Sciencemadness Discussion Board

How do the reductive properties of Zinc and Zinc Amalgam differ?

FireLion3 - 27-5-2014 at 09:05

Both are used as reducing agents but the Zinc Amalgam is considered to be more powerful. Beyond this, I am not able to find much information on the differences in their reducing properties. As far as amalgams, the mercury is supposed to facilitate the electron transfer? But in the end what is the overall effect of the presence of the mercury? Zinc is traditionally a two-electron reducing agent since it can only be in oxidation state 0 and 2, where as mercury can be in 0, 1, and 2. Does the mercury enable a more broken down electron transfer, or something else?

smaerd - 27-5-2014 at 09:28

Looks like you have stumbled onto one of those organic synthesis mysteries.

Turns out the exact mechanism for say a Clemmenson reduction is not currently known.
http://www.masterorganicchemistry.com/2011/12/09/reagent-fri...

The Reactivity subsection of the Aluminum Amalgam wiki (http://en.wikipedia.org/wiki/Aluminium_amalgam) is pretty interesting and may provide some clues to what you're asking.

Wish I could be of greater help. My assumption would be firstly that the success of over-all electron transfer would be very substrate/conditions dependent(of course...). Then I would assume that there are thermodynamically favorable electron transfer coupled reactions driving things things between the substrate zinc(multisteps here sure) and mercury(again pretty useless to say).

I would look for clues in the electrochemical studies of the amalgams, but again I don't know that anyone is exactly sure how this proceeds.

Some relevant articles I found:

Reductions at Metal Surfaces. II. A Mechanism for the Clemmensen Reduction.James H. Brewster. J. Am. Chem. Soc., 1954, 76 (24), pp 6364–6368
DOI: 10.1021/ja01653a035
http://pubs.acs.org/doi/abs/10.1021/ja01500a031?journalCode=...

Studies on the Mechanism of Clemmensen Reduction. III. The Relation of Clemmensen Reduction to Electrochemical Reduction. J. Am. Chem. Soc., 1960, 82 (15), pp 3909–3913
DOI: 10.1021/ja01500a031
http://pubs.acs.org/doi/abs/10.1021/ja01500a031?journalCode=...

Studies on the Mechanism of Clemmensen Reduction. II. Evidence for the Formation of an Intermediate Carbonium Ion.
Tadaaki Nakabayashi. J. Am. Chem. Soc., 1960, 82 (15), pp 3906–3908 DOI: 10.1021/ja01500a030
http://pubs.acs.org/doi/abs/10.1021/ja01500a030

FireLion3 - 27-5-2014 at 10:22

Sure, but the Clemmensen Reduction is but one of the infinite uses of zinc as a reducing agent. My recent investigations have mostly been concerned with the effect of zinc on aryl and alkyl halides. Zinc powder seems to be suitable for effecting reduction for homocoupling or making of organometallic zinc compounds, but I am curious what effect adding a pinch of mercury might have. My guess is little-none seeing as zinc readily reacts with these, though, to the speed of which I am not sure. Mercury may facilitate the electron transfer even further speeding up the interactions.

One possible mechanism is that since zinc can only transfer two electrons at once, the amalgam complex may allow the mercury to facilitate a one electron transfer, or rather, increase the time allowed that can exist between two electron transfers. This could greatly enhance the reactivity of zinc as a reducing agent. Pure speculation, not much else, but the mercury does play some role.

Nicodem - 28-5-2014 at 08:17

Quote: Originally posted by FireLion3  
Zinc is traditionally a two-electron reducing agent since it can only be in oxidation state 0 and 2, where as mercury can be in 0, 1, and 2. Does the mercury enable a more broken down electron transfer, or something else?

Zinc, as all metals in their elemental state, are single-electron donors. Dissolving metal reductions proceed via single-electron transfers. The oxidation states have no relevance to this, as elemental metals generally do not exist in the strictly zero oxidation state (they can exist in a metallic state only, where the metallic atoms in the crystal lattice are in a cationic state). Hence, elemental zinc or other metals can never "transfer two electrons at once".

https://www.sciencemadness.org/whisper/viewthread.php?tid=25...

FireLion3 - 29-5-2014 at 20:58

Quote: Originally posted by Nicodem  
Quote: Originally posted by FireLion3  
Zinc is traditionally a two-electron reducing agent since it can only be in oxidation state 0 and 2, where as mercury can be in 0, 1, and 2. Does the mercury enable a more broken down electron transfer, or something else?

Zinc, as all metals in their elemental state, are single-electron donors. Dissolving metal reductions proceed via single-electron transfers. The oxidation states have no relevance to this, as elemental metals generally do not exist in the strictly zero oxidation state (they can exist in a metallic state only, where the metallic atoms in the crystal lattice are in a cationic state). Hence, elemental zinc or other metals can never "transfer two electrons at once".

https://www.sciencemadness.org/whisper/viewthread.php?tid=25...


But, if Zinc metal is a single electron donor, does that mean it can undergo two, single electron transfers? Zinc Chloride for instance exists only as ZnCl2, never as ZnCl. If zinc transfers a single electron, and the result is a chloride ion in solution, then what is this intermediate called? Zinc (I) Chloride doesn't exist. What is the intermediate called after zinc transfers one electron, but still has one electron to give up, if it can't exist in the (I) state?

I don't want to seem like I'm arguing with you, as I do agree with you. Some papers I was reading over did seem to show that when zinc reacts with halides, radicals and halide anions are formed, indicating a single electron transfer, but I do not understand how this is possible?

Nicodem - 30-5-2014 at 07:43

Quote: Originally posted by FireLion3  
But, if Zinc metal is a single electron donor, does that mean it can undergo two, single electron transfers? Zinc Chloride for instance exists only as ZnCl2, never as ZnCl. If zinc transfers a single electron, and the result is a chloride ion in solution, then what is this intermediate called? Zinc (I) Chloride doesn't exist. What is the intermediate called after zinc transfers one electron, but still has one electron to give up, if it can't exist in the (I) state?

You are asking as if you believed that zinc is a non-metal. There are no Zn atoms that give off electrons. The electrons come from the metal, not from the Zn atoms.
Quote:
I don't want to seem like I'm arguing with you, as I do agree with you. Some papers I was reading over did seem to show that when zinc reacts with halides, radicals and halide anions are formed, indicating a single electron transfer, but I do not understand how this is possible?

In a situation when you simply cannot understand how something is possible, you can always help yourself by applying the scientific method. This says that when you cannot understand something, then your assumptions about it must be wrong. You appear to assume that zinc behaves as a non-metal, which is wrong. Metals (in their elemental state) transfer electrons from the surface of the crystal lattice and not from the atoms as is the case with non-metals. This means that the electron transfer itself does not necessarily change the oxidation state of metal atoms, it just defines the number of these atoms that can solvate as cations to maintain electric neutrality.

leu - 30-5-2014 at 08:48

The attached document explains simplified metallic valency physics rather well :) As always, the end results from the effort applied :cool:

Attachment: Free_Electron_Model.pdf (101kB)
This file has been downloaded 688 times

smaerd - 30-5-2014 at 12:23

Thanks for that link Leu! I have never seen such a direct link between chemical potential and quantum mechanics. Had never heard of the Jelly model or the free electron model. Very interesting!


[Edited on 30-5-2014 by smaerd]