Analytical chemistry provides chemists with a number of tools for qualitative analysis of selected chemical compounds. These tools are called “characteristic reactions”. They enable to identify a selected chemical species based on the observation or measurement of a change in a characteristic feature. Trommer’s test is an example of such a characteristic reaction.
Trommer’s test
It is an example of a characteristic reaction that can deliver either a positive or a negative result – like a “test” – all based on observations and the resulting conclusions. Trommer’s test is one of several methods to identify mainly water-soluble aldehydes and differentiate them from ketones. The former compounds undergo the reaction, while the latter do not. Furthermore, the Trommer’s test confirms the reducing properties of such compounds. Aldehydes are substances classified as organic compounds. They are characterised by the presence of the aldehyde group (-CHO) attached to a carbon atom in the chain. Ketones, however, have molecules which contain the ketone group, made of a carbon atom attached by a double bond to an atom of oxygen (C=O).
Not only aldehydes are susceptible to the Trommer’s test. It may also be so-called reducing sugars (such as maltose or lactose). Actually, all compounds that are capable of oxidising and reducing copper (II) ions at the same time (e.g. hydroxylamine, L-ascorbic acid).
Interestingly, originally the Trommer’s test was used for more than just identifying and testing the properties of aldehydes. It was also used for detecting sugars in urine.
Trommer’s reagent
The Trommer’s test uses mild oxidising agents (which are being reduced). The main role is played by Cu2+ copper (II) ions in the form of copper (II) hydroxide. This is what we call the Trommer’s reagent. The hydroxide forms a blue gelatinous precipitate. It is freshly precipitated because of its instability. Importantly, the Trommer’s test additionally requires an alkaline environment, which can be created by adding e.g. a small surplus of sodium hydroxide already at the Cu(OH)2 precipitation stage.
Trommer’s test in action
The Trommer’s test is a characteristic reaction that allows for a qualitative identification and testing of the reducing properties of aldehydes, sugars and other organic substances.
The Trommer’s test is carried out in the following steps:
- The first step involves precipitation of fresh copper (II) hydroxide. This compound can be obtained, for example, in the result of a reaction between sodium hydroxide (NaOH) and copper (II) sulphate (CuSO4). In aqueous solutions, both substrates readily dissociate into ions. The substitution reaction that ensues leads to the precipitation of a gelatinous precipitate of blue copper (II) hydroxide (Cu(OH)2 is insoluble in water). A surplus of NaOH is added to ensure an alkaline environment for the Trommer’s test.
- Next, aldehyde, for example, is added to the prepared Trommer’s reagent (it is advisable to ensure a 1:1 ratio of the two reactants). Any surplus Cu(OH)2 will lead to incorrect observations because of the formation of a black CuO precipitate. You might want to carry out the reaction in a water bath, gently heating the mixture. This accelerates the occurring processes.
- The reaction results in a change of colour of from blue to brick-red (positive Trommer’s test). The change is a consequence of the occuring redox reaction. The aldehyde, or more specifically its characteristic aldehyde group (-CHO), is oxidised to a carboxyl group (-COOH) that can be found in the molecules of carboxylic acids. At the same time, copper (II) ions are reduced to copper (I) ions that have a brick red colour.
- If the test tube contains compounds that are not susceptible to the Trommer’s test, heating of the mixture of copper (II) hydroxide and the test substance forms a black precipitate. This is caused by the decomposition of Cu(OH)2 to copper (II) oxide and water.
Modifications of the Trommer’s test
There are two known modifications of the Trommer’s test: the Fehling’s test and the Benedict’s test. Compared to the original experiment, they turned out much more effective and more sensitive in practice. This is why the Trommer’s test is rather rarely performed in laboratory practice today. Historically, all three methods were mainly used for analytical determination of sugar (glucose) content in urine.
Fehling’s test
The modification in the Fehling’s test consists primarily in that the (mild) oxidant used for the reaction is not copper (II) hydroxide. In this test, a complex of copper cations is used along with potassium sodium tartrate (which makes the Fehling reagent blue). This modification makes the reaction more effective than the Trommer’s test because the complex of tartrate and copper (II) ions is more readily soluble and therefore more reactive. If the test is positive, it forms brick-red precipitate derived from Cu+, too.
Benedict’s test
The Benedict’s test is used to differentiate reducing from non-reducing sugars. A complex of copper (II), or the Benedict’s reagent made of copper (II) sulphate (VI), sodium citrate and sodium carbonate, is added to the test solution. Compared to the reagents used in the Fehling’s test, it is more sensitive and more resistant to interfering compounds. It is also is more stable. A mixture of the test compound and the Benedict’s reagent is brought to a boil. When reducing sugars are present, brick-red copper (I) oxide precipitate is formed.