Understanding Tollens' Mechanisms in Chemistry

[Zynwyx]

Hi :)

I've been having some trouble getting my head around Tollens' solution, but especially the mechanisms in its preparation. There is next to no information on the internet regarding its mechanisms and my chemistry knowledge isn't advanced enough to be able to really make any idea of what's going on. Hopefully someone here with a knowledge of chemistry can help me out?

So, in the first step of its preparation a few drops of a strong alkali (e.g. sodium hydroxide) is added to some aqueous silver nitrate, and a brown precipitate of silver oxide forms. This is the first place I am unsure about what happens; precisely, how the silver oxide is formed. All that I know conclusively is that the Ag⁺ ions in the silver nitrate solution are hydrated as [Ag(H₂O)₂]⁺ ions, then that the OH^- ions from the alkali protonate the aqua ligands to form [Ag(OH)₂]^- ions. But from here how is Ag₂O formed? My textbook says the hydroxy ligands are further protonated (wouldn't that give improbable [Ag(O)₂]^3- anyway?) but since the silver oxide formed is a binary oxide i.e. two Ag to each O, I first assumed that there would need to be an interaction between two individual Ag complexes in order to provide the two Ag needed for each O.

But silver oxide is a polymer according to Wikipedia so would there be a kind of "build up" of silver oxide crystals/polymers on nucleation sites using the Ag in solution kind of like Lego blocks?

The next problem is the addition of ammonium hydroxide to the mixture. This "dissolves" the silver oxide and diamminesilver(I) ions are formed in solution [Ag(NH₃)₂]⁺. By dissolve I assume it means the silver oxide is split into aqueous ions, ie [Ag(H₂O)₂]⁺ again and some other species (I'm thought OH^-)? Or is there some sort of ligand substitution reaction going on between Ag₂O and NH₃ (even though Ag₂O isn't a complex I thought... but in an analogous way)

Any clarification would be really helpful on this, I know my thinking is deeply flawed on all of this. Tbh I am getting more confused the more I think about this and its causing many sleepless nights :P. Hopefully being able to see the mechanism for this will help me think clearer about how the actual Tollens' test mechanism works as well!

 

[Borek]

Contrary to organic chemistry, inorganic chemistry very rarely deals with detailed mechanisms. The way I see it after adding strong base colloidal AgOH precipitates - so you already deal with solid, it is unstable and easily loses water, thus producing colloidal Ag₂O.

 

[Zynwyx]

Thanks for the reply Borek. So would it be something more like:

Ag[(H₂O)₂]⁺ (aq) -> AgOH (s) + H₃O⁺ (aq)

and then:

2AgOH (s) -> Ag₂O + H₂O

This is a complete guess btw, and here I cannot see where the OH^- ions would come into play.

Also I'm interested in exactly how the AgOH becomes Ag₂O. I presume the suspended silver hydroxide colloid drops are in crystal lattice form or other solid form of some sort i.e. a polymer, suspended in the solution, and that removal of the protons (hydrogens) from this lattice/polymer gives the overall ionic ratio for the resulting silver oxide Ag₂O. But by what mechanism would this happen; do OH^- ions react with hydrogens on the outermost layer of the AgOH lattice/polymer, slowly until all the hydrogens in the AgOH is gone, leaving only Ag and O behind?

Maybe I am looking too deep into this without sufficient foundation knowledge? What topics would it be advisable to cover to fully understand what's going on here; I'm guessing crystal structure since solids are involved, as well as ligand substitution. I have touched upon these two topics but I am still lost in what way to approach this mechanism. If I wanted to learn the more detailed mechanism for these reactions, where would you recommend would be a good place to begin? Or is it possible that the mechanism for this silver oxide precipitation reaction hasn't been deduced yet, if inorganic chemistry does not place much emphasis on mechanisms?

Again, sorry if my assumptions are completely wrong. :shy: I really am shooting in the dark here!

 

[Borek]

Ag[(H₂O)₂]⁺ (aq) -> AgOH (s) + H₃O⁺ (aq)

If anything, more like

Ag(H₂O)₂⁺ + OH^- -> AgOH + 2H₂O

But this is just an overall reaction, no hints at mechanism. Whenever you see water and OH^- or H⁺ you can be sure protons will jump between molecules/ions.

Or is it possible that the mechanism for this silver oxide precipitation reaction hasn't been deduced yet, if inorganic chemistry does not place much emphasis on mechanisms?

I guess we may know something, but it is not that important.

If my guess is correct and you came to inorganic after course in organic, good advice: forget about mechanisms. That's not what inorganic is about.

 

[Zynwyx]

You are right, I came to Tollens' from studying organic carbonyls and the tests used to identify them like Fehling's, Brady's etc. But I was having trouble remembering them as I usually remember things by mechanism e.g. like the iodoform reaction. But the mechanism for this seems very complicated even just from the preparation stage, I don't think I will attempt looking at the mechanism for Fehling's! Still it would have been interesting I think to get an idea of exactly how the silver oxide polymer "builds up" and precipitates out of solution; but that is a question in itself I guess.