What is the difference between tollens and Fehling’s test?


Tollens and Fehling’s tests are both chemical tests used to distinguish between aldehydes and ketones. They rely on different reagents and principles, and here’s how they differ:

  1. Tollens Test (Silver mirror test): Tollens test is used to detect aldehydes. It involves the reaction between an aldehyde and Tollens’ reagent, which is a solution of ammoniacal silver nitrate (AgNO3) complexed with ammonia (NH3). The reaction proceeds as follows:

Aldehyde + Tollens’ reagent → Silver mirror + Carboxylic acid

If an aldehyde is present, it reduces the silver ion in Tollens’ reagent to metallic silver, resulting in the formation of a silver mirror on the inner surface of the reaction vessel. Ketones do not react with Tollens’ reagent, so no silver mirror is observed.

  1. Fehling’s Test: Fehling’s test is used to detect reducing sugars, which include certain aldehydes and ketones. The test is based on the reduction of Cu(II) ions in Fehling’s solution to Cu(I) ions, which results in the formation of a reddish-brown precipitate of copper(I) oxide (Cu2O). The reaction proceeds as follows:

Reducing sugar (aldehyde or ketone) + Fehling’s solution → Cu2O precipitate

Fehling’s solution is a mixture of two separate solutions: Fehling’s A (aqueous copper sulfate) and Fehling’s B (sodium potassium tartrate dissolved in sodium hydroxide). The two solutions are mixed in equal volumes just before the test is performed. When a reducing sugar is present, it reacts with Fehling’s solution upon heating, resulting in the formation of the characteristic reddish-brown precipitate.

It’s important to note that Fehling’s test can detect both aldehydes and ketones, whereas the Tollens test is specific for aldehydes. Additionally, Fehling’s test is commonly used to detect reducing sugars in solution, not just aldehydes and ketones, which makes it useful for identifying carbohydrates.

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