Electra
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The step by step progression of electrochemical reactions?
I've been reading quite a bit about how certain products can be attained through Electrochemical oxidation(or reduction). One thing I've noticed, in
electrooxidations for instance, is that many of the products that are obtained in high yields(90-95%) can be further oxidized into another product,
but are not during the reaction.
Why or why not does this happen? I recall reading somewhere that these reactions precede in steps. For instance, A1 can be oxidized to A4, by being
first turned into A2, then A3, then to A4. Though, I've read that before any of the A2 molecules can go to A3, all of the A1 molecules must first hit
A2.
Is this true? A perfect examples is the indirect electrooxidation of Toluene -> Benzaldehyde. http://link.springer.com/article/10.1023%2FA%3A1024175515730...
High yields are supposedly obtained, but benzaldehyde should be further oxidized to Benzoic Acid(if I remember correctly). Benzaldehyde being obtained
in high yields by electrooxidation of toluene would suggest that these electroreactions do precede in steps, where as, the benzaldehyde could not go
to Benzoic Acid until all of the Toluene is first converted.
If this is true, then how exactly does this happen? Is there a sort of balancing of net potentials in the solution? Maybe for instance if one
benzaldehyde molecule oxidizes to Benzoic acid, then that benzoic acid oxidizes a toluene to benzaldehyde, while subsequently being reduced back to
benzaldehyde? I remember reading this idea somewhere but I do not remember ever seeing it explained.
Does the more times a molecule get oxidized make it increasingly harder to be oxidized? Which would mean the molecules more prone to being oxidized
will always be oxidized first? I'm trying to picture this on a micro particle-particle interactive scale but I'm not sure how these interactions work.
[Edited on 21-12-2013 by Electra]
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WGTR
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I've messed around with this stuff over the years, but I can't explain it as intelligently as some of our other members here
could. There are a lot of books online now, that are available to be freely downloaded and printed. One that I printed out
was Lob's Electrochemistry of Organic Compounds, which is rather dated, but still very much interesting. Another was
Organic Electrode Processes. There are a lot of resources here in our library, as well as on Google Books, and on other
archive sites. One other book is not available for download (as far as I know), but I thought it valuable enough that I bought it
in hardback. It is A. P. Tomilov's Electrochemistry of Organic Compounds.
Having said all of that, I'll attempt to answer your question. Some electrodes provide a lower overvoltage for one reaction
over another. For example, while you might expect oxygen to be oxidized at a graphite anode, chlorine will in fact be oxidized
preferentially if the chloride ion concentration is high enough in solution. There's also reaction kinetics. Some reactions
happen faster than others. Some electrode materials act catalytically, favoring one pathway over another.
As an example of this, in Tomilov's book he records data on the reduction of acetone on various cathodes. Depending on the
material of the electrode, how its surface is prepared, the pH of the solution, and the current density; the product can either be
mostly hydrogen, isopropanol, propane, or pinacol.
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Electra
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That's interesting. I do believe in the above study that there is a mention saying that changing the type of electrode had little effect on the
resulting products, in the case of toluene->benzaldehyde. I figure having higher or lower current densities, ie more or less energy hitting each
molecule that touches the electrode, would effect the reaction.
I've given the theory that the reactions precede in steps some thought, and it does seem to make sense, to an extent. Basically since there is direct
electron transfer going on, usually, with no meditator(such as Al/Mg or LAH), the reactions precede much more calmly. The electrons are most always
going to be trying to go to the source that attracts them harder. So hypothetically if a benzaldehyde molecule was further oxidized via
electrooxidation in the mist of a bunch of toluene molecules, then is there not the possibility that the benzoic acid molecule gets reduced to
benzaldehyde when it comes into contact with a toluene molecule, effectively oxidating the toluene to benzaldehyde?
I wish I could find the text that talked about it. This paper I saw somewhere, which I have been searching for all day, seemed pretty insistent that
due to the mechanism of the electrooxidation/reductions the reactions precede in steps as the mixture is constantly trying to balance out the
potential oxidation states of all the molecules interacting.
I could always experiment and try this out but I lack the proper equipment for fine control and I also lack access to equipment to accurately
determine the % content of the products from a reaction.
One example I had seen was the coupling of radical products via oxidative kolbe electrolysis. Some of these resultant products are also products of
molecules that have been reduced, so in theory they should be able to become oxidized during their formation, yet, they are rarely oxidized as the
resulting yields from the electrolysis are pretty high. If the step-by-step preceding mechanism for electrolysis is true then in the
kolbe-electrolysis example those molecules could potentially be oxidized further if the reaction is allowed to go for longer than it should.
I'm going to go search for that paper and try to find it because I really dislike talking about this mechanism when I don't have the text
Edit:
Okay I managed to find it. Not sure what paper this was originally from, but quoted the section of relevance:
| Quote: |
In the acid reduction of the nitro ... compound to the amino
compound the mechanism of reduction proceeds first to the nitroso
compound, next to a hydoxylamino compound and only then to the amino
compound as represented below:
RN02 ----> RNO ----> RNHOH ----> RNH2
Thus, the reaction is a step wise one and the reaction therefor,
can be no faster than the slowest or most limiting step, nor can it be
complete unless the intermediate products are preserved from the
side reactions until they can be made to undergo the desired reduction
reactions. I have found that the limiting step in the preparation ... is
usually the last one in which reduction of the hydroxyamino compound
to the amine takes place. Under a wide variety of conditions the nitro
and nitroso compounds react swiftly and completely to form the hydroxy-
lamino compound, but on the other hand, very little amine compound
forms until after nearly all the nitro compound has been transformed to
the hydroxyamine compound and then only under carefully controlled
conditions which not only tend to prevent side reactions of the auxiliary
materials present, but also which protect the aminohydroxyl compound
from undesired reactions with the other materials present in the cell,
including dehydration, deammonation, reactions ofthe nitro compound
with the hydroxylamine compound and the like ...
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I suppose there is some credence then to the idea that there is a constant net potential in the reaction that is trying to be balanced out, meaning no
single molecule can be over-oxidized until every other molecule of its like in the solution have reached that point as well. It makes sense... but
someone correct me if I'm wrong.
[Edited on 21-12-2013 by Electra]
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