tollens

Tollens commonly refers to the German chemist Bernhard Tollens, after whom Tollens' reagent and the Tollens test are named. The term also denotes the qualitative chemical test he developed for distinguishing aldehydes from other carbonyl compounds.

Tollens' reagent is an alkaline solution of the diamminesilver(I) complex, typically prepared by dissolving silver nitrate in aqueous ammonia. In the presence of an aldehyde, the silver ion is reduced to metallic silver, which deposits as a mirror on the inner surface of the reaction vessel. The aldehyde is oxidized to the corresponding carboxylate, while the diamminesilver complex is regenerated in the basic medium, allowing the reaction to continue.

The test is primarily used to differentiate aldehydes from most ketones, since most ketones do not reduce silver under Tollens’ conditions. It is, however, not universally specific: certain compounds such as enols, alpha-hydroxy ketones, and some reducing sugars can give positive results. False positives and negatives can occur depending on concentration, reaction time, and the presence of interfering substances. The Tollens test is qualitative and relatively simple but can be sensitive to experimental conditions and reagent aging.

Historically, Tollens’ reagent played an important role in organic qualitative analysis and education. Although modern analytical methods have supplanted its routine use in many laboratories, the test remains a classic demonstration of redox chemistry and aldehyde reactivity. Handling involves standard laboratory precautions due to the caustic ammonia and the oxidative nature of silver nitrate.