Tollens' reagent

Tollens' reagent is also a test for alkynes with a triple bond in the 1-position.

This reaction is a variation of the Tollens' reagent for aldehydes.

Ammonia solution can dissolve silver oxide residues, such as that formed from Tollens' reagent.

Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask.

Ketones may be distinguished from aldehydes by giving a negative result with Tollens' reagent or with the Fehling's solution.

The diamminesilver(I) complex in the mixture is an oxidizing agent and is the essential reactant in Tollens' reagent.

Tollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone.

Examples of these are Fehling's reagent, Millon's reagent and Tollens' reagent.

A negative result is the absence of the red precipitate; it is important to note that Fehling's will not work with aromatic aldehydes; in this case Tollens' reagent should be used.

For example, in the reaction of acetaldehyde with the Tollens' reagent to acetic acid (shown below), the carbonyl carbon atom changes its oxidation state from +1 to +3 (oxidation).

At this point the mixture will be clear, and there are now aqueous silver ions existing as [Ag(NH)] complexes in the mixture, which is the main component of Tollens' reagent.

Both Tollens' reagent and Fehling's reagent also give positive results with formic acid (methanoic acid - HCOOH), which is fully oxidised to water and carbon dioxide.

Another method that can be used to apply a thin layer of silver to several objects, such as glass, is to place Tollens' reagent in a glass, add Glucose/Dextrose, and shake the bottle to promote the reaction.

Importantly, there is a special case in which Tollens' reagent will give a positive for a ketone; if the ketone is an alpha-hydroxy ketone, then the Tollens' reagent will react.

Once it has been ascertained that there is a carbonyl group on an organic molecule using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH), Tollens' reagent can be used to determine whether the compound is a ketone or an aldehyde.

The reducing end of an oligosaccharide is the monosaccharide residue with hemiacetal functionality, thereby capable of reducing the Tollens' reagent, while the non-reducing end is the monosaccharide residue in acetal form, thus incapable of reducing the Tollens' reagent.