Ketones and aldehydes can be distinguished using which reagent?

Aldehydes and ketones are two important classes of carbonyl compounds in organic chemistry. Although both contain the carbonyl group (C=O), they differ in structure and chemical behavior. Aldehydes have the carbonyl group attached to at least one hydrogen atom, while ketones have the carbonyl group bonded to two carbon atoms. This small structural difference leads to major differences in reactivity.

One of the most common laboratory methods for distinguishing between aldehydes and ketones is the use of Tollens’ reagent. Tollens’ reagent is an ammoniacal solution of silver nitrate ([Ag(NH₃)₂]⁺). When this reagent reacts with an aldehyde, the aldehyde undergoes oxidation to a carboxylic acid, while the silver ions (Ag⁺) are reduced to metallic silver (Ag⁰). This reduction of silver ions produces a thin, shiny silver coating on the walls of the test tube, known as the silver mirror test.

The reaction can be summarized as:
R–CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R–COO⁻ + 2Ag (mirror) + 4NH₃ + 2H₂O

In contrast, ketones generally do not react with Tollens’ reagent because they cannot be easily oxidized under these mild conditions. This selective reactivity provides a reliable way to differentiate aldehydes from ketones.

Other reagents like Fehling’s solution and Benedict’s solution can also be used to test aldehydes, as they oxidize aldehydes to acids while reducing copper(II) ions to brick-red copper(I) oxide. However, Tollens’ reagent is considered more specific and visually distinctive because of the silver mirror.

Key Points to Remember:

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  Aldehydes give a positive Tollens’ test (silver mirror).

  
  


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  Ketones do not react with Tollens’ reagent.

  
  


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  Fehling’s and Benedict’s solutions can also test aldehydes but are less commonly used for ketone differentiation.

  
  

Therefore, the correct answer is Tollens’ reagent (Option 3).