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Mar-Vell
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[*] posted on 21-12-2019 at 07:56
Organic Electrochemistry Questions


I have some very basic questions about the process of organic electrosynthesis that I'm hoping someone here can answer.

Even educated guesses or answers based on simple observation would be helpful.

1. Often I have seen in the literature the terms "constant voltage" and "constant potential". Are they interchangeable and synonymous? If not, why not? And if not, how can information about each help in carrying out an organic electrosynthesis?

2. In the book "Electrochemical Data" and other literature sources, the "E 1/2 (V)" has been posted to indicate something about the voltage of a molecule. For example, for acetophenone, the E 1/2 (V) is listed -1.99. In simple ordinary practical terms, how can the -1.99 figure be used (translated) to carry out an electrochemical reduction? And what does the "-" negative figure signify? Is the voltage on a typical variable DC power supply somehow supposed to be set to a negative number? If so, how is this done?

3. A lot of older literature references call for calculations of the surface area of the electrodes (in cm2). And at least one good fairly recent online video presents very simple straightforward calculations using the molarity of a molecule to determine the constant current rate and intensity of an organic electrochemical reaction. Is the molarity of a molecule all that is needed to perform an this type of experiment properly? Or, for example, must electrode surface area be calculated as well?

4. Regarding surface area, when it is called for in old literature references, did they mean surface area in contact with the solution? Or did they mean the surface area of the whole electrode (even the part not contacting the solution)?

5. What happens if you increase the size of an electrode relative to the volume of the reacting solution? What happens to the current and voltage? Does it go up or down? And is more or less current or voltage needed to carry out the reaction successfully?

6. If an organic electrochemical reaction calls for a certain distance between the anode and cathode, what happens if this distance is changed, say with the electrodes further away from each other than specified? How would this affect the reaction and also how could one adapt to the change? For example, would more time, voltage, or current be needed to get the same results?

7. If one is using say a flower pot to create a divided cell, will the resistance created affect the current or voltage (or both)? How can such resistance be overcome? And how does such resistance affect the reaction overall, especially if trying to carry out a constant current or constant voltage experiment?

8. The term "overpotential" comes up a lot in old literature references. Is the overpotential of a metal still relevant to modern-day organic electrochemical reactions? If not, how has the situation changed?

9. If the overpotential of a metal is still relevant, then how can a lower or higher overpotential metal be used interchangeably with another higher or lower overpotential metal in an organic electrochemical reaction? For example, lead has a greater overpotential rating than copper. But say all one has is copper to work with. How can one make copper perform as well as lead? Can the reaction conditions somehow be modified to do this successfully?

10. Can TLC be used to measure the reaction rate of an organic electrochemical reaction? And regardless of the answer, is there an easy way to measure the change in reaction conditions (distribution of products, etc.) that is on the simple DIY side of things?

11. Some literature references call for magnetic stirring of an organic electrochemical reaction, while others say that the gasses formed during the course of the reaction are enough to agitate the solution. In terms of actual practical experience, is there any preference to either way of doing things? What are the pros and cons of each?

12. What are the observed properties of the typical gasses created in organic electrochemical reactions? Are they typically flammable or corrosive? What sort of hazard do they create and how can they be dealt with in a simple cheap and effective manner? (While on the same subject, does anyone have any experience building and using Nurd Rage's mini fume box to pump out the gasses of an electrochemical reaction system? If so, does it work well?)

13. Is the evolution of hydrogen or oxygen supposed to indicate that the reaction is working? Why or why not?

14. What conditions should one be on the lookout for as an indication that the reaction is working? Does the voltage or current level stay the same whether or not a reacting substance is present? In other words, does the presence of a reducible or oxidizable substance increase or decrease the indicated voltage or current level? And if the reaction is nearing completion, does the voltage or current level go up or down?

15. In anyone's experience, what is the best (classic or modern-day) solvent and electrolyte or conducting solution (percentages and all) that should be used for an organic electrochemical experiment?

That's all of the questions I have for now. Thanks in advance to any and all who provide answers.

