Chemistry Lab Demonstrations – Silver Mirror

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This week’s reaction was the HWE-modified Wittig reaction (and here) between para-anisaldeyhde and triethyl phosphonoacetate.  Saponification, followed by recrystallization of the acidified product gives nice needles of p-methoxycinnamic acid.  Funny thing, though, p-methoxycinnamic acid is supposed to be a white solid.  Half of the lab had their reactions turn bright yellow.  Good times.

No exicting Wittig demonstrations that I can think of, but since we used an aldehyde as a reagent, I showed off Tollens’ reagent and its utility in qualitative determination of an aldehyde.  Tollens’ reagent is a solution of silver nitrate, ammonia, and potassium hydroxide.  This produces a silver diamine complex [Ag(NH₃)₂]⁺. This is important because simple aqueous silver(I) is much easier to reduce than the silver diamine complex (RSC).  Silver nitrate on its own would be reduced rapidly and colloidal silver would develop.  These more controlled conditions allow for the controlled formation of elemental silver.

The basic solution is mixed with a sample of an aldehyde and shaken.  The silver oxidizes the aldehyde (but not a ketone!) to a carboxylic acid.  Simultaneously, the silver is reduced to elemental silver.  This production of elemental silver slowly coats the flask with a thin layer of silver.  This qualitiatively proves the presence of an aldehyde.  More importantly, it makes a very pretty silver-coated flask.  If rinsed thorougly with water, the flask can be capped and kept indefinitely.  I gave the one I made in the demo away to the student with the highest percent yield for the previous lab.  Here’s the reaction that takes place:

CH₃CH₂CHO(aq) + 2[Ag(NH₃)₂]⁺(aq) + 3OH^-(aq) → 2Ag(s) + CH₃CH₂COO^-(aq) + 4NH₃(aq) + 2H₂O(l)

The demo is often called the sliver mirror because the layer of silver, well, looks like a mirror.  In fact, this is one way mirrors were actually made historically.  A sacrificial adehyde (such as glucose) was poured over glass submerged in a solution of Tollens’ reagent.  The reaction takes place and deposits a nice layer of silver on the glass.  Wash and you have yourself a mirror.

Here’s the prep I used for preparing the demo, but here are some others if you want more references (I like that last one).  And, of course, a video (not me).

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