8.3.3 Testing Aldehydes & Ketones

Aim:

In this practical, you will distinguish between an aldehyde and a ketone using a weak oxidising agent called Tollens' reagent. The aldehyde will be oxidised to a carboxylic acid, while the ketone will not undergo oxidation, which allows these compounds to be differentiated.

Key Concepts:

Aldehydes can be oxidised into carboxylic acids.
Ketones cannot be oxidised by weak oxidising agents like Tollens' reagent.
Tollens' reagent contains the silver(I) complex ion, $[Ag(NH₃)₂]⁺$ , which acts as an oxidising agent.
When an aldehyde is oxidised, the silver(I) ions in Tollens' reagent are reduced to form solid metallic silver, resulting in the characteristic silver mirror.

Method: Preparing and Using Tollens' Reagent

Prepare Tollens' Reagent:

Add 2 cm depth of 0.1 mol dm⁻³ silver nitrate solution to a clean test tube.
Add 2.0 mol dm⁻³ sodium hydroxide dropwise to the solution until a brown precipitate of silver(I) oxide forms.
Add concentrated ammonia drop by drop to the mixture, shaking gently after each drop, until the brown precipitate dissolves completely. This forms the silver(I) complex ion, $Ag(NH₃)₂⁺$ , which is colourless. Now the Tollens' reagent is ready for use.

Test the Suspected Aldehyde or Ketone:

Add a few drops of the suspected aldehyde or ketone to the prepared Tollens' reagent in a test tube.
Gently warm the test tube in a water bath (do not overheat).
Observe the results.

Expected Results:

Positive Test for Aldehydes (Silver Mirror Test):

When an aldehyde is gently warmed with Tollens' reagent, the aldehyde is oxidised to a carboxylic acid.
The silver(I) ions in the reagent are reduced to form solid metallic silver, which deposits on the inner walls of the test tube, creating a silver mirror.
This formation of the silver mirror is the positive test result for aldehydes.

Negative Test for Ketones:

Ketones cannot be oxidised by weak oxidising agents such as Tollens' reagent.
As a result, no silver mirror is formed when Tollens' reagent is gently warmed with a ketone.
This absence of reaction indicates a negative test result for ketones.

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Reaction Summary:

For aldehydes:

{RCHO} + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → {RCOO}⁻ + 2Ag(s) + 4NH₃ + 2H₂O

The aldehyde {RCHO} is oxidised to a carboxylate ion { $RCOO$ ⁻} while the silver(I) ions are reduced to silver metal** $Ag(s)$ For ketones:
No reaction occurs, as ketones cannot be oxidised by Tollens' reagent.

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Practical Tip:

Clean glassware: Always use very clean glassware for the Tollens' test to ensure accurate results and to avoid contamination that might affect the formation of the silver mirror.
Gently warm the test tube. Do not overheat the mixture, as high temperatures might cause decomposition of the reagent or alter the reaction.

Safety Considerations:

Ammonia and silver nitrate can be hazardous. Ensure you perform the experiment in a well-ventilated area, wear gloves, and avoid contact with skin or eyes.
Dispose of chemicals safely: Tollens' reagent can form explosive silver compounds if left to dry. After the test, neutralise the reagent with a small amount of dilute acid and wash the apparatus thoroughly with water.

Testing Aldehydes & Ketones Simplified Revision Notes for A-Level AQA Chemistry