[Edited on 21-12-2019 by Mar-Vell]
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[*] posted on 21-12-2019 at 12:11


Quote:
1. Often I have seen in the literature the terms "constant voltage" and "constant potential". Are they interchangeable and synonymous? If not, why not? And if not, how can information about each help in carrying out an organic electrosynthesis?


Yes, they are the same.

Quote:
2. In the book "Electrochemical Data" and other literature sources, the "E 1/2 (V)" has been posted to indicate something about the voltage of a molecule. For example, for acetophenone, the E 1/2 (V) is listed -1.99. In simple ordinary practical terms, how can the -1.99 figure be used (translated) to carry out an electrochemical reduction? And what does the "-" negative figure signify? Is the voltage on a typical variable DC power supply somehow supposed to be set to a negative number? If so, how is this done?


That's not the voltage of the molecule, but the reduction potential (well, I'm assuming it's reduction, since that's what most tables provide). That means that the E(1/2) for the half-reaction RCOCH3 + 2 e- + 2H+ --> RCHOHCH3 is -1.99 V. You have to combine this with an oxidation half-reaction to get an overall reaction, with an overall voltage. If your overall voltage is negative, that means that the reaction is non-spontaneous, and you have to put in work (in the form of an applied voltage) to get the reaction to go. If it's positive, it will go by itself.

If, say, we're reducing acetophenone at the cathode, and the anode is copper (which is being oxidized to Cu(2+)). The reduction potential for copper is +0.34 V. Reverse the half-reaction (because we're oxidizing copper, not reducing the ion), and we get E(1/2) = -0.34 V. Add that to your acetophenone half-reaction, and we get Cu + RCOCH3 + 2 H+ --> RCHOHCH3 + Cu(2+) with an overall voltage of -2.33 V. So we have to apply a minimum voltage of 2.33 V to get anything to happen.

If we used magnesium instead of copper, then we'd look up the reduction potential of Mg(2+) to be -2.37 V, which would give us an overall potential for our reaction of +0.38 V. That tells us that the reaction of Mg + RCOCH3 + 2H+ --> RCHOHCH3 + Mg(2+) is thermodynamically favourable, and we should be able to just toss some magnesium in with our acetophenone, and the reaction should go without any applied voltage.

That probably won't happen, though, because redox reactions with organic compounds tend to have high activation energies, and generally have to be guided and forced (which is fine with most organic chemists, because if they had lower activation energies, they'd all spontaneously combust as soon as they saw oxygen, which would make ordinary life difficult).

(Note- the numbers I looked up are for aqueous solution, which you probably won't be using for your acetophenone, particularly since the reduction potential of water is a mere -0.83 V. This means that water is much more easily reduced than acetophenone, so any water present will react with the reducing agent instead of the letting the acetophenone react with it.)
Quote:
4. Regarding surface area, when it is called for in old literature references, did they mean surface area in contact with the solution? Or did they mean the surface area of the whole electrode (even the part not contacting the solution)?

Just the part in solution.

Quote:
5. What happens if you increase the size of an electrode relative to the volume of the reacting solution? What happens to the current and voltage? Does it go up or down? And is more or less current or voltage needed to carry out the reaction successfully?


The applied voltage is being set externally. Increasing the size would increase the current.

Quote:
8. The term "overpotential" comes up a lot in old literature references. Is the overpotential of a metal still relevant to modern-day organic electrochemical reactions? If not, how has the situation changed?


Yes, I believe so. An overpotential will stop a particular reaction from happening until an excess of voltage is applied, so instead of having to apply the calculated 2.3 V, you might need to set a voltage of 2.5 V.




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[*] posted on 21-12-2019 at 16:30


Over potential is used in reference to electrodes. You want them to have an over potential so they are not consumed by the reaction, if that is what you want to achieve.
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[*] posted on 22-12-2019 at 21:43
EMF instead of E 1/2 (V)?


Thank you all for your answers.

In other textbooks, I've noticed that instead of an E 1/2 (V) figure, an EMF (electromotive force) number is given. Does anyone know how EMF data can be applied to an organic electrochemistry reaction the same way, perhaps, as an E 1/2 (V) figure?

[Edited on 23-12-2019 by Mar-Vell]

[Edited on 23-12-2019 by Mar-Vell]
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[*] posted on 22-12-2019 at 21:46


Quote: Originally posted by Mar-Vell  
Thank you all for your answers.

