Testing for aldehydes and ketones

Distinguishing between carbonyls and other organic compounds is quite easy, simply add 2,4-DNPH  as above. The trick is to distinguish between aldehydes and ketones themselves.

Tollens’ Test

Simply put, this uses the reducing power of aldehydes to reduce silver (I) to metallic silver. This is seen as a silver mirror on the inside of the test tube. Hence the test is often known as the silver mirror test.

The creation of the reagent in this case is quite complex:

• Ag⁺ has to be in alkaline solution for this reaction to occur
• Addition of hydroxide ions (OH^-) precipitates silver oxide (Ag₂O)
• Ammonia is the complexing agent

1.                  Add ammonia carefully to about 3cm³ of silver nitrate in a clean test tube

2.                  Stop adding drops when the precipitate is just redissolved

3.                  Careful when adding ammonia – XS is not good!

This clear solution now contains the complex ion [Ag(NH₃)₂]⁺.

• A couple of drops of the aldehyde are added to the clear solution
• The solution is warmed gently in warm water

A positive result = silver mirror. A dirty test tube will give a black or grey ppte.

CH₃CH₂CHO + [O] → CH₃CH₂COOH

2[Ag(NH₃)₂]⁺(aq) + H₂O(l) → 2Ag(s) + [O] + 2NH₄⁺(aq) + 2NH₃(aq)