Tollens' reagent
The Tollensprobe (named after the agricultural chemist Bernhard Tollens ) or silver mirror test is a proof of aldehydes or reducing functional groups.
The Tollensreagenz is an ammoniacal silver nitrate solution, which is prepared by added dropwise to a solution of silver nitrate as long concentrated ammonia solution until the resulting brown precipitate of silver (I ) oxide ( Ag2O ) in the soluble Diamminsilber (I) - complex ( [Ag (NH3 ) 2] ) passes.
For detection is added to an aqueous solution of the test substance in the test tube, the ammoniacal silver nitrate solution and heated in a water bath for a few minutes to about 70 ° C.
The proof is positive when the solution turned black by precipitation of elemental silver and is deposited on the inner wall of the test tube of silver, resulting in the formation of a reflective coating.
The reaction equation for the oxidation of acetaldehyde (acetaldehyde ) as an example of a reducing substance with ammoniacal silver nitrate solution was added to acetic acid is:
With the Tollensprobe can also easily be the aldehyde groups of reducing sugars such as glucose or lactose prove. Although the open-chain form ( aldehyde) of these sugars in aqueous solution only a very small share in addition to the closed ring forms (here, the aldehyde group is bonded as a hemiacetal ) is present, the reaction still runs practically from complete, as the open-chain form via a chemical equilibrium of the ring shapes is simulated.
Eg
The ammoniacal silver nitrate solution should always be freshly prepared because the high explosive silver nitride Ag3N would precipitate on standing.
The Tollensprobe - a redox reaction
Since the oxidation of the test substance is carried out by reduction of the silver (I) ion, the overall reaction can be separated as in all redox reactions in an oxidation and reduction reaction. Here, in the example below is not considered for simplicity, that the silver ions actually present in a Silberdiamminkomplex:
The redox reaction can also be ( here on the carbon atom) follow using the oxidation numbers:
Since the reaction in alkaline medium takes place, the resulting carboxy group is (in the example, the carboxy group of the acid) is deprotonated by the hydroxide ions, carboxylate group in the sense of an acid -base reaction. It arises in the example, the acetate:
Further detection reactions of aldehydes:
- Fehling's test
- Schiff test with Schiff's reagent
- Benedict 's reagent