In other textbooks, I've noticed that instead of an E 1/2 (V) figure, an EMF (electromotive force) number is given. Does anyone know how EMF data can be applied to an organic electrochemistry reaction the same way, perhaps, as an E 1/2 (V) figure?

[Edited on 23-12-2019 by Mar-Vell]

[Edited on 23-12-2019 by Mar-Vell]


It's the same thing.




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[*] posted on 23-12-2019 at 07:42
Thank you, DraconicAcid.


So then, to be clear, the E 1/2 (V) and EMF figures are both used in the same way when applied to the half-reaction calculation you posted before using acetophenone as an example? Both E 1/2 (V) and EMF represent the reduction potential?

What about when a reference says, for example, "-120 V vs S.C.E."? Does the "-120 V" represent the reduction potential in a half-reaction calculation too?

And if all three do represent the reduction potential, why is much of the reduction potential reference information from various sources different for the same molecule? Depending on the reference, I've found the E 1/2 (V), EMF, and S.C.E. figures to be quite different from each other.
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[*] posted on 23-12-2019 at 11:22


Quote: Originally posted by Mar-Vell  
What about when a reference says, for example, "-120 V vs S.C.E."? Does the "-120 V" represent the reduction potential in a half-reaction calculation too?


120 V? Surely you mean a much smaller number.

"vs. SCE" means it's with reference to a saturated calomel electrode, whereas most other E(1/2) values are given with respect to the hydrogen electrode. The difference should be 0.248 V.




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[*] posted on 23-12-2019 at 13:05
Sorry if I wasn't clear...


My mistake. It should be "-1.20 V vs S.C.E.". I got that figure from an experimental section of an article I was reading that has nothing to do with the acetophenone reduction we were discussing. I just posted it as is to show what that kind of a figure looks like in case you might know what it is.

But I'm still confused. If when I see something like the above S.C.E. figure in an experimental write-up, does that mean that all I have to do to try to reproduce the experiment is use that "-1.20" figure to represent the reduction potential of a half-reaction and then add it to, for example, the Cu anode oxidation potential mentioned before, which you calculated earlier as -0.34 V? And after doing the math, the result should be -1.54 V for the overall voltage (the minimum voltage needed to get the reaction going)?

Just wondering if you just plug and play the S.C.E. data as the reduction potential the same way you do when using E 1/2 (V)?
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[*] posted on 24-12-2019 at 10:56


Quote: Originally posted by DraconicAcid  

"vs. SCE" means it's with reference to a saturated calomel electrode, whereas most other E(1/2) values are given with respect to the hydrogen electrode. The difference should be 0.248 V.


I guess this answers part of my previous question as "most other E(12) values" implies that the S.C.E. value is also an E(12) value.

But I still don't know for sure how one can use the S.C.E. value to determine what voltage a power supply should at least be set at in order to induce an organic reduction reaction. All the E(12), EMF, and S.C.E. values (as they are listed in various literature references for organic substances) are all different from each other. And if plugged them each, one at a time, into the calculation you first mentioned, they're going to give different answers. I'm sure it's pretty easy for those of you who understand this stuff, but I'm still trying to work it out.

Also, I don't know what you meant by "the difference should be 0.248 V". How did you get the 0.248 V value?

Thanks again for any help you can give.

Hope you have a Merry Christmas!
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[*] posted on 24-12-2019 at 14:19


These E(1/2) values (or E.m.f-s) are referenced to the so called standard hydrogen electrode (Google it, wikipedia. etc) The potential of this electrode was choosen arbitrarily to be 0.000 V under some standardized circumstances (25 C, 1 atm H2 pressure, 1 M H+ conc in the solution, Pt electrode in the solution, etc. etc.)

The copper/Cu2+ system has +0.34V vs. the above standard H2 electrode. Zn/Zn2+ is around -0.76V compared to the H2 electrode. When you have a reference you can measure all other half-cells against it!
S.C.E. is "just another electrode", its potential when measured against the standard H2 electrode is that +0.248V that Draconic wrote. Google "saturated calomel electrode" and you will see why and when it is +0.248V and when is not.
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