Testing Aldehydes & Ketones (AQA A-Level Chemistry): Revision Notes
📚 Revision Notes
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,, 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, , 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.
infoNote
Reaction Summary:
For aldehydes:
-
The aldehyde {RCHO} is oxidised to a carboxylate ion {⁻} while the silver(I) ions are reduced to silver metal** 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.