Tollens' Fehling's and Benedict's Test

 Three Visual “Tests” For The Presence of Aldehydes: Benedict’s, Fehlings, and Tollens’ Tests

We’ve seen previously that aldehydes are a functional group that can be oxidized relatively easily to carboxylic acids. For example, oxidation of alcohols with a “strong” oxidant like chromic acid (H2CrO4) results in an aldehyde that is quickly oxidized further to a carboxylic acid.

During this process, the aldehyde is oxidized and the oxidizing agent is reduced. Another way of framing this is to say that the aldehyde is the reducing agent in this process.

 The list of reagents that can be used to oxidize aldehydes to carboxylic acids is loooong. Of these, a few methods stand out in providing a particularly clear visual indication that the reaction has proceeded to completion. 

Three “visual” tests for aldehydes that you might encounter in an introductory organic chemistry lab are the following:

• Fehling's Solution:- where an aldehyde changes the color of a blue Cu(II) solution to red Cu(I)  [as Cu2O]. 
• Benedict's Solution:- a slightly modified version of Fehling’s solution
• Tollens' Test :- where aldehyde oxidation results in a beautiful “mirror” of silver metal to precipitate on the reaction vessel.

Importantly, ketones don’t react under any of these conditions.  The above tests were also a useful way of distinguishing aldehydes from ketones in the dark days before IR and NMR spectroscopy made this routine.