Class 12 Chemistry Aldehydes Ketones Acids Test for Aldehydes and Ketones

Test for aldehydes and ketones

  1. 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 .

 Class_12_Aldehydes_&_Ketones_Tollens_Reagent

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.

 Class_12_Aldehydes_&_Ketones_Benedict's_Reaction_1

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:

          Class_12_Aldehydes_&_Ketones_Benedict's_Reaction_1

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:

         Class_12_Aldehydes_&_Ketones_IodoForm_Test_1

Uses of Aldehydes & Ketones:-

Class_12_Aldehydes_&_Ketones_Uses_Of_Aldehydes_&_Ketones

       Class_12_Aldehydes_&_Ketones_IodoForm_Test_2

  • 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 –

              Class_12_Aldehydes_&_Ketones_Limitation_Of_IodoForm_Test

 

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