The Tollens’ test, named after the German chemist, Bernhard Tollens, who developed it, is the best-known aldehyde detection reaction. It is also used for other substances with reducing properties. These include, for example, sugars. Under laboratory conditions, the Tollens’ test is most commonly used to distinguish aldehydes from ketones, but the test is also used to distinguish reducing sugars from non-reducing sugars.
Silver mirror test
The Tollens’ test is widely known as the silver mirror test due to the silver coating that appears on the walls of the laboratory vessel in which it is carried out. Elemental silver emitted from the reaction environment is responsible for the metallic silver colour. The process has found application not only in the chemical industry. The silvering of Christmas tree baubles is also carried out in this way, although the process has little use today.
The Tollens’ test is a chemical reaction that is primarily used in chemistry to detect the presence of aldehydes, among other things. Aldehydes form a class of organic compounds whose defining characteristic is the presence of an aldehyde group, or -CHO group, in the molecule. With the Tollens’ test, it is also easy to distinguish between aldehydes and ketones (with -CO functional group). Ketones are considered to be chemical compounds with almost identical structure and physicochemical properties compared to aldehydes. However, this is a misconception and the fundamental differences between the two can be demonstrated, for example, in the Tollens’ or Trommer’s test. Ketones do not oxidise as easily as aldehydes. This is caused, among other things, by the absence of a hydrogen atom in their molecule. Therefore, they will give a negative result in the Tollens’ and Trommer’s tests.
How are aldehydes detected in the Tollens’ test?
The Tollens’ test is mainly known as the characteristic reaction for detecting the presence of aldehydes. It is an example of a redox reaction, otherwise known as an oxidation-reduction reaction. The Tollens’ reagent used is an ammoniacal solution, obtained by dissolving a black precipitate of silver (I) oxide in an aqueous ammonia solution. During the dissolution process, a complex ion is formed which reacts directly with aldehydes. This reagent is not stable and must be freshly prepared, just before the test is performed. Such solutions should not be stored for long periods of time, also due to the possibility of forming, among others, Ag3N, which has explosive properties.
The Tollens’ test consists of two main stages:
- Preparation of Tollens’ reagent: a selected silver salt that readily dissociates into ions in aqueous solutions (e.g. silver nitrate (V)) is added to an aqueous ammonia solution. The reaction of silver cations with hydroxide anions takes place. The result is silver hydroxide, which is very unstable and decomposes almost immediately with the precipitation of a silver (I) oxide. The Ag2O precipitate reacts with ammonia and water molecules, with the formation of a complex ion.
- Oxidation of the aldehyde with simultaneous reduction of the Tollens’ reagent to metallic silver: in the second, key step of the Tollens’ test, the aldehyde molecule reacts with the complex ion. The aldehyde group is oxidised to the carboxyl group (a carboxylic acid is formed) and the silver ion from the complex is reduced to metallic silver, which forms deposits on the walls of the vessel. At the same time, ammonia and water molecules are formed. The appearance of a metallic precipitate on the walls of a test tube or other vessel is evidence of a positive Tollens’ test. Otherwise, the result is considered negative.
Reactions of the Tollens’ reagent with aldehydes boil down to their oxidation (they can only be oxidized to the corresponding carboxylic acids), which confirms the presence of the carboxyl group. The metallic silver is reduced, acting as an oxidant. Increasing the temperature facilitates the reaction. The entire reaction is performed in a slightly alkaline environment.
A positive result of the Tollens’ test is also obtained in the reaction of the Tollens’ reagent with formic acid. This acid, like aldehydes, contains a -CHO group, which exhibits reducing properties. Formic acid is the only carboxylic acid to exhibit such properties. Also, some carbohydrates which do not have an aldehyde group may give a positive result in the Tollens’ test due to their isomerization in alkaline conditions.
The Tollens’ test or the Trommer’s test?
The Tollens’ test and the Trommer’s test are used to detect the presence of aldehydes and other selected compounds with reducing properties. These two chemical tests are very often confused with each other, due to the similar principle of detecting the -CHO moiety. Both the Tollens’ test and the Trommer’s test oxidise the aldehyde group to the carboxyl group. The fundamental element that distinguishes the two tests is the type of reagent used. While the Tollens’ test uses a complex ion containing a silver atom, the Trommer’s test uses freshly precipitated copper (II) hydroxide. It is a blue, gelatinous precipitate. At elevated temperatures, it behaves like an oxidiser. Copper in the second oxidation state is reduced to copper in the first oxidation state. This manifests in the formation of a brick-red copper (I) oxide precipitate. Similar to the Tollens’ test, ketones also give a negative result in the Trommer’s test due to their weak reducing properties, caused by the absence of a carbon atom in the functional group.