Test for aldehydes and ketones
- For aldehydes
- Tollen’s reagent – It is a solution of Ammonical silver nitrate and it has formula [ Ag (NH3)2 ]+ OH- ] .Now ,Let’s take any aldehyde like :Formaldehyde and react with it as shown below .
Therefore, called as “Silver mirror test”. This test of oxidation that is Silver mirror test is not given by ketones.
- Fehling solution test
- It is prepared by missing equal amount of Fehling solution A and Fehling solution B.(Fehling A has Aq. CuSO4 and Fehling B has Rochelle salt) .
- We take test tube and add both solutions A & B equally and resulting solution is blue due to CuSO4).
- Now, when we add aldehyde this blue solution turns red. The medium is
alkaline due to Fehling solution B.
- The following reaction occurs.
Identification: The Colour changes from blue to red .This test is not given by ketones.
- Benedict’s solution test- It is alkaline solution of cupric ion complied with citrate ions. When we react with aldehyde the following reaction occur:
Identification: The Colour change from blue to red .This test is not given by ketones.
Test for Ketones
- Iodoform test
- By performing this test, we get yellow precipitate due to Formation of Iodoform.
- In this, we react aldehyde with Iodine in presence of NaOH. In this 3H are substitute by Iodine as shown below:
Uses of Aldehydes & Ketones:-
- Limitation of this test: This is given only by methyl ketones i.e. one of the R in ketone should be CH3 ,and also by ethanol as it also contains methyl group.
- See we have few examples Which will give and which will not give this test –