on the right metal the underlying electrode would form salts instead of the mercury if things go wrong? It's been a LONG time since I used Zinc-mercury for anything, but if I remember correctly the zinc gets oxidized in preference to the mercury and sort of leaches out as a salt.


Well Melgar, here's a reference you'll just love.

Graphite anodes and a stainless steel cathode are used in an electrolytic cell to form an epoxide from a double bond in one of Uncle Fester's books.

He says it is a process from " European patent 0,247,526 "

The patent does come up on google patents, and the process is essentially as he describes it, using graphite and stainless steel for the electrodes. (EP 0,247,526 A2 )

I know it's an oxidation, but at least it uses SS and graphite.

Edit: I've wondered about the ubiquity of lead in these processes myself.
perhaps they just think of it first because lead/H2SO4 is used in car batteries?

What really makes me wonder is the reactions where the lead is in the other 1/2 of a divided cell away from the reaction making the target compound.

I know little about electrolytic organic reactions but would have thought you could use almost anything for the other 1/2 of a divided cell.

[Edited on 26-10-2017 by SWIM]

Plastic membranes from car batteries and other sources are discussed on some of the electrolysis threads around here. Several posters were quite enthusiastic about them.

Sulaiman has found some very low priced divided cell porous pots for his experiments. They were just a few dollars each. He also points out that they are nothing like terracotta flowerpots but are MUCH more porous.



[Edited on 26-10-2017 by SWIM]

[Edited on 26-10-2017 by SWIM]

[Edited on 26-10-2017 by SWIM]
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Melgar
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[*] posted on 26-10-2017 at 11:10


I did figure out some interesting things about lead that makes it work well as an electrode material. First, its sulfate and chloride salts are both insoluble in water, meaning that with the most common electrolytes, you won't have it dissolving into the solution. Second, if you have a lead anode, most of the time, it'll coat with amorphous lead oxide, which has enough unoxidized lead in it to conduct electricity. Higher voltages produce lead dioxide, which also tends to adhere to the electrode and is a strong oxidizer. So if you're trying to oxidize something that's in solution, lead conveniently forms a layer that actually assists with that oxidation, rather than forming soluble salts like most other metals do.

These things all make it work well as an anode material, but I'd have expected that any metal that won't dissolve in the electrolyte would work well enough for a cathode. Clearly lead is the material of choice though, but I'd need to know why that is, in order to figure out what the second best or third best material might be.




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[*] posted on 26-10-2017 at 11:28


From the description you just gave, I think silver is a strong candidate, being resistant to oxidation and insoluble as all salts except the nitrate, fluoride and certain sulfonates.



[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 1-11-2017 at 17:06


I recall in the electro disinfecting of water, there was a proposed use of carbon electrodes derived from burnt coconut shells. The latter are apparently widely available in more tropical zones.

Here is one paper discussing electrode properties: https://www.frim.gov.my/v1/JTFSOnline/jtfs/v25n4/497-503.pdf

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