Tollens
Tollens' reagent, also known as ammoniacal silver nitrate, is a chemical reagent used to test for the presence of aldehydes, aromatic aldehydes and alpha-hydroxy ketones. It is a clear, colorless, alkaline solution prepared by dissolving silver nitrate (AgNO3) in aqueous ammonia (NH3).
Preparation and Composition
Tollens' reagent is typically prepared fresh, as it can form explosive compounds upon standing. The general procedure involves first adding a few drops of dilute sodium hydroxide (NaOH) to aqueous silver nitrate. This results in the formation of a silver oxide (Ag2O) precipitate. Ammonia is then added dropwise until the silver oxide dissolves, forming the Tollens' reagent. The active ingredient is the diamminesilver(I) complex, [Ag(NH3)2]+.
Reaction with Aldehydes
The key reaction of Tollens' reagent is its reaction with aldehydes. When an aldehyde is added to Tollens' reagent and warmed, the aldehyde is oxidized to a carboxylic acid. Simultaneously, the silver ions (Ag+) in the diamminesilver(I) complex are reduced to elemental silver (Ag). The elemental silver deposits on the inner surface of the reaction vessel, creating a silver mirror, which is a positive test for the presence of an aldehyde. The reaction can be represented (in simplified form) as:
RCHO + 2[Ag(NH3)2]+ + 3OH− → RCOO− + 2Ag(s) + 4NH3 + 2H2O
Where RCHO represents the aldehyde, RCOO- represents the carboxylate anion, and Ag(s) represents the elemental silver.
Applications
Tollens' reagent is primarily used as a qualitative test for the presence of aldehydes. It can differentiate between aldehydes and ketones, as ketones generally do not react with Tollens' reagent (although alpha-hydroxy ketones are an exception). It is also used to distinguish between aromatic and aliphatic aldehydes; however, aromatic aldehydes can require longer reaction times or elevated temperatures.
Beyond its analytical uses, Tollens' reagent has been employed in the silvering of mirrors and other glassware, using similar principles to those behind the aldehyde test.
Safety Considerations
Tollens' reagent is a potentially hazardous substance. The reagent itself, especially when allowed to stand for extended periods, can form explosive silver compounds, such as silver nitride. Therefore, it is crucial to prepare the reagent fresh and to dispose of any unused reagent immediately by adding dilute acid to decompose the silver compounds. Contact with skin and eyes should be avoided, and appropriate personal protective equipment should be worn when handling the reagent.